There are probably several factors at play here. Make sure the barrel fits snug in the body. The gap between the outside of the barrel and the inside of the body affects how far the nubbin gets pushed into the barrel. If the gap is large, you need to add some artificial depth to the nubbin. This is usually done by adding something to the back of the nubbin where it contacts the body since this is the pivot point that pushes the front of the nubbin into the barrel. It can be a drop of epoxy glue or even a piece of tape wrapped around the barrel and nubbin at that spot. As long as the nubbin gets pushed farther into the barrel it will work.
A lot of the twist lock barrels have a larger bore because the paint was bigger when they were in use. Now that may paint producers are releasing smaller paint, any nubbins that are weak or not fitting the barrel/body combination will have even greater issues with the nubbins not contacting the ball properly and allowing the ball to roll out of the breach with minimal push by the hopper or even by tilting forward.
If you can actually fire a single ball, it should shoot straight. If it does not shoot straight when fed one at a time, then the issue is usually bad paint or a dirty barrel.
Except for the Automag in front, its usually the man behind the equipment that counts.