Your barrel is leading.
Remington Thunderbolt is notorious for doing this. If you are shooting fast groups or even slow singles, the soft lead will melt, coat the barrel and the result will be like having a smooth bore. What you describe happened to me with 2 .22 target pistols. I don’t know why but fat barrels don’t seem to have this problem where thinner ones do (.750 vs .920). My Kidd heavy barrel 10/22 and my Lothar-Walther heavy barreled AR22 don’t have this problem. Twist rate could also a potential problem as traditional .22 LR barrels utilize a slower 1:16 twist whereas most AR barrels are between 1:7 and 1:9. There is a ton of information on the web about the Remington Thunderbolt problems. JB Bore paste and elbow grease is your friend. In the final analysis, stop using Thunderbolts in that rifle.
Good luck friend, you have a lot of work ahead of you.