The Caliber Wars Revisited
Today the shooting world has more or less settled on 9x19mm Luger as the default handgun caliber for a variety of reasons. Capacity, effectiveness, recoil, and more are all have combined to make 9mm the default round. However, I being a bit of a number’s nerd want to revisit this question with some comparisons. In this three-part series I’ll be looking at some of available ballistics data and attempt to draw some conclusions from it.
First well start by looking at how each of the popular calibers, .380 ACP, 9mm, .40 S&W, .357 SIG, and .45 ACP all perform in defensive loadings tested under the FBI ballistics standards.
Then we’ll look at barrel length and how it affects performance for each of these calibers.
Finally, we’ll try to create some kind of objective measurement for a particular round, in a particular handgun based on factors such as its performance in the FBI testing and how capacity might affect its performance to create a hopefully objective rating that you can use for ammunition not covered in the data we’ll be presenting.
The goal for this new standard of scoring that I am going to be presenting is to create an objective picture of what you are carrying, ammunition, pistol, and spare magazines into one total view that does not just focus on one aspect of performance, but as many as possible.
But first, I think it’s best if we lay some ground work and look at some of the things that we should be looking at when judging performance for a given handgun caliber.
Table of Contents
Self-Defense Performance of .380 ACP vs 9mm vs .40 S&W vs .357 SIG vs .45 ACP
Let’s set the stage. There are some key elements that we need to talk about before we get in too deep in this discussion.
I want you to think of this as a meta analysis of the testing data that I could gather, unfortunately, it is expensive and difficult to rigorously test a wide range of calibers and loadings out of many different types of pistols, so we’ll be focusing on a few specific data sets as major sources of information.
When comparing common handgun calibers for civilian self-defense, many of the same elements as Law Enforcement Officer use should be considered. However, intermediate barriers such as car doors or glass are not as important to consider as your average civilian defender is highly unlikely to need to be engaging a threat through a car door or a windshield. The “urban” nature of your neighborhood notwithstanding.
That leaves us with more “normal” key factors, including, penetration (meeting the FBI-recommended 12–18 inches in ballistic gel), bullet expansion, kinetic energy, and the trade-off with magazine capacity.
Lucky Gunner’s extensive ballistic gel tests of hollow-point ammo (through 4 layers of denim) provide a useful data set for these comparisons, supplemented by FBI insights and other reputable sources. When comparing these calibers, the data we are going to be most closely examining does use different barrel lengths, however, the discrepancy is because calibers such as .380 ACP tend to be used in smaller pistols with shorter barrels. We’ll be examining this more closely in a follow up article. However, this does not invalidate the data, as you are unlikely to find or carry a pistol in .380 ACP with a 4″ or 5″ barrel.
What About Other Calibers?
We could evaluate additional calibers in this piece, however, to keep this information more digestible we’ll leave aside other calibers such as .32 ACP, 10mm, or .44 Mag. These calibers, while potentially effective, make far more sense for more specialized use cases. Older shooters with weaker grip strength in the case of .32 ACP or wildlife defense in the case of 10mm and “similar” calibers. In this piece we’re trying to be as general as possible, with our analysis.
We are conceding that it is unlikely that you’ll have to shoot through car doors or similar barriers, you most likely won’t have to worry about grizzly bears, nor are you likely to encounter an attacker with body armor. That is reflected in the data set we have chosen to focus on, one that does meet the FBI ballistics standard. That said, if you have a legitimate reason why you would need to consider these elements, it might behoove you to look at other research and testing, in addition to this to find your ideal caliber.
So, with that out of the way let’s continue.
Penetration
(FBI 12–18″ Standard)
FBI standards call for 12–18 inches of penetration in calibrated gel as the ideal range for consistent stopping effectiveness. All of the service calibers (9mm, .40, .357 SIG, .45) can generally meet this standard with quality modern hollow points, whereas .380 ACP is on the borderline. Lucky Gunner’s ballistic test data (using short-barreled carry guns) found the following average penetration depths in 10% gel (after heavy clothing):
Average Penetration by Caliber -
.380 ACP Penetration
Approx. 10–14 inches. Many .380 ACP defensive loads struggle to consistently reach 12″ of penetration. In Lucky Gunner’s tests, the median penetration was about 13.3″, just over the FBI minimum. This suggests some .380 rounds meet the bare minimum, but others fall short if the bullet expands too much or slows quickly. (In fact, some .380 JHPs that fully expand only penetrate ~8–11″ in gel, and only loads that under-expand or FMJ reliably exceed 12″.) In short, .380 ACP can just barely satisfy the penetration requirement with select loads, but it leaves little margin for error.
