5.56 or 300 BO

[thom]

What is the easiest way to figure out if your upper is 5.56 or 300 BO? I have 2 almost identical uppers and 1 has the 300 blackout stamped into the barrel that I can read. The other upper is covered with a full length handguard and a suppressor mount.
Thanks
Thom

 

[QuietM4]

Easiest way, or the safest way?

Remove magazine, rack BCG several times, visually inspect chamber for any rounds. Then look at the muzzle. A .30 cal hole is much larger than a a .22 cal hole.

 

[thom]

Thanks. I figured that out couple minutes after I wrote it, doing exactly what you said. I also dropped a 300 BO loaded cartridge in after taking out the BCG. It didn't go all the way into the chamber. Then I dropped in a 5.56 and it went in to the chamber.