I was recently shooting groups with different loads to evaluate a barrel, what barrel and what loads aren't important to this question (I don't want to get derailed ).  Anyway I struck up a conversation with an older gentleman who ended up spotting for me.  He noticed that the first shot of several of my groups was off from the rest of the group and he said in his experience that's common when switching powders.  Apparently he was a long time competitor of long range silhouettes and used a different load for the rams than he did for the closer targets.  He said every time when he switched loads the first shot would always go 8" high, the second 2" low, and then the rest would settle into a good group.  He attributed it to the difference in fouling between the powders.  

I thought about this for a while, and I think I understand his theory.  Let's say powder A makes fouling with a different amount of friction than powder B's fouling.  Bullet/load A fired through flouling A gives a certain point of impact, and bullet/load B through fouling B gives a certain point of impact, but bullet/load B through fouling A is going to give yet a 3rd point of impact.  So when you switch from load A to load B it takes a couple shots to replace fouling A with fouling B and you end up with first shot flyers.

Does anybody else's experience reflect this? Or is the guy crazy?

It's commonplace, don't worry about it. 

It is a well studied phenomenon.
It is real.

I'm a regular metallic silhouette competitor. One of the things I had to do when setting up my match gun was tune my ram loads to not need any fouling shots. When I got the rifle the previous owner had done that already but used a powder and bullet for the ram loads that I didn't normally deal with and had no other use for. I wanted to use one my preferred powders and one of my preferred bullets so I had to build a load that would shoot to POA right off the bat without any fouling shots. Since I try to start on the pigs that ends up necessary. My coach prefers to start on rams for related reasons (but also so he takes all the hard recoil early and finishes the match on lighter kicking chicken/pig/turkey loads). I think this is one of the things that made fast 6's like 6XC and 6Wally and 6CM so popular in the last few years. Just load up some 115gn DTAC's running ~3000fps and you've go enough for a super light recoiling load that will go across the course, no POI changes and stupid fast running 6mm's seem to have a hard time ringing targets, they go right down. Personally I'm not a fan because those loads chew the snot out of the chickens. I personally use a 7BR right now in standard gun class and I'm building a 6.5grendel for hunter gun class.

Wow, I just learned something new. I guess I've just never really switched bullets mid-session. Thanks.

You have to post or let me know how the grendel does on rams. I was thinking of the same thing but figured it would be marginal at rams. I've been looking at the creedmoor instead.

Thanks