http://www.futurefirepower.com/myths-about-the-nato-556-cartridge
-quote
"Other calibers of bullets travel through the body on, more or less of, a straight line after some fragmentation. When the 5.56 round was first designed by Remington, it was meant to tumble through a target, not kill with brute force. It did this not only by the relatively blunt shape, but also by using a rifle barrel with less of a twist. Next time you look at an M-4 or an AR-15, notice it says 5.56 NATO 1:7 on the barrel. This literally translates into; the bullet will make 1 full rotation for every 7 inches of this barrel. This was not always the standard twist set for the new NATO round. The first AR-15 made by Armalite, had a 1:14 twist making it a very, very unstable round. One can only imagine the orientation of the entry and exit wounds. Now if you havent figured it out already, the less the twist, the more unstable the round is. (1:14 twist is less than 1:7) It is said in firearm enthusiast legend that the first tests were done on pig carcasses and that the entry wound could be on the lower right stomach with an exit wound coming out of the back upper left shoulder. It left horrific wounds and terrible internal damage to its intended target, immediately drawing the interest of the US Military, in particular USAF General Curtis Emerson LeMay. Thats right folks, you can thank we in the United States Air Force for the M-16/M-4 legacy (I say this without sarcasm). He thought it was an ideal weapon for his deployed members of the USAF Security Forces for guarding the perimeters of Air Force installations in such places as Korea and Vietnam. Before military trials, Armalite increased the barrel twist to 1:12 to improve accuracy. But when tested in frigid Alaska, accuracy was decreased because of the increased friction from the denser, colder air. Therefore, the barrel twist was eventually increased from 1:12 to 1:9 and eventually to the 1:7 you see it today. Although some bull-barreled AR-15s and Stoner Sniper Rifles can be found in a 1:9, most issued M-16s and M-4;s are primarily a 1:7 twist."