I did a search and found this on an auction site:
**FANTASTIC NEW UN-ISSUED HARRINGTON & RICHARDSON U.S. PROPERTY MARKED M16A1 MACHINE GUN (FULLY TRANSFERABLE).
SN 2242452. Cal. 5.56MM. 21-1/4? bbl with bird-cage compensator. Fantastic near perfect specimen which according to the consignor came out of the museum display when they de-accessed their firearms. Classic and most recognized configuration and with tri-angular hand guards absolutely un-issued, un-fired condition. No mag present. Bbl marked “MFR 26978 MP CHROME BORE”. Marked on left side with “Lion” H&R logo and “Property of U.S. Govt.” Firing mechanism moves smoothly when operated by hand, this weapon appears fully functional. This is a National Firearms Act item and requires BATF approval prior to transfer. This weapon is fully transferable on an ATF form 3 or form 4. CONDITION: As new un-fired, un-issued. Upon closest inspection one might find a rub mark on the finish, but otherwise, orig factory finish on metal with composite parts in same condition. This is a dream come true for the advanced U.S. Military modern firearm collector. 50313-2 JWK44 (18,000-22,000) – Lot 3002
It looks as though your barrel is a M16A1 barrel that was installed on early H&R M16A1s. Of course, it could have been a replacement barrel like the SAK barrels. I am sure that someone with a lot of barrel knowledge will be along to help with this.