Jerry, I'd give you a simple, straight forward answer if there was one. As it is, I can only say that I see no reason for chamber pressure to be reduced, good reasons for barrel pressures to be slightly reduced, and MV to be reasonably constant. It could happen that the powder is not burning well in the enlarged volume (reduced barrel pressure) of the 10 bore barrel. You might want to try PB. Remember, as bore goes up, we need faster powder. However, we still must deal with keeping chamber pressure sufficiently low. I have a spiffy old Scott 10 bore hammer gun, so I'll be going through this same issue in a short while.