I bought the upper for my first AR at a gun show. The upper is marked "LK" which is from FN or Kaiser, according to the info found on this site.

My question is, how can I tell if I am able to fire 5.56 out of the barrel. The barrel has no markings on it at all.

Thanks,

DW

Just shoot it... and examine fire cases for signs of flattened primers.  Way I see it, if it is reliable, you are good to go.  I fired many a 5.56 round out of a .223 SAAMI chamber with little issue.  Occasional stuck case or popped primers will tell you if you can live with it or not.

Thanks for the info. I guess I'll try that and see how it goes.

BTW, what's the worst that can happen if I fire some 5.56 and it turns out to be a .223 barrel? Could it blow up the rifle?

Thanks,

DW

No, you are not going to blow up your rifle.  I have never heard of a case.  I have heard lots of forum talk, but no proof.

The SAAMI warning is very general, it does not specifically say it is dangerous to fire 5.56 thru your .223 AR-15.   The SAAMI specs are based on worst case, lowest common denominator.

I have also fired lots of 5.56 thru .223 barrels.  No problems.

FALARAK and I are in full agreement on this.

Great. Thanks a lot for the info, guys!

DW