How is my Oly Arms barrel chambered?

Q:  How is my Oly Arms barrel chambered?

A:  To make it quick and clear: all button rifled barrels are chambered in 5.56 NATO; all SUM (Ultramatch) barrels are chambered with Clymer® .223 Remington reamers cut to minimum SAAMI specs.

Additional details about identifying your Olympic Arms barrel can be found in the FAQs.