I was told,as fact, that the Vietcong would scour the battlefields for the NATO ammo, and could fire it, single fire, from their Soviet Bloc bolt action rifles. As the NATO round did not have a rim, after firing it, to extract the shell casing, they used a cleaning rod, and would knock the spent casing from the chamber.
I read a book by a Navy Seal who served in Vietnam. He relates the same story. Dimensionally the 7.62x51 will
fit in a 7.62x54mmR chamber and maybe fire. But the 7.62x51's dimensions are still wrong plus the pressure is much greater. Anyone trying this might wind up wearing part of their rifle. The VC were in combat and didn't know better, so don't try this at home.