9x19mm Luger (9mm) Penetration
Typically, 12–18+ inches. Modern 9mm hollow points are very good at penetrating deep enough. Lucky Gunner’s tests showed a median of 17.9″ penetration for 9mm, near the upper end of the FBI’s ideal range. In practice, most premium 9mm self-defense rounds easily hit 12–18″ even through heavy clothing. In fact, 9mm’s smaller diameter and high velocity often give it slightly deeper penetration than larger calibers, since it faces less resistance. It’s not uncommon for 9mm +P loads (e.g. 124–147 grain JHP) to land around 14–18″ in calibrated gel – well within the safe zone. This penetration performance has been a key factor in the FBI (and many police agencies) standardizing on 9mm.
.40 S&W Penetration
Typically, 14–17 inches. The .40 S&W (180gr, 165gr JHPs, etc.) was designed to meet FBI performance specs, and it does. Median penetration in tests was about 16.5″. In heavy-cloth gel shots, .40 often ends up in the mid-teens (inches) of penetration – comfortably within the 12–18″ window. It generally penetrates a bit less deeply than 9mm of similar design (because the .40 expands to a larger diameter and carries more mass, slowing it down slightly in the gel). But effectively, .40 meets the FBI standard with a bit of a safety margin against under-penetration. Over-penetration is usually not a concern; .40 JHPs tend to stop well before the 18″ mark in calibrated tests.
.357 SIG Penetration
Typically, 16–18+ inches. This bottleneck high-velocity round tends to penetrate toward the upper end of the FBI range. Lucky Gunner’s median was about 18.3″, meaning many .357 SIG loads went right up to ~18″ of penetration. In gel tests, .357 SIG 125gr JHPs often expand and still hit 15–18″ consistently. In some cases, if the bullet doesn’t fully expand or is a bonded core designed for barriers, .357 SIG can exceed 18″ (indicating a risk of over-penetration). But with modern duty bullets, it reliably meets the minimum. Essentially, .357 SIG will penetrate as deep as needed — and sometimes almost too deep — due to its high velocity (it was designed to mimic .357 Magnum revolver performance in an autoloader).
.45 ACP Penetration
Typically, 13–17 inches. Despite its larger diameter, .45 ACP’s heavy bullets (185–230gr) penetrate well. In Lucky Gunner’s test data, .45 ACP JHPs had a median penetration around 16.7″. Many standard-pressure .45 hollow points penetrate 14–16″, and +P or bonded loads can reach the upper teens. Because .45 bullets start out big and expand wide, they encounter a lot of drag in gelatin, often stopping in the mid-teens of inches (which is ideal). The .45 usually doesn’t over-penetrate when using JHPs – it tends to “dump” its energy and come to rest well within the 12–18″ zone. Even lighter 185gr +P rounds that expand aggressively will get close to 12″ penetration. In short, .45 ACP meets the FBI standard with a comfortable penetration depth, similar to .40 S&W.
Penetration Summary
With quality hollow-point ammunition, 9mm, .40 S&W, .357 SIG, and .45 ACP all achieve the FBI-recommended 12–18″ penetration range in ballistic gel. The difference is that 9mm and .357 SIG often penetrate on the deeper side (closer to 18″), while .40 and .45 tend to stop a bit shorter (around 14–17″). .380 ACP is the one caliber that struggles – many .380 loads only reach 9–11″ unless they sacrifice expansion. So, for penetration, any of the service calibers (9mm and above) are adequate; .380 ACP can be adequate with the right load, but it’s the least consistent in meeting the 12″ minimum.
Expansion
(Bullet Diameter Growth)
Bullet expansion increases the wound cavity size, as the hollow point opens up to cut a wider channel. The FBI protocol ideally wants a bullet to expand to at least 1.5 times its original diameter in gel. Modern premium JHP designs (bonded cores, +P versions, etc.) are very good at achieving this. Lucky Gunner’s tests measured expanded diameters, and the typical expansion for each caliber was:
.380 ACP Expansion
~0.45–0.55″ expansion is common, roughly 0.475″ median. Since .380 bullets are 0.355″ in diameter unfired (same as 9mm), this is about 1.3–1.5× expansion. That’s on the lower end of FBI’s ideal. In practice, .380 JHPs often expand less or irregularly, because at .380’s lower velocity some bullets only partially expand or deform. Some .380 rounds do expand well (~0.5″), but then they usually have shallow penetration as noted. Others might barely expand (or fragment), penetrating more but acting almost like FMJ. In short, .380 creates the smallest expanded bullet of these calibers – not much bigger than an unexpanded 9mm.
