can a rifle scope be used on a muzzleloader?

beagle...

As far as I know it is simply a matter of the parallax being different on scopes identified as for shotgun or muzzleloader use. Also the reticle maybe a design better suited for the firearm.

Parallax is visible as an apparent movement between the reticle and the target when the shooter moves their head or, in extreme cases, as an out-of-focus image. Most scopes with a fixed parallax are set at 100-yards. Scopes identified as for use with a muzzleloader or shotgun have the parallax set at 50-yards. I bought a Tasco 2X shotgun scope and the parallax was 70-yards. The specs of the scope usually state what the setting is.

I deleted the other of your dual post to save server bites and bits...