They are separate organizations. You need to join IDPA to be classified by their rules, but you could shoot USPSA forever as unclassified without joining, however you can only compete at local clubs, not higher level matches. In IDPA you are only allowed to have a mag in the gun and 2 on the belt, in USPSA, wear as many as you want. They are both games, with different rules. IPDA was invented by Bill Wilson, an IPSC champion back in the day. Shoot both, see what you like.
Last edited by 9x45; 04-06-2010 at 19:08..