First AR
 

S the Palmetto State Armory PA-15 with a 20"
heavy barrel, an A2 stock, and is a flat-top with no rear sight any good and what caliber is it?

Good for what? Caliber? Check their site. Do a little homework for yourself. You'll be better for it.

PSA is a good vendor. The rifle will be a heavy gun capable of MOA accuracy out to 800 yards or more. You need to buy a QUALITY rear sight. DD, LaRue, Colt, YHM. Don't expect anything from it if you spend less than $100 on the rear sight, or $500 on an optic system. I recommend the Trijicon TR24 with LaRue one-piece Mount.