You arent mentioning how you are sighting it in. You want to be sure you are sighting it in from a benched rest, padded sand bags, etc. Make sure you are wearing a good heavy jacket, wearing ear protection, etc. I see alot of folks at the range still getting upset over thier crappy groups, when they are blasting away off hand.
Be sure to give the bore a good scrubbing with a good bore cleaner after all that shooting,dry, and fire a fouling shot. Pace your shooting. All semi, pump, etc. guns are sensitive to heating. Although seldom noticeable when hunting, at the range it is. Fire, wait a few minutes for the barrel to cool and fire the next one. Sure it takes time, but what of it? you are trying to get it right. Make sure the shooter isnt getting flinchy. Even the "tough guys" start to after awhile.