This post has been in draft form for a long time. It really doesn't have much to do with firearms, and more to do with the things to think about when confronted by a barrier.
An ethical hunter will not take an obstructed shot on game. The rules of good sportsmanship prohibit taking risky shots that might wound and animal.
A Soldier or even Law Enforcement Officer may need to engage another human being who is using cover or concealment. Let us define each.
Cover is something that stops the bullets aimed your way. They might be able to see you (through bulletproof glass or a big block of ice), but they cannot hit you with direct fire.
Concealment simply hides you from direct observation by your opponent. The enemy can't see you, but if they have enough bullets to shoot through the tin shack or apartment door you will learn the difference between cover and concealment the hard way.
The definitions are fairly simple, but what could be "cover" for a 9x19 could only be "concealment" to a 357 Sig. In fact this little bit of wisdom is why the Chicago Subway Police are issued firearms in 357 Sig, it can launch a bullet fast enough to penetrate a subway seat with enough terminal velocity remaining to do significant damage to someone behind the seat.
The last few posts in the Firearms Lethality series have dealt with bullet construction and velocity. Barrier penetration is something that has been touched on with Armor Piercing rifle ammunition and KTW pistol rounds. The barriers designed to be defeated were made of steel. But barriers can be anything, from sheetrock to tempered "bulletproof" glass. The guys at the "Box O' Truth" showed that bulletproof glass really is bulletproof, for a while anyways http://www.theboxotruth.com/docs/bot48.htm.
The truth is that in order to defeat "cover" or a "barrier" all you need is time and ammunition. I don't want to think about how many 22LR cartridges you would need to defeat NIJ level 3 rifle bulletproof glass. But you would need significantly less 50 BMG to get the same effect. Even standard ball 50 BMG would be fine, but specialty rounds are available just for punching through armor steel or bulletproof glass.
But, bullet design is nothing without some terminal velocity bringing kinetic energy into the act of turning cover into concealment. Toss a rock at a bulletproof glass viewport and you might not even make enough damage to obscure vision, but hit it with a precision 12 gauge slug a couple times and things get interesting to anyone behind the glass. Conversely, if you don't want to go the "big and slow" route, you can always go with the "screaming fast" method. These two schools of thought are not mutually exclusive, and so you can get some really screaming tough bullets. In a low mass, high velocity cartridge like the 5.56x45, making the projectile "tougher" is the easy answer. If you want to compare two bullets against each other, test them against some barrier and find the "V50" numbers (lower is better). V50 describes the velocity where 50% of the bullets defeat the armor in question. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA567525&Location=U2&doc=GetTRDoc.pdf One should note that using standard M855 or M855A1 bullets in a higher velocity round like the 22-250 or 223 WSM would increase the range at which the bullet would remain "armor piercing." What exactly that extra velocity would do at closer ranges is beyond my ability to predict, but assuming the bullet can behave as designed, ballistic performance against barriers should be improved.
Now, there are different types of barriers. Bulletproof glass (and most non-homogenous armor in general) uses alternating layers of hard and soft to redirect, absorb, and disperse the kinetic energy and momentum of the projectile. Other types of barriers, such as sandbags or snow (do not underestimate the ability of an unpacked snowbank to stop a bullet) simply use a lot of mass to bleed energy and momentum away from the projectile. Composite materials (carbon fiber reinforced ceramic, resin impregnated aramid fibers, fiberglass reinforced concreate) use a bit of both properties (mass, and multiple materials) in order to spread out the impact to an area larger than the diameter of the projectile.
The hole in a sandbag will be the same size hole as the bullet. The hole (or impact crater) on bulletproof glass will be orders of magnitude larger than the size of the bullet. Some materials (like homogenous steel) behave more like sandbags than like bulletproof glass.
In the race between small arms and armor, armor has won. That is why explosively formed projectiles and warheads have been developed, for everything from Rocket Propelled Grenades to anti tank missiles. Because in the end, if your small arms fires simply aren't working, several pounds of explosives usually help out quite a bit.
Explosives are a lot like machine guns, if you aren't effective, you need to use more.... And if you always choose the biggest, fastest, toughest thing you have, all that remains is to ensure you have ENOUGH of it to defeat the barrier you want to defeat.