Originally Posted by KevinDooley

...most 1:9 barrels (regardless of markings) are actually a .223 chamber. Unless you're going for a high accuracy gun I'd look for something with a 5.56 chamber and a 1:7 twist rate which will a. mostly guarantee that the 5.56 marking is valid and b. allow you to shoot 69gr and heavier(longer) bullets at all temperatures with no problems...