Shooting 5.56 in a .223 CAN be problematic if excessively long.
Some of the military 5.56 being sold today has excessively long necks: Dangerously long. i've found LC cases with necks that are .030 longer than spec. Started finding these long necked cases on ranges several years ago. This has not always been the case: i have an ammo bunker half full of 5.56 ammo made from 1964 to about 1979. None of those cases have long necks. Much of LC made 5.56mm ammo being sold is military rejects. One never knows why a lot was rejected.
Ten years ago i saw very few popped primers from firing US military ammo in .223 chambers. Now there is a rash of popped primers. There is a race to the bottom when it comes to labor costs and quality control. Most chambers are not reamed by gunsmiths, day labor is wielding the reamer. i bought a .223 Handi rifle with an out of round oversize chamber and a very short throat. After much hassle that barrel was replaced.