Never "pinch" a cap because it doesn't fit properly. Throw the tin away, (or lose your hand or trash your gun)?
From an old Colt Industries pamphlet:
"Percussion caps are now made in sizes from nine to thirteen. Ten and eleven are the best numbers for the small and medium-sized arms, and twelve for the larger sizes, although, as different-sized nipples are sometimes met in specimens of the same model, no hard and fast rule can be given. It is better to have caps slightly too large than too small, as large caps can be pinched together at the bottom enough so they will stay on the nipples,
but small ones must be driven down on the nipple by the blow of the hammer, and this process frequently cushions the blow to the extent of producing a misfire."