Unless a shooter screws up a barrel, maybe from not cleaning it properly after shooting, that barrel should be ok. The OP is confident he let the barrel cool between shots.
I've had two Encore rifles that each had over 4,000 rounds through the barrels. With the exception of replacing breech plugs, the barrels shot as good after 4,000 rounds as they did with the first 100 rounds. Nothing was used in the barrels, short of cleaning them properly and making sure they were lubricated properly. JB nor any other product was needed or used. That may not be the case in everyone's rifle, however it is an example in two of my past Encores.
Each suggestion/recommendation pointed out above can be a contributing factor to bad accuracy or groups. Assuming that the OP is a reasonable shot with a good rest, properly maintained his rifle and the rifle shot great in the first place, the clues indicate it is most likely not the rifle. It could be related to the propellant and how humid it may be, how old it is or, maybe a bad lot of sabots or bullets. It certainly could be related to the scope, especially if the groups are not following adjustments. If you make a 3" down adjustment to your scope and the bullets/group doesn't follow, with all other things equal, I'd look real hard at the scope.