Letter code (P S ) and date (81) plus the primer sealant (red) indicate a military cartridge or one made for military use.
Lots of rounds came on stripper clips. Unfortunately only seeing the base of the round with nothing for scale makes ID difficult, as it could be lots of things.
I assume its a .30 Carbine round, but without seeing anything else it could be a .30-06 or almost anything else using a rimless case.
Another picture, showing the case body and bullet would greatly help positive ID.
Measurements would also help, a lot.
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All else being equal (and it almost never is) bigger bullets tend to work better.
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