how do you offset the choice between a small snubnosed semi auto with a 8-15 round capacity that will never equal the ballistics of the same cartridge shot from a full size barrel?
If you consider all defensive handgun cartridges under powered, then you accept that it will take multiple center of mass hits to stop a determined attacker withing some number of seconds after the bullets crush vascular tissue cause hemorrhaging and blood pressure loss. This will take many seconds.
The loss of power due to a shorter barrel is something to consider. How do you calculate what is lost?
To put the calculation process that I use I will try to present an academic set of assumptions that may bear no actual resemblance to actual tested fact:
1) Let us assume
a 124gr. 9mm para bullet will have a muzzle velocity of 1200fps from a 5" barrel. Let us also assume
that you need three center of mass shots to stop a determined threat in 30 seconds from a full sized barrel handgun.
2) Let us assume
a 124gr. 9mm para bullet will have a muzzle velocity of 1000fps from a 2.5" barrel. Let us also assume
that you need three center of mass shots to stop a determined threat in 40 seconds from a short barreled handgun.
Of course I am making assumptions which fit my appreciation for how little effectiveness will change due to the loss of a couple hundred feet per second. Basically I am trying to demonstrate that since they (defensive cartridges) are all under powered, a little more under powered will not change much.
Since there are an infinite number of variables that may be involved, no two shootings will ever be the same; so comparison is extremely difficult. Basically, a given 9mm para load fired from a 5" barrel should be slightly more effective than that same load fired from a 2.5" barrel. But in actual fact it depends on an infinite number of variables and one threat might not be stopped as quickly as another due to factors beyond the performance of the bullets.
Handguns are a compromise of power and availability. We trade size and power for being able to get a package small enough that we will have it at hand if needed. Personal preferences will determine which limits in size and power you