Did you take the forearm off? What size is the hole in the end of the barrel? Do you have anything for ammo you can try and say yay or nay this or that does or does not fit and perhaps narrow it down? I'd start with bore size, and check any short action ammo you have that bore size.

Or you could just take a 22-250 cartridge and touch it off, see what it comes out! grin (KIDDING! DO NOT DO THIS)

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An 8 dollar driveway boy living in a T-111 shack

LOL