Since your rifle has a very nice bore, I would try rechambering to 25/20 with a reamer pilot sized to the bore.
You would end up with a cleaned up chamber for a slightly bottle necked cartridge with a neck length of .235" if I'm thinking correctly.
That is if your existing neck area of the chamber is in good condition - you say the roughness is about halfway down the chamber.
Would be a 30-20 or if in keeping with the British way, maybe call it a 300 #2 Rook