Written by Sierra Bullets Ballistic Technician Paul Box
Over the last year or so we’ve had hundreds of calls on our 800 line concerning the difference between the .223 and the 5.56. So what is the real difference ?
In reality these two are the exact same cartridge. The case dimensions are the same. The difference is in a combination of throating and how they are loaded. Maximum working pressure for the .223 has been established by SAAMI at 55,000 PSI. Maximum pressure for the 5.56 has been taken up to 60,000 PSI. This was accomplished by using a longer throat in the 5.56. Much in the same way Weatherby has done for years with their long free bore. This longer bullet jump allows for heavier powder charges and naturally higher pressures.
So is it safe to shoot a 5.56 in a .223? Probably not if that rifle has a common throat length. Pressures could run high enough to blow a primer and etch a bolt face. Smoke boiling out of the action and can get pretty hairy in general. So what’s the safe thing to do ? Use .223 in .223 and 5.56 in 5.56. It will be easier on your nerves.