Set of three Marksman Barrel Backs (inside diameter .683, .686, .689) for the M5 and M17 line of magfed paintball markers. Set is optimal to add to Marksman Barrel giving you an ideal range of bore sizes for paintballs which are used today. A5/Milsig rear thread. Opposite A5/Milsig inside the front thread. Install it between your barrel and marker and extend your stock barrel.