Pressures are an issue as is heat in rapid fire. I have heard stories of the plastic cased .223 sticking in chambers. OTOH, the plastic case will insulate the barrel/chamber from some of the heat of the burning propellant so it may not get as hot. The makers of the plastic/metal hybrid .223 are out of business. Maybe because they could not sell and make a profit. Maybe because they made junk. I don't know.