This is a problem that cannot be solved by "keyboard" advice. Evidently you have a non-standard chamber. Firing anything other than ammunition made specifically for this rifle can damage the rifle and possibly injure the shooter. Do nothing further with this rifle before taking chamber casts, slugging the bores, and getting expert advice as to identification of the proper cartridge followed by having custom reloading dies made. I recommend RCBS.

If all this sounds foreign to you, then wipe off your rifle and store it safely until you feel comfortable with this advice.

Curl