Sounds like a pressure issue. .303 sav headspaces on the rim as all rimmed cartridges do. Is your bolt not in battery?
If your headspace was excessive you would be getting blow by if your OAL was long your brass would show Markings from the chamber. Your saying your backing out primers?
Solid frame or takedown?
Original barrel? Original lever?
What's the ammo make and load?
Partial Ser #, example 123,XXX or Date code
Primer is driven out? Not flattened?
