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.
Or drop a .30-06 in the chamber to show that you know
it is definately not a 7.7x58
and only know that it is at least as large as a .30-06. Go on to tell him that it is not a .30-06 since the bore is at least 7.7mm.