If you have checked everything out, like scopes, mounts, rings, using the same ammo as the time that your rifle shot good, and have checked the screws that hold the barrelled action in the stock, and the barrel does not have a build up of copper in the rifling, then I would say it might be the shooter. I am not trying to be a smart ass, but some days we just shoot better than others. The 300 RUM is a lot of cartridge with a lot of recoil. You might want to get a Caldwell Lead Sled from Midwayusa.com and try shooting your rifle in it. You control the recoil by adding 25 pound bags of lead shot to the sled. Then if your rifle is inconsistant, you might need the services of a good gunsmith.
Dave (Bubba) Thornblom