Definitely get the chamber of that rifle identified. It is not 7.7x58mm Jap.
If you're not familiar with the process of taking a chamber cast (or don't have the proper measuring tools), take it to a competent gunsmith. They'll take a cast, ID the chamber, and can stamp the barrel for the appropriate cartridge (required by law).
Take it back to the shop you bought it from, and tell them you're getting hit in the face by debris, every time you shoot it.
If they don't offer to take the chamber cast for free, push the issue. If they still refuse, you have a very shady dealer that should not be trusted.
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."