It's simple. You do what the Govt (FBI) did)
You start with a 38 special. Then after that doesn't work. You go to 9mm. Heck, that is no good. Then go to 10mm. Well that is to potent, So you scale back to the 40 SW. Now that is to snappy for lots of recruits, so you go back back to the 9mm.
I may have the order messed up, but something like that.
The ballistics guys will calculate it for you.
Smaller bullet going fast or bigger one going slower.?
Somewhere there is something that will work if you actually hit the target!