I have a 7.62x39 upper which I was told was S&W. I can't see any markings of any kind on it. The Standard Catalogue of S&W 4th edition doesn't list 7.62 x 39 in uppers or complete rifles. Could it be an S&W upper? Is there any way to tell if it is?
S&W offered a 5.45x39mm upper, but I don't recall them ever offering it as a complete rifle.
As others have mentioned, if its a 7.62x39mm upper, its either a built or purchased upper that is not S&W. Doesn't mean its bad, but its not a factory S&W item. I like the 7.62x39mm AR rifles...just wish the magazines were a bit more reliable. CProducts mags are the best in my experience. ASC mags not so much.
I have a Stoner 7.62x39 upper on my S&W lower. Works great. Holds 2” groups at 100 with Wolf FMJ. Use only C-Products mags or Stoner mags. Stoners are made my C-Mags. Great hog gun with Hornady ballistic tip loads and FMJ works to!!
If it's a forged receiver, there should be a forge mark (like a hall mark) on the upper somewhere (usually near the forward assist) that will identify the forge. That won't tell you who machined/assembled it, but may give you a clue as to source of the receiver.
A broken A is Anchor Forge, a hollow square is Brass and Aluminum Forging Enterprises, a stylized cardinal (bird) is Cardinal Forge, etc. If you find one and post it, someone here can probably identify it.