I used to be of the mindset that ammo can't have that much impact on groups, at the ranges you're talking about.
That is until I got my 35 Whelen. It shot groups like that with Rem ammo. I was about to sell it when someone suggested I try different ammo. The Fed Fusion was night and day. First group I shot a 3 shot ragged hole. When Gander went out of business I bought every box they had. That gun should be in ammo now for a few generations.
Remember, shoot straight and shoot often.
Good... Bad... I'm the guy with the gun.