A 3 1/2" 10 ga and a 3" 12 ga shoot the same fps , just more pellets in the 10. A 31/2" 12 ga is Peeing in the wind. Your trying to get a larger amount of pellets out of a constricted barrel. That's why they kick so darn bad and still have lower velocities. They shoot 3" 12s fine but the gun weighs too much.