Unmarked Upper….?

[BigMoose]

It was built in the mid 90’s so I’d bet it is either 5.56 or 223. Were there other calibers available 30 years ago?

Really great information. I appreciate everyone’s input helping me.

I mean it's always possible someone could chamber it in something weird.. with it having no markings, its hard to tell..

Its also possible someone hand reamed this chamber.. and matched it to whatever pet load they wanted to fire.

Twist rate is of course another unknown.

At the very least I would headspace it. You can get 5.56 and 223 head space gauges.