I use copper solids myself and get excellant groups at 75 yards, thats what shoot the best out of my 1300. Its a rifled barrel w/ iron sights.
As far as your breathing, take a breath and exhale about half and hold it, before you sqeeze the trigger. As far as you flinching, try dryfiring, if you have one of those dryfire rounds (they sell them in Cabelas) but put a Quarter on the top of your barrel near the muzzel and dryfire and dont let the Quarter fall off then when you get that down do that with your eyes closed. These will help with breathing control and flinching. Just have to practice cause it wont cure it overnite. Good Luck!! (these were sometricks they taught us in the service)