For starters, the .308 is almost 1/4" shorter then the 7mm Mauser.
And the case body taper is different.
It could be the magazine follower doesn't match the new cartridge shape, and after the first one hits it, it throws the whole stack off kilter as the rest try to depress it further.
It could also be your stripper clips are goofy and not the right ones.
Also, you have to have a case-hardened cast iron thumb to smash 5-rounds of stripper-clip ammo down any box magazine rifle in one fell swoop.
Maybe your thumb isn't hard enough yet.
rc