Unfortunately, that isn't a measurement that can be given to you by another shooter. Even with a brand and model, the diameter of the individual bores will vary .001-.002. Most of the Knights have a tighter bore, which would be closer to .500, but that isn't necessarily the case. If you're trying to use Thor bullets, which is what most people are sizing their bore for, you can order a sizing pack for a couple bucks. It's a pretty simple process.