Looking at the picture of the shells in the chamber it is hard to tell but looks like shell heads are higher than barrel face at bottom and fit correct at top. Shell rim cuts might have been done at an angle to cause this .
If this is the case get rims cut slighty deeper at bottom.
The rim cut is cut with part of the chamber reamer. It goes round and round, in line with the bore. The only way the cut would be deeper at top than bottom or vice-versa is if the breech end of the barrels was not square with the bore. In any case, the cut is too deep now. Cutting it deeper would make that part of the problem worse.
