I've got one of the centerfires in 45-70. Exceptionally well made, nice looking gun. Heavy, but decently balanced. It's very accurate with each barrel, but getting a load that groups both barrels together continues to elude me (after several years of trying!), so I'm forced to use a bit of Kentucky windage for the second shot. You might have an easier time of it with a muzzleloading version, even if you have to vary the load slightly for the two barrels. Bottom line, it's a lot of fun to shoot.
They have within the last few years released a much larger bore size in this gun, something like .69 or .72 as I recall. Now THAT would be a plinker to end all plinkers!
Good luck
John