Check it out: http://members.idpa.com/Content/Rules/g0hunysc.eoc.pdf

These rules expired on 10/1/13 and I have to find a new set of rules: http://members.idpa.com/Content/Rules/g0hunysc.eoc.pdf

The two things that jumped out at me instantly were things like a tungsten guide rod or something heavier than stainless and stippling certain areas. I also run the same gun out of the same holster in both games so picking an IDPA holster is also a smart move to run in USPSA IF you want to play in both sports with the same equipment. Everything is the same the way I play both games for equipment. You might want a specific gun for USPSA and another for IDPA.

It's a game, go have fun.