.40 is only popular in limited. Production is 9mm. Open is 9mm Major or .38Super normally.
I like Arredondo basepads.
.40 is a viable option for production i
f you reload, and can be made softer shooting than the 9mm at equivalent power factors. It is somewhat popular in New Jersey due to the 15 round magazine max capacity. Admittedly, I know some that used to shoot .40 and now shoot more 9mm, particularly after some of the price hikes on bullets.
As for the Arredondo base pads, I bought a few to make IPSC length (not USPSA). I followed the instructions to a T. One was very brittle and just broke. The others worked fine. While he did replace the one that broke, he insisted that I did something wrong. Was kind of turned off by that.
For USPSA, I much prefer the Dawson pads over other models. Very to easy to assemble / disassemble, no tools needed.