5.56 & .223 info


Short and sweet: the 5.56 and the .223 are the same bullet (.224 diameter) but the brass case and primer are different. The brass is thicker on the 5.56 and the primer is less sensitive. yes, the 5.56 is loaded to a slightly higher pressure, but not a significant one.
So, you can safely fire .223 in a 5.56 chamber. But, trying to fire 5.56 in a .223 chamber can lead to a cartridge jammed in the tighter chamber, a “slam fire” when the round jams not quite in place and the firing pin hits the primer while the rifle is still out of battery. This is where the damage can occur to the firearm, or the shooter!
Lastly, the reason for the less sensitive primer on the 5.56 is because the firing pin is not retained by a spring. This means that when the bolt slams forward, the firing pin will contact the primer. To see what I mean, in a safe location (range etc) place the firearm on safe, chamber a round from the magazine and let the bolt slam forward. Do not fire the cartridge, just remove the magazine and eject the unfired cartridge. Now look at the primer. I guarantee you will see an indentation from the firing pin in the primer. A sensitive primer would have fired. When you reload this is why it is critical not to have a high primer that is not fully seated. Also, this is true of ANY military full or semi auto rifle from the U.S. and any semi auto copy such as the AR, the M1a or the M1 Garand. Ruger Mini 14 is the only other rifle I know of off hand where it is perfectly safe to fire either .223 ot 5.56 in it. This is also stated by Ruger.
Note: The 5.56/.223 has nothing to do with designating the rifling of the barrel. Early M16’s actually had a 1 in 14″ twist. This caused the bullet to become unstable, and when it struck anything it began to tumble rapidly which caused devastating wounds, but was not as accurate at longer ranges. Next came the 1 in 12″ twist which also produced devastating wounds. Then the 1 in 10″ twist (which I think is the best) gave more longer range accuracy but still allowed the tumble to start about 2″ into a body. After that, many changes in bullet weight and a tighter twist in rifling began to stabilize the bullet too much for it to tumble and the bullet fails to tumble at all now (which is why the bad guys in the big sandbox are continuing to fire even when hit multiple times). But it did gain longer range accuracy and penetration into body armor etc. Actually, if a hollow point bullet is used with the faster twist and heavier bullet, the wounds go back to the devastating ones caused by the early rifling. This is why all of our enemies use a heavy bullet with a steel core and a hollow point. It gives them the best of all worlds. Accuracy at longer range, penetration of body armor and rapid expansion in soft tissue. Personal note: if the U.S. would withdraw from the outdated pact signed (no not the geneva convention, but a later one) banning “dum dum” bullets, we could provide our soldiers with proper performing ammo and the 5.56/.223 would really shine on the battle field.

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