9×19mm (9mm) Expansion
~0.50–0.70″ expansion, around 0.58″ median in tests. Starting from 0.355″, a good 9mm JHP typically doubles in diameter to ~0.60″ (≈1.6× expansion). Many of the best 9mm defensive loads (e.g. Federal HST, Speer Gold Dot, Winchester Ranger) expand to about 0.60–0.70″ consistently in gel, even through clothing. This is a significant diameter – roughly the width of a nickel – and not far behind the larger calibers. The expansion ensures a larger wound channel than the original small 9mm profile, while still preserving penetration. Overall, 9mm meets the FBI expansion goal and then some, with reliable uniform expansion in modern designs.
.40 Smith & Wesson Expansion
~0.60–0.80″ expansion, with a 0.686″ median. A .40 caliber bullet is 0.400″ unexpanded, so it often opens up to 1.7× its size. That typically yields ~0.65–0.75″ expanded diameter in real tests (some lighter .40 JHPs can top 0.80″). In Lucky Gunner’s data, .40 had a larger average expansion than 9mm or .357 SIG. That wider mushroom means a broader wound path. Essentially, .40 S&W’s primary advantage is that it makes a bigger hole than 9mm – about one-eighth to one-fifth of an inch wider on average. This gives it a slight edge in crush cavity area. Importantly, .40 achieves that expansion while still penetrating adequately (as seen above). So, expansion performance is excellent for .40 S&W with quality ammo.
.357 SIG Expansion
~0.50–0.70″ expansion, around 0.60″ median. Despite its high speed, .357 SIG uses 9mm-diameter bullets (0.355″) and generally expands similarly to 9mm. In fact, many .357 SIG JHP loads are just slightly tweaked 9mm designs – they expand a bit more violently or reliably due to higher impact velocity, but the final diameter is comparable. Lucky Gunner’s tests showed a median ~0.60″ expansion for .357 SIG, virtually the same as 9mm. Notably, some lighter or faster .357 SIG rounds can fragment or over-expand, which could lead to a very large expansion but at the cost of penetration. However, standard 125gr bonded hollow points tend to expand to around .60–.65″ in gel, very effective. In summary, .357 SIG expands well — on par with quality 9mm – but generally does not out-expand a .40 or .45 (its advantage lies more in energy and penetration than bullet diameter).
.45 ACP Expansion
~0.70–0.90″ expansion, with a 0.731″ median. Starting from the largest bullet diameter (0.451″), .45 ACP JHPs often expand to around 0.70–0.80″ in gel, roughly 1.6× their original size. This is the largest expanded diameter of the group – roughly the width of a quarter coin. Many .45 hollow points routinely hit 0.75″ expansion. In Lucky Gunner’s test data, .45 was the clear leader in expanded size. This means a .45 can create a wider permanent wound channel, all else equal. The heavy bullet and larger frontal area contribute to robust expansion. Even the FBI’s original testing back in the 1980s found .45 JHPs could expand to around an inch in bare gel (though usually a bit less through cloth). Modern .45 designs easily meet the 1.5× expansion goal. The trade-off, of course, is that pushing such a wide bullet can limit penetration to the mid-range of the spec (which, as noted, is usually fine).
Expansion Summary
All of these calibers (besides marginal .380) are engineered to expand roughly 1.5 times their original diameter or more, which they do in testing. As a reference, a 9mm round is .354 inches in diameter and should expand to roughly .531 ideally.
That means 9mm, .40, .357 SIG, and .45 ACP all produce a significantly expanded bullet in tissue simulant – increasing their effectiveness over FMJ. .45 ACP has the edge in sheer expanded size (often ~0.75″), followed by .40 S&W (~0.65–0.70″). 9mm and .357 SIG tend to expand to around ~0.55–0.65″, a bit less in absolute terms, but still very effective. And .380 ACP trails with ~0.45–0.50″ expansion if it expands at all. In practical terms, a well-designed 9mm JHP can reach about the same expanded diameter as a lower-end .45 JHP – modern 9mm bullets are that good. The larger calibers’ expansion advantage is real but not enormous: for instance, a .45 might expand ~0.15″ wider than a 9mm (Lucky Gunner Gel Test Median Results by Caliber | The High Road), which is an increased wound area but not a night-and-day difference. This fact underlies the ongoing debate about whether the bigger bullet’s slightly larger expansion is worth the other trade-offs.
