Yep, the primer pocket is the wrong size. You have to stick something in the pocket that has the correct inside and outside diameter because the primer still has to fit it. As 7MMinaTree said, you could have primer leaks and in my book that is never allowed.
I think, don't quote me, the key to this is casting your own or buying cast boolits and shooting them at lower velocities, hence lower pressure, hence no primer leaks. And when you consider that some of the more esoteric euro calibers are berdan primed, the steel they use may not support all that much pressure anyway. So casting your own and doing the primer thing makes sense.
Now, all that being said, it sounds ridiculously difficult but it is not the case. I just gotta look it up.