The PzB.38 has 1x 10 round box connected to the gun, and then there are 2x 10 round boxes in your pockets. However the “Upgrade” or “advantage” of the PzB.39 is that it has 2x 10 round boxes on the gun, making the reload “faster” (which isnt even true as the reload and fire speed is the same), however instead of 2x on the Gun and 2x in your pockets, you just have 2x on the gun and no more, simply reducing your overall ammo from 30 rounds to just 20.
A Way to make the PzB.39 just a tiny bit less worthless (beyond the 38), or simply removing the useless nerf, would be to give it 20 additional rounds, to a max of 40 rounds, 2x on the gun and 2x in your pockets.