BACKBORING Gunsmiths "backbore" shotgun barrels by enlarging the bore diameter with a reamer. Traditionally, 12-gauge bores measure .729 inch, give or take a ten-thousandth of an inch. Opening the bore to a larger diameter (usually around .740 inch) reduces friction as the shot charge travels down the barrel, increasing payload velocity, improving patterns and--in theory--reducing felt recoil. The gain in velocity is modest--about 20 to 40 fps. The reduction in recoil, to my shoulder, anyway, is unnoticeable in a hunting shotgun that may be fired no more than half a dozen times in a day.