Huh? The .223 round is exactly .22 caliber, and is used in M16 and M4 assault rifles. The difference, as has been mentioned, is a longer, heavier bullet and a much more powerful cartridge. With all that, the M16 actually has less recoil than 9 mm pistol, and much less than a .357 (same caliber), which can actually hurt your hand it kicks so hard. The reason is a much smaller bullet to gun mass ratio of the M16. Also the power of a bullet mostly depends on its energy, which is proporitonal to its mass and to velocity squared. Thus a relatively light bullet like the .223 has a tremendous amount of energy because of the velocity it is able to achieve. Nevertheless, I completely agree with your idea of doing what works best for the game, instead of what's most realistic.