Changing powders has been known to effect accuracy but only for a few rounds . After 5 rounds or so of the new powder . All the old powder fouling and it's effects are gone . For me that works out great because I've never found the first minimum load as being the perfect load so I use those to wash my pains away .
Now that's all with in reason . If you just put 100rds down range testing loads and plan to run another 50+ . I'd clean the rifle but if I've only shot 25rds from a clean bore . I'd just shoot my next powder knowing the first test loads will clear out any powder fouling left behind from the last load .
There was a point in time that I tried the copper equilibrium theory . Which is (all rifles are different) At some point in the round count you are removing just as much fouling from the bore as you are leaving behind . Most rifle need a few fouling shots to settle in ( some more then others ) . Then you hit a point where the rifle is pretty much GTG for the next X amount of rounds before accuracy drops . I would go 300rds before cleaning with out any real noticeable change .
The theory is you don't clean the rifle so the next shot shoots to the same POI as it did 2 weeks ago when you put the rifle up after the last time out . Think about it , You're at the range and you get your rifle shooting dead nuts . Then go home and clean it . How do you think those first few shots next time out are going to shoot . Not the same as that last group you shot two weeks earlier .
Now this is not the way to go for competition but if your in a situation where you need your rifle to shoot a dead nuts cold bore shot . Which would you want , a rifle that's sparkly clean that you're not really sure where exactly the POI will be or a rifle you just finished shooting a few days ago that was shooting great .
Now I did find this to work , After 150rds and the rifle put up for a month . The next time out my cold bore shot was almost always dead center of what I was aiming for .
OK now all that said , DON'T DO THAT
. You can get a lot of build up in the throat area and It takes for ever to clean the rifle after 300+ rounds . I was getting so much fouling in the throat it actually effected my COAL . That's when I realized it's not worth the effort for the type of shooting I do . how ever IMHO the theory has some truth to it and in the right situation it's valid .