Shortround6
Major General
It's strange that UB has better penetration of ShVAK, something is not right
Well, the ranges are not the same but that is minor.
next try work out the energy applied per unit of frontal area (size of the hole).
I don't think the right ammo is listed?
API came in serval types for both guns but one sources says thq 127mm used a 52gram AP projectile with 19,200 joules of muzzle energy.
In trying to punch a 12.7mm hole it is trying to remove 126.6 sq mm of material, this means 151.6 Joules per sq mm of area.
The 20mm used a 99 gram AP projectile with 36,600 joules of muzzle energy. However a 20mm hole has 314 sq mm of area and so the projectile only has 116.5 joules of energy per sq mm of area.
In theory Penetration is accomplished in one of two ways (or a combination). 1 is pushing the material that was occupying the hole to the side, the other is hit the armor hard enough that a plug of material is punched out of the armor matching the size of the projectile. In reality it is usually a combination of the two but the energy per unit of area formula usually works.
Just as a comparison, the Hispano has about 46,900 joules or higher and would have 149.4 joules per sq mm of area.
The American .50 has 17,400 joules for the M2 AP round.
The ShVAK and it's short/ light projectiles didn't do well at armor punching.