the 55 boys is so close to the 50bmg case (If you forget the belt, this is only for headspace because the 55 boys havn't such a clear shoulder like the 50bmg) that I thing the bmg case was the mother case. in all the old ww1 13,2x92SR is the mother or better say the model of all anti tank cartridges. Brouwning work first time with the mauser anti tank rifle and this cartridge and than double the dimensions of the 30 06 cartridge. but test with the 50 bmg was done in rebarreled mauser anti tank guns.