A SIMPLE KEY FOR COLT 16 INCH UPPER UNVEILED

I will test to look for some. The piston rings seem like the bolt rings but at smaller sized. The outer diameter is about .375 of the inch as well as inner diameter about .310 of an inch.Good news, MK18 upper arrived currently from Key Arms.  Appears to be like terrific.  I want to finalize the remaining parts I should get to complete this Develo