Tom, you got it correct. From the end of the chamber to the start of the rifling is .510" 
I'm seating the bullets with two exposed grease grooves and the third groove and base still in the case. I guess l can try sticking it out that last groove. I want to use the rifle in the couple of ram bash gong matches in my area so I don't want to have to use a slug the length of my pinky. Shooting a few big slugs of the bench is one thing but 50 in a row offhand is a little different.