Energy Transfer and Kinetic Energy
Bullet energy (usually measured in foot-pounds, ft-lb) affects the velocity and the potential for creating a temporary cavitation in tissue (though handguns are generally below the velocity threshold for true hydrostatic shock in human targets). Energy can indicate how “powerful” a round is, but energy alone doesn’t guarantee better stopping power – it must be coupled with sufficient penetration and expansion to be effective. All of these calibers are relatively low-energy compared to rifles, but there are notable differences among them:
.380 ACP Energy
Muzzle energy is comparatively low, often around 150–250 ft-lbs. In Lucky Gunner’s short-barrel tests, .380 loads delivered between ~115 ft-lb up to ~200 ft-lb of energy This is only about half the energy of a typical 9mm. In plain terms, .380 ACP is anemic in energy by service caliber standards (Handgun Bullet Muzzle Energies by Caliber). It simply launches a lighter bullet at lower velocity. This lower energy contributes to .380’s poorer penetration and expansion (the bullet often uses up its limited energy early). For self-defense, .380’s energy can still be lethal with good shot placement, but it leaves less room for error or barrier penetration.
9×19mm (9mm) Energy
Standard pressure 9mm loads (115–147gr) usually run in the 300–400 ft-lb energy range, while +P rounds can approach 450 ft-lbs. For example, a common 115gr 9mm JHP at ~1,225 fps carries about 383 ft-lb of energy. A heavier 124gr +P can be around 400+ ft-lb. This level of energy is considered adequate for self-defense – and it is efficiently used when the bullet expands and stops in the target (dumping energy). In fact, 9mm is the baseline: other service calibers were developed to get more energy or bigger bullets, but advancements in 9mm ammo have narrowed the gap. The FBI notes that as of the late 2010s, improved 9mm projectiles can deliver terminal performance on par with the larger calibers. In short, 9mm gives you moderate recoil with a muzzle energy that has proven effective in real-world shootings, especially with today’s bonded hollow points.
.40 S&W Energy
This caliber was originally adopted to boost energy and effectiveness over 9mm. We’ll skip the rabbit hole that you can go down when it comes to the history of its development for now. Typical .40 S&W loads (165–180gr) produce 400–500 ft-lbs of energy (higher with lighter bullets). For instance, a 180gr .40 at ~1,000 fps has ~400 ft-lb, and a 155gr at 1,200 fps can exceed 450 ft-lb. One comparison showed .40 S&W (180gr) at about 450 ft-lb vs a similar 9mm around 330–350 ft-lb – roughly 20–30% more energy. In Lucky Gunner’s data analysis, it was observed that in short barrels .40 S&W, .45 ACP, and .357 Mag all had about the same muzzle energy (on the order of ~400–500 ft-lb). In other words, .40 sits in the middle of the pack energy-wise; it clearly hits harder than 9mm, but not as hard as a hot .357 SIG. The extra energy can contribute to slightly larger temporary cavities and perhaps more “shock” to the target, though again, handguns rely mainly on penetration and permanent cavity. The key is that .40 uses its energy to drive a wider bullet (as seen above) into the target, ideally without over-penetrating.
.357 SIG Energy
The .357 SIG is loaded to high pressure and fires a light bullet very fast, yielding around 500–600 ft-lbs of energy. A typical defensive load is a 125gr JHP at 1,350–1,400 fps (from a duty-length barrel), which is about 500–550 ft-lb of energy. For example, tests have shown a 125gr .357 SIG with ~1,395 fps velocity carrying 540 ft-lb at the muzzle. That is roughly 50% more energy than a standard 9mm round. In the Lucky Gunner data, .357 SIG out of a 4.5″ barrel had some loads around 700–760 Joules (520–560 ft-lb), making it one of the most energetic common pistol rounds. This energy translates to very high velocity impact, which can cause bullets to expand rapidly or even fragment. The temporary stretch cavity in gel from a .357 SIG can be more pronounced than that of 9mm or .45. However, if the bullet is well-designed, that energy is spent on expansion and penetration rather than exiting the target. Essentially, .357 SIG gives you magnum-like energy in a semi-auto format. The downside is more recoil and muzzle blast, which can make follow-up shots harder. So, it remains a viable, but not popular option.
