I would say close to the 12, it really depends on the ammo you use. You wold have to study the ammo and see how accurately the rifle shoots it. From what I can see most of the new slug offerings in a good rifled barrel are as good if not a tad better than an Inline muzzle loader. I would say 150 easy with the right load if it shot accurately. I would for sure shoot it at that distance to see how it groups and find the actual amount of drop for your set up.

I know I asked at the gun shop if there was a big difference between the 12 and the 20. They said not really and most prefered the 20 over the 12 slightly. From what I can see from the research I have done lately the 20 guage may be a bit more accurate. That is not a given however. They will probably have less recoil depending on weight as well.

Had they not had a used a 12 guage deluxe for $219 I would have gotten the plain jane 20 gauge ultra slug instead. It was about the same price. 12 guage has more ammo offerings currently and might be easier to find in a pinch as well.

It's a nice gun when you find the right load. Mine didn't like the lightfields at all, but most others seem to have great luck with them.

Paul