In 1968 i had my beautiful .222 Sako re-chambered for .223. Since that time my Sako has been fed a steady diet of 5.56mm US military ammunition with no ill effect. It is now on its third or fourth re-barrel. My .223 guns have fired at least 200,000 rounds of M193 US military ball ammo with no problems.
What you knew about .223/5.56mm ammo may be wrong: