With regards to Federal Law, AP rounds, as defined to be regulated, are specified as Hand Gun bullets. Rifle rounds are exempt, with two exceptions, those being .308 and 7.62x39mm. Someone made a hand gun to shoot them. State laws may ban AP rifle rounds, Conneticut banned them not ong ago.

Many .50 BMG shooters shoot AP because it is cheap (relatively) and it is among the most accurate milsurp projectiles available. The hardened steel cores are lathe turned. Most of it is old, before Tungsten was used.

A solid steel projectile is possible, but highly impractical. In the early days of .50 BMG competition, many competitors shot monolithic solids, CNC turned from a steel/lead alloy (97.5%/2.5% if I recall correctly),called leadloy steel. The small lead content is needed to prevent galling and the alloy machines real nice, too. Most monolithic solids in .50 BMG are now made from bronze, or similar.

Not only that, the bullets are slightly sub-caliber (.502"), except for a driving band at the rear(.511"), a cannon projectile type design. This type of projectile is called a borerider. A borerider chamber has a 2nd throat. The projectile slip fits into this throat when loaded into the rifle. The actual bearing surface is the small driving band at the rear. The forward part of the bullet shank behind the ogive "rides" the bore, with only .001" engraving, compared to the usual .005"(.50 BMG)

Any projectile that you get turned in any volume will need to be done on a CNC machine. To make it worth it, we are talking a big volume. You better have a lot of barrels. You will need them. Unless your barrels are reamed with a 2nd borerider throat, your projectiles will need another driving band up front called a bourrlet. How wide the driving bands? How to load for such a round so it will not blow up? I have not got the slightest idea.