It can be a long process that's for sure...
Alternate an aggressive Copper Solvent with a Carbon killer to find the bore metal then work it with JB Paste.
There is likely corrosion as well.
It will either clean up and shoot or it won't.
Groups may or not be related to the bore...it could be the crown.
We used to have to bore into the muzzle end of Garands maybe an inch or more to find clean rifling at the muzzle.
That's why CMP rifles are graded as to muzzle diameter. it it's a reasonably accurate measure of how many rounds were on the barrel..
