Two words....
Shot Placement
That is for sure. The back of the eye orbit is paper thin bone and the only thing separating the soft tissue of the eye from that of the brain. With a carefully chosen shot to the eye, any projectile (including a BB or pellet) could potentially kill someone. As a medical student, I have seen this part of the skull a number of times and it has always amazed me how vulnerable this part of the body really is. It truly is like a piece of cardboard protection for the brain. I use this example when illustrating to young people the inherent dangers of using airsoft plastic pellets without eye protection, so of course the same holds for ALL firearm (including .25ACP, .22LR, .17HMR) calibers.
We can talk about "manstopper" calibers all we want. Virtually all modest size projectiles that enter the central nervous system (which includes the brain
), will stop a person "dead in their tracks". So the old adage that it is about "shot placement" really rings true with me. Shooting anywhere else in the body may simply cause the individual to "bleed out" which may happen slowly (in fact, so slowly that they may be able till still attack you) or quickly, if you happen to be using a large caliber bullet.
I guess my point is that ALL projectiles (including air guns) can be deadly, but multitudes of factors come into play on whether a firearm is ideal for ones defense. This includes the accuracy of the gun, the skill of the shooter, how the shooter reacts to recoil, where the shot is placed, the mindset and behavior of the BG, the energy of the projectile (bigger is USUALLY better here), the number of shots, etc.
And finally and most importantly what gun is always available when you need it! Cause if it is big, it ain't gonna do a darn thing sitting in your gun safe.