I guess I should update this. I broke down the rifle and noticed the barrel was caked with lead. In my opinion that's unacceptable for less than 1000 rounds. I called Umarex before doing anything in case they wanted it "dirty" to look at it. The CS agent told me to clean the barrel good and try different ammo. I explained the whole situation to him and he still said the clean the barrel. I told him it was going to take a ton of work to clean the barrel and it may get messed up trying to push all the lead out. He assured me it would be fine and to shoot it more after it's clean and see if it leads up again.

I took the whole upper apart and pulled the barrel. I've surprised it was shooting at all with all the lead in it. I ended up having to pound a cleaning rod through it to get out the lead. There was basically a whole .22lr bullet worth of buildup in there. After countless passes with a brush and solvent patches it started to look clean. After a few more passes I was satisfied and reassembled the gun.

We finally got a chance to shoot it last weekend and used only 40gr Mini Mags. After a couple hundred more rounds the barrel looked alright. I put the bore snake through it to clean out the burnt powder but I'm not going to clean it until it gets over 1000 Mini Mags through it. I'm starting to think the Rem. Thunderbolts are just garbage.

Also it shoots great again after cleaning it. It's no tack driver but with a red dot at 35 yards it's a hoot to shoot.