New (to me) AR... How to tell if 5.56?

[YALE]

Pull the hg, I'm sure it is marked somehow, somewhere. If not, rebarrel it if it makes you feel safe. Aim has several spikes barrels pretty cheap with a 5.56 chamber.

Sheeit! Headspace it before you rebarrel it!

[ThreeFingerPete]

You guys make shit complicated.

[David]

Tie a shoe string around the trigger, load, step back a couple feet and give it a yank and see what happens. What I always do.

[Downs]

lol

[HOOCBB]

The barrel on my AR is stamped with the caliber and twist rate closer to the flash hider.