You can modify how far the nubbin sticks into the barrel. There is a small nub on the back of the nubbin. This usually stick up past the outer diameter of the barrel. If you have a barrel that has a slightly deeper nubbin groove or smaller outside diameter the inner surface of the body may not provide any force on this nub to keep the nubbin pushed far enough into the barrel to adequately keep balls from pushing past when there is a bit of force applied. If the nub is sticking up, wrap a piece of tape around the barrel over the nub to take up some of the slack to help push the nubbin farther into the barrel. If the nub doesn't stick up past the outer edge of the barrel, you may need to put as smaller piece of tape on it first or apply a tiny drop of epoxy on it to build it up. Once you get more of the nubbin pushed into the barrel, your problem should go away.
Except for the Automag in front, its usually the man behind the equipment that counts.