However, if it says "5.56mm," that does not necessarily mean that it is safe to fire all variants of 5.56mm NATO out of. If it says ".223 Remington," then you can be pretty sure that is what you have. On most other rifles, look on the receiver or the side of the barrel near the chamber. On an AR-15, look on the top of the barrel near the muzzle. To avoid this, ensure that you only fire the appropriate ammunition in your rifle. Obviously, this is not good and can result in a frustrating and dangerous situation. More likely, it will blow the primer out of the primer pocket on the case and it will end up somewhere in the internal workings of the rifle. The rifle probably won't blow up, though it is possible. However, combine ammunition that is toward the top end of acceptable safety limits with hot summer temperatures and a little too much fouling in the chamber, and you stand a good chance of running into some serious trouble. For this reason, it is absolutely possible to fire 5.56mm NATO ammunition out of. Yes, it is true that not all chambers are the same, and the same is true for ammunition. 223 Remington chamber can produce dangerous amounts of pressure. This means that firing 5.56mm ammunition (which is already loaded to a higher pressure) out of a. In addition to producing increased velocity, this also had the advantage of improving reliability by allowing more buildup of carbon and other "gunk" without adversely affecting the function of the rifle.Īll other things being equal, a shorter leade results in increased pressure when compared to a longer leade. Originally designed as a military cartridge, the 5.56mm NATO was meant to be used with a longer leade with a shallow angle. Generally speaking, a shorter leade results in increased accuracy at the expense of increased pressure and decreased velocity, while a longer leade produces increased velocity with lower pressures at the expense of reduced accuracy 223 Remington chambers have a shorter leade (the distance between the mouth of the cartridge and the point where the rifling engages the bullet) with a steeper angle than 5.56mm chambers. A 5.56x45mm NATO round (center), flanked by.