The .223 (5.56x45)
 

Nation	Year	Max. press.
U.S.A.	1964	55000psi

    This is actually a pretty good all around cartridge if kept within it's 300 yard effective range. In the right rifle with the right load, it can be extremely accurate. The Thompson contender has a barrel for it, and there are many varmint and sporting rifles chambered for it, but of course this round is most strongly associated with the M-16, and it's semi auto AR-15 counterpart. I have already gone into some of the history and theory of the round in my AR-15/M-16 section, but in general, this was an attempt to design a small, light round for a small light rifle, which would have the same killing power as the larger rounds then in use. The 30-06 was a 1000 yard cartridge, and the .308 was maybe an 800 yard cartridge. The first thing the round's designer did was to apply tactical studies to the characteristics of the cartridge. He picked up where the German designers left off, and decided that the new round would not need to be lethal much past 300 yards which was the distance beyond which fighting rarely ever occurred. He made the slug itself very small and light, and he went beyond the formula of using mere energy to determine bullet lethality. There area many ways to do this, but all of the accepted ways require frangible, or special soft point or semi jacketed bullets. These rounds present certain problems in terms of reliable feeding in rifles exposed to the strains of battle, particularly when they are expected to fire in semi or fully automatic mode. These rounds are also all against the Geneva Convention.
    I will go into much greater detail in my section on bullet lethality, but essentially what was done was to make the bullet marginally stable so that it would tumble after hitting it's target. In it's early versions, before the military procurement system ruined it, the "black rifle" was dreaded by the Vietnamese against whom it was first used. There are a number of different barrel twists available for this caliber in the M-16/AR-15 rifle. The original was a 1 in 14", but twist rates of 1 in 12" and even 1 in 7" are available along with a 1 in 9". The twist rate, along with almost everything else about this cartridge and the M-16 rifle is controversial. The reason for this is that the faster the rate of twist, the more stable, and therefore the more accurate, the bullet. The problem with this is that the bullet was designed to do it's best work when only marginally stable. An M-16 with a 1 in 12" twist will leave a nice clean quarter inch hole in whatever it hits, before exiting. The same rifle with the same round using a 1 in 14" twist will leave a nice clean quarter inch hole in the front, and a ragged fist sized exit wound in the back, that is if it exits at all. Depending on how the subject was hit, sometimes the round would not exit but would simply "rattle around inside" until it expended all of it's energy.
    After the army "improved" the round by the use of faster and dirtier ball powders, and then "improved" the rifle by increasing the twist rate, it was still a dreaded weapon; this time the dread was felt by the soldiers who carried it. In more recent times, the military has decided that the .223 round lacks penetration at longer ranges. This should be no surprise considering what the round was designed to do, and the ranges at which it was designed to do it. The solution to this supposed problem is to use a heavier bullet (69 grains). The new load is much better at longer range than the old one, but of course, another problem has been created. A heavier bullet in the same caliber is longer, which makes it less stable. A less stable bullet requires a faster rate of twist, which in turn means that the old rifles, with their twist rate set for the lighter rounds, will not accurately fire the new heavier rounds. The solution to this was to design the new M-16 with a 1 in 7" twist rate. This would seem to solve everything. You just get a rifle with a faster twist rate, and if you want increased lethality you simply use soft point or hollow point bullets, after all, hunters and civilians are not prohibited from using them. This is not ideal either. It is entirely possible to over stabilize a bullet. This section has already gone into more detail on ballistics than I intended. Please see the section on bullet lethality for a more complete explanation of these factors. The point that I am trying to get across is that the .223 has for all practical purposes become two different rounds, which must be fired in two different rifles. Before you buy ammo or work up reloads for your .223 rifle, you should determine which twist rate you have. The best overall twist rate is considered to be 1 in 9".