A couple of months ago, there was an all-10mm issue of Firearms News (fka Shotgun News), and they tested the same rounds through multiple guns, including some AR-based 10mm pistols with longer barrels, and the velocity difference between 5" and 7" is negligible. Selecting slower-burning powders for the longer barrels would probably eke out a few more FPS, but for a packing gun, I don't think a 7" barrel really gets you anything in exchange for the additional weight and bulk.
