Yep, not all these bores are the same. Just because they say .50 caliber doesn't mean that is what they will measure. I can slip remington sabots right down my bore for the most part, but I own a remington. My buddies that have knights can barely squeeze them down theres with a clean barrel.

You may also have plastic fouling as well if the gun has been shot alot. I have never noticed this with my gun though.

Paul