Well...your question will generate some discussion and some disagreement. The answers will depend on the theory being used.
The Hatcher rating for example dates back over 100 years and it holds that it is diameter and momentum (mass times velocity) that mostly does the work with point shape having some effect. If I recall correctly a round ball had a form factor of 1.0, with
0.9 Jacketed Round Nose
1.0 Lead round nose and unexpanded soft point
1.05 Small Flat Point
1.10 Large Flat Point and unexpanded hollow point
1.25 Lead Semi Wadcutter (and other “Keith Types”)
Hollow points and soft points were added later.
1.35 Jacketed Softpoint and Hollow point (expanded)
Hatcher’s relative stopping power formula it generally favored heavier bullets in comparisons like a 185 gr or 230 bullet of the same point type. It was an attempt to try to develop a formula that would account for bullet performance on live stock and cadavers that would then allow meaningful comparisons between different cartridges and loads.
Despite the extensive of terminal effects on living livestock and wound analysis of hI am cadavers, the Hatcher RSP fell out of favor.
Many folks, and many of them claiming to be “authorities“, place all their faith in ballistic gel tests and denigrate the data folks as using “anecdotal“ evidence. That’s a mistake as it fails to take into account the fact that the FBI‘s ballistic gel penetration standards were based on reports of officer and citizen involved shootings and the loads used. Looking at a large number of shoots they then determined which loads tended to be most effective in the real world and then tested them in ballistic gel to see how they performed.
The FBI then created its acceptable performance envelope based on the performance of those rounds that showed good real world effectiveness. The ballistic gel results are thus just used for comparison of relative performance between different loads, and against a performance standard that correlates with good performance in real world shoots. It’s also convenient as it is repeatable due to the use of a consistent test media.
The slower kids in the class miss the fact that those gel penetration performance standards were developed based on analysis of real world shoots - data analysis of real world shoots. The data they themselves denigrate.
In truth both the data approach and the gel testing approach have serious limitations.
On the data side, there are a lot of factors that are not considered or that can only be considered in fairly broad terms given the limitations in the data collected. Where the individual was shot is a big wild card and at best you might be able to separate out torso hits from extremity hits.
There are also other factors that could, come into play such as lighter recoiling calibers being able to be fired with greater accuracy and or more rapidly. In some cases, those can cancel each other out. For example a shooter who can accurately place 3 rounds of 9mm in the vital zone in the same period of time he could place 2 shots in the same zone with a .45 ACP, may be just as effective with a 9mm, but the greater number of shots he is able to fire in the same period of time would make it appear as if the 9mm required more shots to stop the assailant. That may or may not be the case, and we don’t really know short of an autopsy and knowledge of what 9mm rounds hit first, and we won’t know that.
On the ballistic gel side, the weaknesses are that the acceptable performance range is based on data analysis of a large number of shoots with all the same limitations described above. Plus some FBI/Law Enforcement centered concerns such as wanting greater minimum penetration to ensure adequate penetration when shooting at fleeing felons from the side, etc where an arm or arm bone may need to be penetrated before the torso is even hit.
Way too many armed citizens regard the 12” minimum penetration as sacrosanct when it really doesn’t have much relevance to the face to face aspect angle that occurs in almost all legitimate armed citizen self defense shoots. Again for the slower kids in the class that means if the assailant is fleeing and you shoot at him, you are most likely soon to be referred to as “the defendant“, followed shortly there after as “inmate”.
There’s also the potential problem of over penetration where that 18” maximum could still be excessive in a face to face engagement.
And of course actual people don’t have much in common with a homogeneous block of ballistic gel.
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Personally, I do my own ballistic gel testing with the loads and handguns I carry as I want to know wha kind of gel performance I really get in my specific situation. Some hollow points have narrow expansion envelopes and how fast they expand at a specific velocity also has a big impact on their actual penetration. Others have much wider envelopes and work well in a wider range of barrel lengths.
For example, these Sig 124 gr V-crown bullets were all fired with the same powder charge in three different barrel lengths, with velocities ranging from 1050 fps to 1350 fps, yet consistently penetrated 16-17”. The higher velocity just resulted in greater expansion, with the expansion from a 3” barrel still being acceptable.
At the other end of the spectrum was this 124 gr Berry’s Hybrid hollow point. They are very attractive from a price perspective and you could easily do all your shooting with your carry load. They are however very, very velocity sensitive.
For example the bullets I tested penetrated between 18.5” and 19” when fired from a 3” barrel at 1049 fps and 1033 fps. They both lookEd like this one and their total failure to expand wasn’t inspiring. I could probably load and shoot both bullets again with reasonable accuracy.
However when fired from my 4” Kimber at velocities of 1097 and 1121 fps with penetrations of 10.5” and 12” and extreme expansion.
One of them was so flat it came apart when it was dug out of the gel. Berry’s indicates a velocity range for expansion and penetration of 1050 fps and 1150 fps, and that’s probably generous. If I were going to use this bullet for self defense I’d do extensive testing and load development to get reliable velocities right around 1100 fps and accept the probable 10” penetration. Berry’s isn’t mis representing the bullet (much) as it will perform quite well but will probably struggle to meet the FBI minimum penetration mark, which isn’t all bad as we’ve discussed why 10” is probably just as good as 12” in a face to face engagement - and it will definitely expand to a very impressive diameter.
Just for the fun of it I shot one out of my 8.9” MP5 at around 1350 fps and the result was pretty spectacular fragmentation with about 5” of penetration.
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.45. ACP wise it’s a lot like a .357 Mag where it’s not hard to find an effective hollow point load.
Below is a 9mm 124 gr XTP on the right for comparison purposes with a 185 gr XTP, a 185 gr Golden Saber and a 230 gr HST. (The 2nd and 4th unfired bullets are transposed.). They all penetrated 14” - 16” with the differences being the expansion going from adequate for the XTP to excellent for the HST. I would not want to get shot with any of them.
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My take on it is:
1) Carry the largest caliber handgun you can comfortably carry all day and
fully conceal;
2) Carry ammunition that is 100% reliable in your hand gun. Terminal performance is secondary. 12“ to 16” penetration with reliable expansion is probably ideal, but I would not turn my nose up at 10” of penetration with excellent expansion.
3) Be able to shoot it well enough to put 3 rounds in a 10” circle at 10 yards in 2.5 seconds. That’s roughly 1.5 seconds to draw from concealment and fire the first shot, with the next two aimed shots leaving at 1/2 second intervals.
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