.45 ACP Energy
The .45 launches heavy bullets but at a slower velocity, so its energy is moderate – often in the 350–450 ft-lb range for standard pressure, and up to ~500 ft-lb for +P loads. A 230gr .45 at 850 fps has around 369 ft-lb, and a 230gr +P at 900+ fps can hit 400+ ft-lb. Lighter 185gr +P loads at ~1,000+ fps can approach 450–500 ft-lb. For example, one chart shows a 230gr JHP at 950 fps with 461 ft-lb of energy – indeed a bit higher than a lighter 9mm round. In general, .45 ACP’s energy is comparable to .40 S&W’s when fired from similar barrel lengths. It’s well above .380 or standard 9mm, but usually not dramatically more than 9mm +P. The .45’s philosophy is a heavy, slow bullet that transfers energy by making a big hole and stopping in the target. It tends to have a lower velocity, hence a less violent temporary cavity, but a large projectile that delivers its energy over a broad front. Many .45 enthusiasts point out that it “hits with authority,” and indeed, when a .45 bullet expands to 0.75″ and penetrates a foot or more, it can impart substantial energy to tissues (often fully dumping its ~400 ft-lb in the target).
Energy Summary
Summary: In raw numbers, .357 SIG is the energy champ here (approaching the foot-pound energy of a mild .357 Magnum revolver and generally exceeding the others by 20–50%). .40 S&W and .45 ACP are next, roughly on par with each other in typical carry guns (around 400–450 ft-lb, give or take). 9mm is a step down but still in the effective range (~350 ft-lb for standard rounds, up to ~400+ ft-lb for +P). And .380 ACP is far below the rest in energy (often <200 ft-lb). It’s important to note that all of these handgun energies are relatively low (a 5.56mm rifle by comparison might have 1,200+ ft-lb). What matters is how the energy is used: a bullet that penetrates to the vital organs and expands will transfer its kinetic energy to the target, whereas an over-penetrating bullet might leave with unused energy.
In this sense, as long as a round meets the penetration and expansion criteria, having “excess” energy offers diminishing returns, causing over penetration or exiting the target. There’s little evidence that a handgun round’s temporary cavity (stretch) causes significant additional damage unless velocity exceeds roughly 2,000 ft/s (which none of these do). So, while .357 SIG’s extra energy might sound impressive, in practice a well-placed 9mm, .40, or .45 will cause similar damage if the bullets expand and penetrate properly. Shot placement and sufficient penetration are far more critical than sheer muzzle energy in handguns.
Magazine Capacity Considerations
Caliber choice also impacts how many rounds your pistol can hold (assuming a given frame size). Generally, smaller diameter cartridges = higher capacity in the same size magazine. This is a major practical factor in self-defense, since no handgun bullet is a guaranteed “one-shot stop” and multiple hits might be needed. Here’s how capacity tends to shake out:
.380 ACP Magazine Capacity
Many .380 ACP pistols are very small concealable guns, which inherently limits capacity (often 6+1 or 7+1 in pocket-sized .380s). However, if one were to build a larger double-stack .380, the cartridge itself is short and slim, so you could fit quite a lot. For instance, some subcompact .380s hold 10+ rounds with extended magazines. That said, in practice, people choose .380 for ultra-compact carry guns where even 6 or 7 rounds might be the max due to grip size. It’s somewhat a moot point to compare .380 capacity to the others, because it shines in small format guns where 9mm might not fit or would recoil too hard. In equal size, a 9mm is almost always a better choice; .380’s only real advantage is that it can be chambered in tiny, easy-to-carry pistols. If capacity is a concern, one could simply carry a similarly sized 9mm with more rounds for a slight decrease in concealability.
9x19mm Magazine Capacity
This round’s small size allows for high magazine capacity. Most compact or full-size 9mm pistols hold between 12 to 17 rounds in a standard magazine. For example, a Glock 19 (compact) carries 15+1 rounds of 9mm, and a Glock 17 (full-size) carries 17+1. Even some subcompacts can manage 10+ rounds of 9mm. This high capacity is a key advantage of 9mm. In fast-moving defensive encounters, having more rounds on tap gives you a better chance to hit vital areas (especially under stress when accuracy drops). Law enforcement statistics show officers often miss a majority of their shots in a gunfight, which is one reason many agencies favor the higher capacity of 9mm. For civilians, capacity may or may not come into play, but it’s certainly peace of mind to have more rounds in case of multiple attackers or if initial shots don’t stop the threat.
