I agree the projectile originally asked about is for the HS case and not the US 20x102mm case. Since the discussion went into the US high pressure test loadings though, I thought I would add to it. As has been shown the M50 series of HPT rounds have gone through many variations, some of which I have shown. I am missing the M54A1 variation with the stannic coated case (silverish appearance) and the wasp-waist projectile (M54E2).
The round on the left is the M99 Ball projectile loaded in the Hispano-Suiza case. I show that because the first variety of 20x102mm HPT loadings used the M99 projectile with 0.26 inch of the tip cut off to make the cartridge the right length. It also used a re-sized cal. .60 case which was then stannic coated. The headstamp on this case is FA* 49 *. It, like the other 20x102mm cases here, is electric primed with either the M52A3 or M52A5 primer. This cartridge is the basic M54 HPT design, originally designated the T156. This round would generate 65,000 to 70,000 psi of chamber pressure well above the maximum 60,500 psi of the other sevice rounds.
The next round (3rd from the left) is the M54E1. Also with a stannic coated case, but it used the M55A2 Practice projectile from the 20x102mm family of projectiles. The projectile is shown next to the round.
The first of the wasp-waist projectiles was painted black and loaded in the M54E2 HPT cartridge. My particular example has a plain brass case, but manuals of the day refer to this round with a stannic coated case. The projectile is turned from a solid steel bar and therefore had to have a portion of the exterior removed to meet weight requirements. The round used the M52A3B1 electric primer.
The final round is the M54A1 which was the last of the series. It is almost identical to the M54E2 except the forward flat portion of the projectile, just behind the ogive, is longer. This projectile was painted violet because of the change in the color coding scheme for medium caliber ammunition that had been made. This cartridge peaked out at 72,500 psi. The projectile weighs 1965 grains.
I can't seem to find my MIL-STD 709's, etc. to determine when the color shift from black to violet was made. Maybe someone else can fill in that blank spot.
