Two things that make the larger bore pattern better, generally. The shot scrubbing against the barrel is one, but the set back forces against the shot is the bigger one. 1 1/4 ounce of shot in a 10 bore does not have as tall a shot column, which means on firing there is less weight or fewer pellets above the bottom layers which causes deformation. When you touch off the bottom of the column pushes against the top. The English used this principle in their double game guns, where many were 12 bore but maybe 2 1/2 chambers and used 1oz to 1 1/16 oz of shot The shot load should also for recoil principles be 1/96 of the weight of the gun, by their reckoning. Do the math and you see 1 1/16 is 1/96 of a 6.5 pound shotgun. Another reason larger bores may pattern better with the same charge is that pressures are generally lower. Turkey shell principle, where they load to low pressures and low velocities to get high center densities. Use of hard shot or plated shot helps to reduce the setback effects as they deform less. Right off the top, three of the most useless cartridge loads, partly due to the lack of cushioning, are the 1 7/8 oz 3 inch 12, 1 1/4 oz 3 inch 20 and the 3 inch 410. As Roster stated about 1 1/2 oz of shot in a 12 is what it will handle. I have handloaded loads in a 3 inch that convince me of this. In a ML as the weight of the shot charge goes up more cushioning may be needed, especially in a choked gun. One of the things Roster wrote about was the use of fiber wads in the shotshell over plastic. Probably holds for ML's as well.
DP
