300 wby mag
180 grain barnes ttsx
78 grains RL19
2 fouling shots
First two 3 shot groups approx 2" at 200 yds
Then pattern went to 4-6 inches and shots started to walk up and right-left progresivly getting worse...
Fell on my ass pretty hard the other day but?.....
It is a 300 Wby magnum and there is part or most of your answer. You don't say what kind of stock or barrel you have or how many rounds you have through that barrel but I am guessing sporter 26" barrel and fully bedded stock. My gun does the same but my initial cold barrel group of 3 shots, (I don't shoot fouling shots because this is a hunting rifle and I go in the field clean and shoot cold), is a tad under 1 1/2" when I do my part. The next 3 shots if taken soon after are just under 2" and after that the group opens up and you could fry a dozen eggs on my barrel. I am using plain old Remington 180 gr Core Lokt ammo nothing special about it. Left dirty or clean it makes little difference, when the gun cools down I am back to 1 1/2" initial groups. The last few years I learned to limit myself to 5 or 6 shots per session and then go to a different gun. This isn't a gun I am going to use for competition. If I wanted to do that I would put on a heavy match barrel.
Just dos centavos from a dinosaur but I think you are expecting a bit much from a hunting rifle that shoots giddy up go bullets. I use 4350 powder in my reloads, not quite as fast but I gain a little in accuracy, something you might want to think about.