.40 S&W Magazine Capacity
Because .40 S&W uses a wider case than 9mm, magazines hold fewer rounds for a given size. A rule of thumb: you lose 2 rounds of capacity going from 9mm to .40 in the same gun design. For instance, the Glock 23 (compact .40) holds 13+1 rounds, whereas the similarly sized Glock 19 (9mm) holds 15+1. Full-size .40 mags often hold 15 rounds (vs 17 in 9mm). This is an inherent trade-off with .40 – you carry ~10–20% fewer rounds compared to a 9mm of the same dimensions. Some shooters accept that penalty for the added power of .40, while others prefer having more rounds. When I first started carrying I accepted this trade off, as I was able to find a .40 S&W pistol that fit my hand comfortably and helped with taming the “snappy” recoil of the .40 S&W. A decision that was made during the years when .40 S&W was still a popular LEO choice. It’s worth noting that after the FBI and many police forces moved away from .40 back to 9mm, one factor cited was capacity: the ability to have more rounds in the magazine (along with 9mm’s easier recoil for faster follow-ups)
.357 SIG Magazine Capacity
The .357 SIG cartridge has a bottleneck shape but uses a case derived from .40 S&W in diameter, so most .357 SIG pistols have identical capacity to their .40 S&W counterparts. Typically, that means a little lower capacity than 9mm. Using Glock again as an example: a Glock 32 (compact .357 SIG) holds 13+1 (same as the .40 cal Glock 23). Some .357 SIG magazines might hold one less than the .40 version due to the bottleneck, but generally it’s the same. So, expect about 12–15 rounds in a service-size .357 SIG pistol. This is still quite good, but you give up a couple rounds versus a 9mm of equal size. Given .357 SIG’s ballistic performance, many consider that a fair trade. However, one reason .357 SIG never became as popular in police use is that it has no capacity advantage over .40 (and shares the same lower magazine count vs 9mm). When capacity and shootability were prioritized, agencies went to 9mm.
.45 ACP Magazine Capacity
Guns chambered in .45 ACP almost always have lower capacity than those in other service calibers. The .45’s cartridge is big and fat (0.48″ case diameter, 0.451″ bullet), so in a double-stack mag you get significantly fewer rounds. Many traditional .45 pistols (like the M1911) use a single-stack magazine of 7 or 8 rounds. Even modern double-stack .45s hold around 10–13 at most (e.g. a full-size Glock 21 is 13+1 rounds of .45 ACP). A more compact .45 (Glock 30) holds 10+1. So you’re typically looking at 40–50% fewer rounds in .45 vs an equivalent 9mm handgun. To compare, most 9mm pistols carry 12–15 rounds, whereas most .45s carry 8 (standard) up to maybe 10, unless you have an extended or enlarged magazine. This is a significant firepower gap. While “tactical” reloads are always an option, one cannot ignore that in the same space where you could have 15 rounds of 9mm, you might only have 8 of .45. This capacity shortfall is one of .45 ACP’s biggest drawbacks in a defensive context. It requires more frequent reloading and gives less room for misses or multiple assailants. Advocates might argue that “you only need one shot” or that the .45’s effectiveness offsets the lower count – but objectively, more rounds on tap is a tangible advantage in a gunfight.
Magazine Capacity Summary
Among service calibers, 9mm offers the highest magazine capacities, often 15+ rounds, whereas .45 ACP offers the fewest (often 7–10 rounds), with .40 S&W and .357 SIG in between. This means with 9mm you may have 5–7 extra rounds compared to a .45 of similar size. Conversely .380 ACP suffers from limited capacity not due to its size, but the size of the guns that it is often chambered in. That can be decisive in a prolonged encounter. The FBI has explicitly cited higher capacity (and resultant higher hit probability) as a factor in favoring 9mm for agents. For civilian defense, capacity might be less critical than it is for law enforcement sweeping the streets, but it’s still a comfort to have more rounds than not. No defensive shooter ever complained about having “too many” rounds available in an emergency. Since handgun wounding capability is limited, no matter the caliber having additional shots to stop a threat is a significant plus in favor of smaller calibers.
Weighing Expansion & Energy vs. Capacity
(Is Bigger Better?)
