Recently acquired a 24" SS bull barrel in, proportedly, 5.56MM.    Now it's mounted and time to go
to the range to test fire it.  The barrel is marked "SS 1-9", with no other markings.   At this point,
in my mind, the yellow flag of caution has jumped up, and I question if I have a 5.56MM or .223 Rem
chamber.  Firing 5.56MM ammo in a .223 Rem chamber, and there is going to be an heightened and
lengthen PSI pulse cycle in the 24" barrel,  which increases the stress factor on the parts and will
cause accelerated wear, and may induce harm to the shooter. (I hate to visualize the bolt coming out
of the rear of the receiver into my face.)   All that being said, how can I mechanically determine
which chamber is present in this barrel.  Having a smith do a chamber cast is kind of pricey and
messy.  GO and NOGO gages do not go up into the chamber throat.   Are there throat gages out
there to check this?   Suggestions?