I would recommed you clean the rifle with a copper cleaning agent, there are several to choose from. Sweets, Barnes & Hoppes #9 Copper solvent come to mind. Make sure you get all of this cleaning solvent out of the barrel before you put the rifle away. I have been told this solvent can do more harm than good, if it is left inside your barrel.
Shoot the exact same ammunition you shot before, when your rifle was shooting its best groups.
My wife shoots the exact same rifle (with a muzzle brake) her best groups are with Federal Premium ammo that is loaded with 180-grain Nosler Partitions. She shoots an honest 5/8 to 3/4-inch group @ 100-yards from the bench. If all else fails you could give the Federal Premium ammo a try.

good luck,