The crux of the caliber debate often boils down to quality vs quantity: Does a bigger, more powerful bullet (.45 ACP, .40 S&W, etc.) provide a noticeable terminal performance advantage to justify fewer rounds and more recoil, or are you better off with more capacity and easier shooting (9mm) once minimum performance is met? Based on the data:
Lower-capacity calibers like .45 ACP do have advantages
In terms of expansion diameter and per-shot energy – but these advantages are not as dramatic as one might think. For example, a .45 ACP might expand to ~0.75″ while a 9mm expands to ~0.60″. Yes, the .45’s bullet is wider, resulting in a somewhat larger permanent cavity. And as noted, a .45 can hit with maybe 100 ft-lbs more energy than a 9mm, potentially causing a slightly larger temporary cavity or greater tissue disruption. However, in realistic terms, both rounds can fatally wound an attacker quickly if they hit a vital area – and conversely, both can fail if they don’t.
The extra 0.15″ of expansion or couple hundred joules of energy that .45 ACP offers have not proven to guarantee faster stops in actual shootings. Indeed, the FBI’s own analysis found “little to no noticeable difference in the wound tracks” between premium 9mm, .40 S&W, and .45 ACP bullets.
All will punch roughly the same depth and cause a similar volume wound channel (any differences are on the order of a few millimeters). Shot placement and penetration trump absolute expansion beyond a certain point. As the report put it, “Handgun stopping power is a myth” – no pistol bullet is so powerful that it drops someone without a well-placed hit. Given that reality, the .45 ACP’s slightly bigger holes or the .357 SIG’s extra energy on paper don’t necessarily translate to a meaningful real-world advantage in stopping determined attackers.
Meanwhile, the higher-capacity (typically smaller caliber) option offers very tangible benefits
The ability to have more rounds in your magazine – and often less recoil per shot – means more opportunities to make effective hits quickly. The FBI notes that their agents (and shooters in general) are usually faster and more accurate with 9mm than with .40 S&W or .45, due to the 9mm’s lower recoil and muzzle rise.
This can be even more crucial for less experienced or recoil-sensitive shooters. In defensive situations, multiple hits increase the likelihood of stopping a threat. If a .45 might neutralize with, say, 2 good hits, and a 9mm might require 3 – the 9mm still likely has 12+ rounds left in a full mag, whereas the .45 might have 5. And if you miss (which is statistically very likely under stress), the 9mm gives you more chances before running dry.
Simply put, capacity (and control) can trump caliber, as long as the caliber meets the basic penetration/expansion threshold. This sentiment is reflected in modern law enforcement decisions: when the FBI switched back to 9mm, they highlighted that advances in bullet design, and powder formulation made 9mm as effective as larger calibers, removing any need to accept the downsides of .40 or .45 (like reduced capacity and increased recoil). The primary concern became placing more rounds on target rather than marginal gains in bullet size.
So, does .45 ACP offer any “compelling” advantage?
For most experts today, the answer is “not particularly, with modern ammo.” Yes, the .45 makes a slightly bigger hole and is naturally subsonic (good for suppressors, though that’s a niche concern). And .45 ACP has a storied reputation for “stopping power” largely due to legacy and perhaps psychological impact. But objective testing and ballistic gelatin data show that a top-tier 9mm JHP and a top-tier .45 JHP cause very similar wound channels and penetration. The .45’s wound might be a bit wider; the 9mm’s a bit deeper – but both are lethal. There is no magic caliber that will drop an attacker instantly without a CNS (central nervous system) hit, and a .45 to the arm is far less effective than a 9mm to the heart.
In that sense, shot placement and sufficient penetration matter far more than an extra tenth of an inch of expansion. The FBI concluded in 2014 that with contemporary bonded hollow points, 9mm Luger offers all the terminal performance of .40 or .45, “with none of the disadvantages of the ‘larger’ calibers.” That’s a strong statement, essentially saying there’s no measurable stopping advantage to .40 or .45 if the 9mm is using a good bullet. Accordingly, many professionals now carry 9mm for the higher capacity and faster follow-up shots – because as long as a bullet meets the 12–18″ penetration standard, the priority shifts to shot placement and the ability to deliver multiple hits.
Capacity vs. Performance – finding the balance
The FBI’s penetration standard (12–18″) is essentially a go/no-go gauge for service ammo. Any load that meets it is considered capable of reaching vital organs from various angles (including through an outstretched arm, etc.). Once a caliber reliably meets that benchmark, having more rounds (capacity) and being able to shoot them accurately under stress arguably become the most important factors. This is why many experts recommend choosing the caliber you can shoot fastest and most accurately, assuming it’s 9mm or above (since .22 or .32, while easy to shoot, don’t meet the penetration threshold reliably).
