If the paint breaks as it is pushed into the barrel, it will result in paint in the breach and on the bolt, resembling a chop. It would still be considered a barrel break and not a chop. A chop is where the ball gets chopped in half when it is only partially in the breach. The hopper has to be slower than your shot rate for this to happen. This is rare on force fed hoppers unless you run them empty.
Given your inconsistencies in velocity, I would look toward paint as the culprit. I'd at least test it first.
If you can single shot your gun using the same trigger pull method and same time between trigger pulls, then you will level out the velocity from the gun's point of view. Then it comes down to paint inconsistency, unless there is a malfunction in one of the mechanisms in the gun. If the paint is all nice and round and the same general size, then look to something in the gun.
When you oil a mag, make sure you blow out all the excess oil by cycling the gun before you put the barrel on. Make sure you clean out the breach. Oil residue will cause inconsistency and erratic shot accuracy. Make sure you don't oil or grease the bolt or bolt spring. That will cause issues as well.
Except for the Automag in front, its usually the man behind the equipment that counts.