Have one like new SIR #50 C available. It was purchased new, put on an M4 and then put in my safe. It has never been out of the house. It is in 100% condition.
A.R.M.S. #50C S.I.R. SYSTEM BILEVEL COMMERCIAL
The S.I.R.(TM) System is designed to fit mil-spec flat top receivers from Colt, Armalite and Bushmaster in the 5.56 carbine lengths. There may be other flat top receivers manufactured that the S.I.R. may fit to properly, but we do not guarantee or warrantee S.I.R.(TM) on any flat top receivers other than those listed above.
This modular receiver sleeving system is a hand guard universal mounting system that leaves the barrel totally free-floating while providing a rigid ultra-light platform on which to mount virtually any mission specific accessory like thermal imager, lasers, night vision, optical sight, M203, etc.