Im new to the AR-15 world and recently purchased an upper receiver that I cannot identify. My pictures are pretty much worthless so ill try to explain what it looks like. 20'' barrel, triangular handguards, dark grey finish. A1 sights, with three prong flash hider, there is a small triangle, and square marking forward of the ejection port (COLT?) Also, the barrel is chrome lined and marked "CMP(?) CHROME BORE" No forward assist! The bolt has no markings, the carrier just has a "J" stamped on the side. The bolt assembly may not be original to the gun. Just wondering what model I have as well as possible year of production?

Sounds like a  mid 60's USGI M16 made by Colt and then rebarreled mid 70's or later.  I don't have a clue about the J marked carrier.  Is the bolt itself marked MP, MPC, or something similar?

Quoted: Sounds like a  mid 60's USGI M16 made by Colt and then rebarreled mid 70's or later.

Or an SP1 barrelled upper. What diameter is the front mounting hole?

The "J" marked bolt carrier is made by J.C. Manufacturing, a contractor for the military.

Thanks for the info guys..i believe from researching and with your imput the rifle was rebuilt a few times. The upper is an M16 with a mid 70s barrel (colt) the bolt is made is marked  "J" and the bolt/ carrier is a USGI replacement part that are are commonly found on rebuilt USGI M16A1 rifles. They were made by Smith Enterprise for JC manufacturing in the mid 1980's.

Thanks again!