- No HBARs are standard issue in the US military

- All modern AR barrels are marked something like "C MP 5.56 NATO 1/7".  The only exceptions are the older barrels, which will be A1/pencil profile.

- Cast front sight blocks often look "prettier", and will have a very thin seam.  There will be no machining marks around the ears of the front sight; you can see the texture from the casting mold and the markings are usually raised.

- A forged part will have a thick (1/4"-3/8") seam that has to be machined smooth, and may show some pitting at some point along the seam.  The groove that forms the sight ears is machined, and the markings there are stamped.  The military uses only the stronger, forged FSBs.

-Troy

Forged



Note the seam on the inside of the "window" and the rounded edges of the FSB.



Note the forging line in the front of the FSB and the milled "ears" on top.



Note the stamped "UP" and the milled ears



Note the milled bayonet lug and proper sling swivel rivet.



Note the rounded edges of the FSB.

Cast

Thanks to Stickman for the awesome photo



Note the square edges of the FSB and the flat (as opposed to convex) front of the FSB and the lack of a forging line there.

Cast FSBs will often have a raised "UP" and as cast ears having the same thickness from top to bottom. The bayonet lugs are also often left as cast.

M16 barrels didn't start being marked until the M16A1 introduced the Chome Chamber followed quickly by the Chrome B (or Chrome Bore).

All M16A2 barrels are marked.

even on the early ones there should be an M (Magnafluxed) and the triangular Colt proof  (Veritas Proof) stamped between the legs of the FSB, usually on the right side, most are very faint. Any barrel dated after the change to the 1X12" twist will be marked at least with the new twist.