300 wsm headaches

[Cossack]

If the barrel is in fact fouled, try some Wipe Out and let it soak all night, won't hurt the barrel and will clean all the crud out.
Shooting copper bullets (Barnes) over leaded bores is a recipe for super fouled barrels. Always clean the barrel down to steel after every shooting session for best results and, especially, shoot copper loads from a clean barrel.
But check the crown for dings. I just had a rifle in here that was doing the same thing. The crown had a barely noticeable ding that I observed with a magnifying glass. A brass round headed wood screw coated with JB paste and chucked in a drill polished it up and accuracy returned.