The Crushed Rib sabot by Harvester should solve your problem. However, Knight Barrels generally run large and generally don't require a CR Sabot.

A couple of things to think about:

1) Is the barrel properly cleaned and free of powder residue? With sabots, you should be swabbing between every shot.

2) You can mix and match bullets and sabots to find the right fit for your gun. A 50 cal will vary from .498 to .504 depending on how new the equipment was when rifling the bore. Again, Knight are generally on the larger size which makes me think something else is going on.