All you need to shoot iDPA is your M&P, three mags, a legal holster and two mag pouches, a cover garment to conceal the rig, and ear and eye protection. About 100 rounds will handle most IDPA matches, Bring 150 in case of reshoots.
Check the IDPA website for clubs in your area, and for the rules about equipment. You will be shooting in SSP - Stock Service Pistol.
Most of the people in the action pistol sports are very friendly and helpful. Go for safety always, accurate and smooth first, speed will follow.
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