Make a lot of copies of your own simple targets on plain paper.
Shop the stores as well as online to obtain at least 20 or more different kinds of ammo. Spend some time at the range firing off 5 rounds of each per target and you will come to see which ones group better. Weed out the poorer performers, then do more comparisons going out further in distance. I did this and found a couple particular brands that worked the very best. Then I went back to my favorite online catalog seller and bought a few bricks to use for my shooting season. Keep track of the brands while shooting and make written notes on the targets. Some might appear to be one-hole-wonders while others could be widely scattered. Buying one box of everything you can find can be a bit costly, but I think it is an excercise well worth it.