Must preface this with sorry for the long post, if you don't want to read my dribble the entire thing could be summed up in my last paragraph so feel free to skip forward......
Don't forget energy transfer.
Lighter projectiles transfer energy faster than heavier ones - something to do with Newton's law that objects in motion tend to stay in motion unless acted upon by an outside force......(er, something like that - any physics majors out there with the right verbage?)
Because of this a 55 gr projectile hitting a mass of jello at 2000 fps won't penetrate as far as a 500 gr projectile at the same speed, right? Pistol weight (and calibre) rounds are three times as heavy (or more) than .223 rounds and penetrate much deeper in similar materials when traveling at similar speeds. Everyone knows that .223 rounds are fired at much higher velocities than pistol rounds though, so the arguement comes into play which one penetrates more. The arguement remains in force because the magic point at which a heavy slow moving projectile out penetrates a fast light weight projectile just happens to be in between the overlapping performance envelopes of the modern pistol round and the .223.
Now throw energy transfer in the mess and you get the answer why pistol rounds out penetrate .223 rounds. Lighter projectiles transfer energy faster than heavier ones. Whatever it is that these objects are running into - steel plate, sheet rock, kevlar, flesh, etc. - are acting on the projectile and causing it to loose energy. The rate which a projectile looses its energy is directly related to its mass and velocity (remember the formula for finding energy is velocity sqared X mass divided by 45036) Not sure how to put this next part not being a physics guru or mathmatician, but energy loss through velocity decrease happens faster than energy loss through mass decrease. So the lightspeed traveling fly that hits your car would penetrate all the way through it and maybe not loose enough of its mass to be hurt, but if he was a little slower - splat! He looses his mass.
This gets even more intersting with varying bullet types. Many .223 rounds are designed to stop and impart thier energy quickly. Imagine a bullet exploding on a coyote's hair - that's instant energy transfer! Even the .223 FMJ or armor piercing rounds don't penetrate much over 13 or 15 inches in soft media because thier velocity is used up. But for the first inch or so they can liquify steel because their mass stays solid.
And so a .223 round traveling at nominal .223 speeds will pucture a kevlar vest, 10 inches of flesh and stop under the second layer of kevlar having expended its energy. A pistol round, lacking the speed to puncture the kevlar expends its energy on the first layer.
A .223 round punctures a 7" piece of hard wood while pistol rounds and a very fast - lightweight 7mm round (probably a frangible or varmint bullet designed to depart its mass instantly) fail to penetrate.
A .223 soft point penetrates sheetrock, insulation, another layer of sheetrock. but wont go through a wet sack afterwards, while the 9mm hollow point grabs a chunck of sheetrock wich stabilizes its mass and goes on to penetrate the insulation, sheetrock, the wetpapersack holding little Dorthy Jane's lunch, the lunch, Dorthy Jane, and part of her friend Mary.
But when talking about flesh - people - the media which can get you sued or killed, the .223 with a regular FMJ penetrates about 7 to 15 inches, less with soft point or hollow points, while the average pistol ammunition (defensive type bullets) penetrates 8 to 15 inches. What gives? Like I said this is the magic point where things get so similar and arguements get started.
Easy solution without this lengthy post (sorry, but I needed to put some of my thoughts on this in print) would be- .223 out penetrates pistol ammunition easily for the first 7 inches and after that if the pistol ammunition is still viable and moving, it being heavier and having more mass, penetrates more, somethimes a lot more.
