jesshodgie, I guess it could be copper fouling. Powerbelts are lead with a thin copper plating on the outside. If you were shooting 150 grains 777 with a light powerbelt, I could see how some of the copper plating might be stripped off and stay in the barrel. It's never happened to me, but I guess if you shoot enough and don't clean out the fouling it could mess up your accuracy. The only way to be sure is to make sure the barrel is completely cleaned out, then go to the range and try it out.

If you want to stick with powerbelts though, I'd recommend using a heavier one (275 grains) and using less powder- you shouldn't have problems with fouling and they should perform better on game.

But if you really want high performance, I'd try out the 200 grain Shockwave/SST - if you can get them to shoot accurately (which they do in most rifles) you can push those really fast and they'll still perform great and shoot flat. Plus you don't have to worry about copper fouling- though you do need to clean it to get the plastic sabot remnants out.