Welcome cnybrian. You've just entered the best and "most comfortable" muzzle loading forum around.
I'm a Leupold guy myself. Have VX-II's on three of my guns and would recommend them to anyone. Don't have a Rifleman model, but would assume they arepretty much standardLeupold quality, which is good.
Don't have any Nikon scopes, but do have two Nikon binoculars and they are great. I don't think you can go wrong with any of those, though I would suspect the SlugHunter would have heavier cross hairs than I would want.
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Welcome to the forum and the sport of muzzle loading.
I want to answer your question with a question. What distances are you planning on encountering when you hunt? If all your shots are 100 yards and less the need for a BDC is really not necessary. Actually with practice a BDC is not necessary so the Nikon Omega would be a little more expensive, but a great scope. If you do take long shots then the BDC might be a good investment. I personally do not use them.
Leupold Rifleman is their bottom of the line, but it is a Leupold. I played with one in Wal Mart and it is a nice clean clear scope. Also for under $200.00 it is a pretty good bargain. The Leupold VX-1 is a good scope. And being a Leupold almost assures you of quality.
The Nikon Slughunter is more set up for Shotgun slugs, so the BDC might not be real close for you with that scope. Although the eye relief would be nice to have. And Nikon is a great scope.
Another scope out there is the Nikon Pro Staff which also comes with a BDC and is a little less money then the Omega. I personally use the Pro Staff on two different rifles and they are great scopes. You can get them for $129.00 so they are a real value. They have excellent clarity, and light gathering. I like the 32mm scopes.
Another great scope I use are the Bushnell 3200 with rain guard. They are a great value. I picked one of them up on sale and mounted that on one of my inlines and it kind is my favorite scope of all of them I use.
What I look for in a muzzleloader scope is clarity, light gathering, parallax, and length of the scope. I like them under 12 inches so that they do not effect the rear sight. Also I like to mount them with QD rings so I can take them off.
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You might want to look at a Leupold Ulitmate Slam muzzleloader scope. Ithas the mulitple lines like the Nikon BDC, but from what ive read on other reviews, it works easier with other loads.
Up until last week - I would have told you are missing the best scope on your list... the Bushnell Elite 3200 with Rainguard lenses and a ballistic reticule - wow! what and awesome scope... but last week I got a new Leupold Ultimate Slam - built for MLs - really think you might want to take look at.
Also - as the rumor mill has it the Rifleman and the VX-1 are going to be discontinued
Take a look @ the Ultimate Slam
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Keep on Shooting Muzzleloaders they are a Blast
Muzzle loaders have a different recoil than High power rifles, I have shelled out quite a few scopes over the years now I stay with scopes designed foe muzzel loaders. The Nikon Omega is very good and the Leupold Ultimate that Sabot loader mentioned would be a good choice. Lee