Guys what is the difference between a rifle scope and a muzzleloader scope; if I have a rifle scope that I want to re-purpose to a ML, what do I have to take into consideration when sighting in?

Mostly eye relief I believe. I switch out my scope of my .308 for the few times I can use a scope in other states, ML hunting. Typically you do not need the higher magnification seen on many center fire rifle scopes for muzzle loader hunting. Some cheaper ML scopes might not withstand heavier recoil of larger center fire cartridges

Parallax on muzzleloader/shotgun scope are set at 50yds (and maybe 75) whereas rifle scopes are normally set at 100yds (maybe 150 also). I've had no problems using rifle scopes on my ML.

I forgot about that point. CRS is creeping in more frequently

LOL Happens to the best of us. To be honest though the only reason I remembered about the parallax is because I've only been into a ML less than two years. Researched ML scopes but had extra rifle scopes around so went with them. Settled on an Weaver K2.5 60C I bought new in the late 60's and had reconditioned by Weaver Scope Repair Service a bit before he went out of business.

Yup, and for a 50 cal inline shooting 290gr Barnes TMZ sabots and 150gr loose 777, the 3-9x40 Redfield Revolution has been perfect for me, that scope has took a pounding and never once blinked.

I have a Leupold pig plex. It was made for muzzle loaders and shotguns. Appropriate eye relief, parallax, and reticle marks for distance. I think my 1.5-4X weighs less than 10 ounces, so it is nice and light.

Can you be a bit more specific? What make/model is the scope? What
muzzle loader do you plan to put it on?

I personally use only rifle scopes on all my muzzle loaders, and all
have adjustable objectives, so parallax is of no concern. But some
of the higher power muzzle loaders can EAT rifle scopes, thus the
inquiry for more info.

Just getting into ML, so picked up a TC Impact. I have an old Tasco that I took off my 30x06 that I've had for 40 years, lol; but the scope is in great shape and never had any problems with it. I can re-purpose (save money) or pick up something else. Basically looking at up to ~150 yards but more likely in the ~50-100 yd range. Will shoot Black Horn with mild recoil, not looking to kill an elephant, lol.

If it's a Japanese made Tasco, you're probably good to go.