A couple of decades ago I had a bad experience with sub-sonic loads in a 1918-vintage Ansley H. Fox B-Grade, 12-gauge, with 28-inch Krupp 4-weight barrels. The loads I tried were the Eley Hawk. The gun would shoot right on both barrels with my 1200 fps 7/8 and 1 ounce reloads, but with the sub-sonic loads the left (improved modified) barrel shot off way left?!? Right barrel was fine. Have stuck with 1200 fps ever since.