For most people, that tends to be 9mm. A heavier caliber like .45 ACP might offer a slight edge in one shot expansion or energy, but if it slows down your follow-up shots or limits you to half the ammo, is it truly better for defense? The general trend in both law enforcement and informed civilian circles has been toward smaller, high-capacity calibers with advanced bullet designs – because they get the job done and give you more chances. No handgun is a death ray; you might need multiple hits – so carry as much ammo as you reasonably can. If a 9mm, .40, and .45 will all penetrate to the same vital depth and expand, you might as well have the one that lets you carry 15 rounds and shoot quickly, rather than 8 rounds and a bit more recoil.
Energy and Capacity Compared
To further drive home the point about how capacity can influence things let’s look at this from a “total energy per magazine” standpoint.
Using the formula “Average Muzzle Energy × Magazine Capacity”, we can determine each caliber’s total kinetic energy in a full magazine. For this we’ll be using average muzzle energies and full-sized handgun capacities.
- .380 ACP: 200 ft-lbs × 10 rounds = 2,000 ft-lbs per magazine
- 9mm Luger: 375 ft-lbs × 17 rounds = 6,375 ft-lbs per magazine
- .40 S&W: 450 ft-lbs × 15 rounds = 6,750 ft-lbs per magazine
- .357 SIG: 525 ft-lbs × 15 rounds = 7,875 ft-lbs per magazine
- .45 ACP: 400 ft-lbs × 13 rounds = 5,200 ft-lbs per magazine
As you can see, with the capacity advantage, even with a larger 13 round .45 ACP magazine we get more energy per magazine with a 17 round 9mm magazine. .357 SIG pulls ahead of both due to the increases in energy from sticking a 9mm round behind what is basically a .40 S&W powder load.
Policing the Brass - Coming to Some Conclusions
In summary, all five calibers in question can be viable for self-defense, especially with modern hollow-point ammunition. Penetration is crucial: 9mm, .40 S&W, .357 SIG, and .45 ACP all meet the FBI 12–18″ standard with good JHPs, whereas .380 ACP is borderline and generally considered the minimum caliber for defensive use.
Expansion characteristics show that .45 ACP and .40 S&W create somewhat larger expanded bullets than 9mm or .357 SIG, but all service calibers tend to expand ~1.5–1.7× their original size, which means in practice they all cause comparably significant wounds. Energy wise, the hotter rounds like .357 SIG (and .40 S&W to an extent) carry more kinetic punch, but as long as the bullet transfers that energy into the target (by expanding and stopping within 12–18″), a higher energy number doesn’t automatically equate to a more effective stop. Magazine capacity and shootability end up being major discriminators: a 9mm gives you more rounds and generally faster follow-up shots, which can be life-saving advantages, whereas larger calibers like .45 ACP sacrifice capacity (and tend to produce more recoil) for a minimal increase in bullet size.
Ultimately, the consensus of testing and real-world experience is that shot placement and sufficient penetration are far more important than caliber. Any of the common service calibers 9mm through .45 ACP, using hollow-point ammo that meets the FBI standards and expands well, will do the job about equally well if you do yours. Since that’s the case, many experts argue you should carry the one that lets you maximize hit probability – which often means 9mm for its high capacity and manageable recoil.
As the FBI Training Division put it, “given contemporary bullet construction, [agents] can field 9mm with all of the terminal performance of any other caliber, with none of the disadvantages”. In other words, once a cartridge meets the FBI penetration/expansion benchmarks, capacity and shootability become the primary concerns. Smaller, faster rounds like 9mm (and even .357 SIG) meet the standard and allow more rounds per magazine, whereas big slow rounds like .45 meet the standard but limit capacity.
There is no clear “knockdown” advantage to the bigger calibers to outweigh that. So, while .45 ACP and .40 S&W remain effective choices, their lower capacity doesn’t provide a compelling ballistic edge in expansion or energy transfer under most conditions.
For the average defender, a quality 9mm JHP, that meets FBI standards in a higher-capacity pistol is often considered the most prudent balance of performance and practicality.
Next up in this series we’ll start looking at how barrel lengths can affect these calibers and then we’ll wrap up with a third part where we try to create an objective measurement for different loadings based on their performance in the FBI ballistics testing.
