Had time to do some checking, and playing today with the #7! Slugged the bore and was surprised to see it is about halfway between a .44-100 and a .45-70! It slugged to .452" with a soft lead ball. Just to be sure I grabbed a .446" bullet and it dropped into the neck of a sized case! Then I took a .458" bullet and ran it through my .452" sizer, and it fit the neck perfectly with good tension.
I then set up the dies to see how deep the 500 grain bullet would need to seat and chamber. Since the factory .44-100 was a 500-535 grain bullet, figured it should work. Well sort of. In order to seat a 500 gr. bullet, it ends up with about 60% of the bullet seated into the case, or the ogive hits the rifling!
The left is the case I got with the gun, and the right is the 500 grain sized and seated, plus the bullet itself beside it. I think I'll fire up the furnace and cast some of my 405 gr. bullets. They may have a more pointed nose shape, and possibly seat out farther. They'll also be easier on the shoulder!
I'm actually glad the bore slugs larger, as sizing down .45-70 bullets will give me a lot more variety of bullets to try, and figure out what the #7 likes best!