Shotgun barrels are noted as having more elevation in the rib due to larger base compared to muzzle diameters (Lt. James Forsyth 1860) and more conversion of the muzzles due to getting the shot loads to print together at close ranges. Both of characteristics are negatives when shooting slugs. Shortening the tubes helps reduce both potential problems. I filled my rib with JB and it worked well and still hasn't shot loose, the current owner now having used it for his moose yearly, since about 1988.