I have trouble with this,my 30/30 336 barrel lately seems to have suddenly, after nearly 50 years of good service,seems to have given up the ghost, can't believe
it because my 1893 built in 1900 still shoots acceptable groups.first the 336 seemed to be suddenly sensitive to heating up after about 6 rounds,then shoot successively higher unless cooled,then at 100 yds it totally lost it with a shotgun type group,I checked the scope mounts they were good and the scope also checked out I ran through the adjustment in square grids it was fine.then I cleaned it with ammonia based bore cleaner,no green of any great consequence.i also cleaned it electrolytically ,for 30 min. Again some slightly greyish green liquid no great green though,I then used a chores boy but no real reason for leading I had lead bullets ,but for the fist time ever, shot 5 maybe the rest were jacketeds ,proven handloads, no tumblers or keyholes but a shotgun had a better group is at 100 yds,after this a normal regimen and losso bore cleaner with marvel,normally I would use Kroil and JB,but these are not available due to the covid sit.so I adapt and overcome.no great result I know this bore is clean ,it's not perfect with some small pitting but still way better than my 1893,and that dog still hunts(the rifle still shoots)!I figure ,in my ownership from about 1970-72 at least 100 rds. year-+5000 rds maybe ?I have to shoot it again although I don't see any reason to believe I accomplished much no evident crud to see.the muzzle looks good,but I might brass lap it and if no improvement I will have my friend,a good gunsmith look at it .seems strange I did find a crack in the forearm but doubt that is an issue but I will tend to it before shooting tomorrow. Any advice?