Hard to find ordnance gauge.  This is PN A-77301.  If your piston slides inside the ring it's toast!  Marked :

30 BAR M18 M18A2

BC30 BMR M22

C-9073 GAS PISTON

REJECTION RING

and on the other side

FOR GAS PISTON

FIELD SERVICE

A-77301 NOT GO 494

ORD. DEPT U.S.A

It appears unissued, although there is some storage wear