Forget the head size, just find a pellet that works. Starting out with springers just get a benchmark group size. Once you're comfortable with that you can then see if batch variation will have any effect, but to be honest once you have a fairly consistent pellet then batches are probably lower down on springer shooting woes than they are on a pcp.

JSB & their rebadged clones like AA etc is the make predominantly used by HFT/FT shooters, almost exclusively. You may see the odd person trying something different but if you look down the lists from results where pellet is stated it will be JSB or AA (made by JSB).