that is an excellent question, you know the Winchester self loading cartridges are only semi rimmed. with other semi rimmed cartridges like 7,65 Browning /32 ACP the small rim is enough to work in a revolver.
probably the size of the 7,62x39 was just good enough it stuck in the chamber of the Winchester 1910 on the right position. after turning the base down a little bit its semi rimmed again.
we will see how it goes in the future.