My best tip would be that instead of concentrating on keeping the rifle absolutely still, steady your breathing (DONT hold your breath as many seem to do) concentrate instead on pulling the trigger as you breath out at the correct time ie: as the cross hair pass your desired point of aim. Concentrating on keeping a rifle still will just make you tense and you will be jerky and more likely to flinch. My dad taught me that when I was a young fella and it is the single piece of advice that made the most difference on my shooting.
Bookmarks