The 5.56x45 was adopted by the US in 1963 using a 55 grain bullet and by NATO in the late 70's using the 62 grain. I don't know exactly when the 55 grain bullet was phased out of our military but the 1:12 twist barrel wouldn't shoot the 62 grain NATO bullet so barrels had to be changed out on all the M16's in the inventory. However it is the longest cartridge in service by our military and that the 55 grain load didn't last very long is relative to the longevity of the cartridge considering it is going into its 50th year of service.
Again because of the change of the military going to faster twist barrels that made the commercial .223 rifles to start using a faster twist barrels as well. While technically not exactly the same cartridge they are close enough most people don't have a problem running the 5.56 through .223 bolt action rifles. A 1:9 or 1:8 twist of most bolt action rifles allow for a larger selection of bullets to be used that the average .22-250 can't.
Last edited by taylorce1; November 16, 2012 at 12:55 PM.