Or aluminum foil.
If they're small rifle, ream 'em out to LR & use some of the mild Wolf LR primers German Salazar's been talking about, or large CCI BR. Be a good test to see how much is the primer, how much the flash hole size.
Or, as long as they have *some* fit, make sure the case doesn't get excessive head clearance --- like over .001 -- and bang away. They'll expand to fill the pocket, and if they can't back out any during ignition, the risk of gas cutting is lessened.
Of course, officially, I say toss the cases.