Not saying this is you, but the biggest mistake I see people make at the range ,when sighting in a rifle or slug gun, is they do not have a solid front AND rear rest. I cant tell you how many guys I see resting their guns forend, or pump handle, on a sandbag ,or 4x4 piece of wood w/ no rear bag. Some guys even rest their barrel on the bag! IMO you cannot get a accurate zero w/ out proper front and back rest, and proper shooting form. I also believe front bag placement is crucial too. I like to place the front rest about 3-4" from the front of the receiver. Also make sure your action screws are tight. MY savage ML was grouping bad a few month ago ,then I realized I did not tighten the rear action screw, after removing the breechplug. Tightened it down and groups went back to an inch.

X2 on barrel cooling, and dont swab your barrel in betwen shots.