Valdada. Lots of choices.Who makes a 40mm ring?
I have a 40 caliber muzzleloader too. Love it. Wears a 4-20x50 STAC SFP. All I need for it and works great out to 740 yards my maximum shot so far. Wish it had an illuminated reticle tho. Don't care for the same model in FFP so one day maybe they'll offer it. 5-50 is alot of optic. My 10-50 is the same way for focus. Get last say 1200-1300 yards and focus on it gets a bit tough.would like to try one on my big .40 caliber Smokeless ML gun, right now it wears a Delta 5x50 and works well but at a mile I can't focus past 25 power, hopefully they have this figured out on the new ones.
I have several Sightrons and they have served me well, hard to beat for the money. I would bet it will work well, has to be 2nd FP & MOA for me though.