It can get a bit tricky. I' m lucky enough to have a good friend who shoots for Specialty and he has several different size and power combinations so we could figure out what I needed. The web sight will give you some generalizations, but probably won' t help you nail down exactly what you need http://www.specialtyarch.com/superpeep.html . It would be great if you had a dealer that carried the stuff (and had an assortment on hand) and could help a guy like your or I find the right combination, instead of taking a stab (at about 18 bucks a whop) and going from there.

Good luck!

-Chie