Caliber arguments tend to have the conclusion that it is the placement of the shot that counts, and it does–just as soon as the projectile can actually get to what it is intended to disrupt. Defensive pistol bullets do not have much energy to operate on, and therefore, the engineering challenge is not complex: the load must continue to penetrate through clothes, tissue, and possibly bone without exhausting its heat.
That is why a very large number of trainers continue using 12-18 inches of ballistic gelatin as an effective test. It isn’t a magic “stop” number. It is a sanity test to determine if a bullet has a deep enough margin when the shot is angled or when the target is moving or when an arm is present.
There are cartridges that fail due to being small. Others due to the fact that the platform option throttles speed. Some do fail, in a less visible manner: they are capable of working, but only in a situation when everything remains in order.
1. .22 Short
22 Short is persistent as it is manageable, well-known and can be carried in little revolvers that fit easily in pockets. Even as a training round or a nostalgia piece, it does make sense.
The margin is meager as a defense mechanism. Modest speed light bullets are likely to fall short as soon as you add clothing and bone to the line and that becomes maybe performance rather than a consistency problem.
2. .25 ACP
25 ACP was a realistic solution to an actual limitation: small pocket-sized pistols that had to be centerfired. It is able to feed with ease and fire more reliably than rimfire in equally small firearms.
But in gelatin the cartridge is seen to have limited depth over and over. The five rounds all went beyond 11 inches in the same test with only one load of .25 ACP in pocket-caliber, and even then the outcome is below the typical minimum target. Any obstacle or even a turn of the road scalds up the little remaining power with so little reserve.
3. .32 S&W (Original)
The original.32 S&W is typical of older, small-frame revolvers and is remembered as easy to shoot. That part is true.
The only difference is the expectation of a defensive round to continue working through realistic obstructions. Practically the cartridges of later .32-family tend to be capable of supporting more usable velocity, and the additional speed will be the difference between a shallow halt and a satisfactory depth. Even without any evidence of expansion, .32 S&W Long loads showed good penetration in pocket-gun gelatin work, and that explains the rapid relegation of the original .32 S&W to the status of an outcast even within its own family.
4. .410 Bore (from a handgun)
There are 410 handgun platforms that sell the concept of shooting more than one bullet when the trigger is released and that is a kind of shortcut when you are close. It is physics that is its limiting factor: there is not much room to fit in barriers with short barrels and low per-projectile energy.
Some 000 buck loads can achieve useful performance in bare gelatin, but heavy clothing is likely to derail that performance. During testing of.410 handgun loads, plated 000 buckshot penetrated 14-16 inches into bare gelatin but was greatly decreased by the FBI heavy clothing barrier. Even the lightweight birdshot and BB-type payloads have a harder time, as shallow penetration is coded into the design at handgun velocities.
5. .22 LR (in pocket-size handguns)
The 22 LR is ubiquitous and it is tempting to consider it as a few-purpose tool. On handguns it provides low recoil and allows a quick follow-up shot and is surprisingly accurate in certain pistols. The two issues that are consistently occurring are reliability and depth consistency. Rimfire ignition is simply not as robust as centerfire and short barrels may restrict speed.
When this was tested under controlled conditions with two barrel lengths, the higher-speed loads did not necessarily give the highest penetration as the longer pistol was found to generate approximately 126 fps more velocity than a snub. The cartridge is capable of performing, but more conditions than most defensive choices are requested.
6. .380 ACP (FMJ as a “penetration fix”)
380 ACP is at the limit of what many carriers regard as minimum and choice of ammunition is where it or doesn’t stay together. Full metal jacket is frequently selected so that hollow-point clogging can be avoided, and penetration pursued.
FMJ can continue to move, but it normally cuts through a narrow path, and can leave without much disturbance, although it is not a sure way of achieving the straight-line depth desired after arms, shoulders and diagonal lines through the torso. The outcome is that one can trade tissue effect and (possibly) depth instead of providing the two consistently in a load strategy.
7. .45 GAP
45 GAP is an engineering: provide .45-caliber performance with a smaller package so that the use of a shorter grip frame on pistols becomes possible. In case of decrease in dimensions, the cartridge operates at a higher pressure in order to maintain performance. The weakness is supporting rather than ballistics. Having less mainstream platforms and more limited popularly distributed defensive load choices makes it more difficult to do performance validation and standardization as is done with common service cartridges. That ecosystem gap turns out to be a practicable problem of reliability with time elapsed: reduced opportunities to test, reduced combinations to verify, reduced routes to remain current.
All these cartridges are not harmless. The question is whether they retain sufficient performance margin when the conditions cease to be clean: layered garments, angled anatomy and imperfect presentations. Gelatin is not a crystal ball, but it makes a man square up. Loads with difficulty in reaching the common depth window in controlled cases are likely to have even lower buffer with actual-world variables appearing in the picture.
