Major and minor(caliber) are for scoring purposes in USPSA.
If you choose to shoot IDPA caliber will have NO bearing with regards to scoring.
In both disciplines IF you re-load your ammo must meet the minimum required feet per second or power factor. At the local level ammo is not checked. At a state or national match ammo is checked using a chrono to see how fast your ammo is moving. If your ammo passes you are good to go. If your ammo fails you are DQ'ed and your shooting is all done for the match.
NRA Life Member, NRA Range Safety Officer, IDPA Safety Officer, USPSA NROI Range Officer
As you are, I once was, As I am, You will be.