I've been firing 5.56mm military ball ammo in .223 chambered rifles for over 40 years with no problems at all. There are several SAAMI myths concerning the alleged differences between .223 vs 5.56mm ammo pressures, case thickness, etc. The US 5.56mm military M193 ball round generates 52,000 psi. See page 10-3 US Army TM 43-0001-27:
SAAMI waited for 30 years before warning us of the "dangers" of firing 5.56mm military ball ammo in .223 chambers. This "warning" came at the same time the US market was flooded with 5.56mm military surplus ammo.
There are about a dozen different .223/5.56mm chambers. Here are the dimensions of eight of them:
SAAMI "warns" us that all military 5.56mm cases are very thick. This is not so. For over 40 years I have weighed the cases for all of my accuracy loads. The thickest us made cases are .223 Federal Gold Medal and American Eagle. Some, if not all, Brit military 5.56mm cases are very thick. Lapua .223 cases are very thick.
Go to Brass Weights:
AR15BARRELS.COM - Technical Documents
One exception: If your gun has a tight match chamber do not attempt to fire military ball in it.