Figured someone would have already chimed in with this suggestion: rather than shooting a group, then moving onto a different ammo and shooting another group, you could shoot them all incrementally.
set one target for each ammo type (I use a 4' x 4' plywood board to set multiple targets).... fire a couple of rounds to get the barrel fouled and warm so that barrel heat can be more or less constant across the range of loads. Then fire one round per type at each target, then go back and shoot round #2 into each respective target, then repeat the process until you have your groups to whatever number you like (I like 5 shot groups, statistically significant but not excessive. some say 7-shot groups are best)
the beauty of this system is that it puts each ammo type on a level playing field, provided you do your part and keep the barrel at a more or less constant temp by allowing it to cool between shots. EVERY ammo type experiences +/- the same degree of fouling throughout the process.
two downsides are keeping the ammo/targets straight, and as mentioned before, it's tough to know when groups are opening up due to barrel fouling and not due to its inherent accuracy in YOUR rifle.