I'm not sure about the MK-86 designation but I know they made a shotgun version of the MK-85. Maybe they redesignated the shotgun version. Anyway it's #11 or musket fired, but can be converted to 209. Pricewise your probably about right.
If it's 12 ga (and I'm pretty sure it is) several companies make plastic wads for them. They aren't necessary though. The way you load a ML shotgun is powder, overpowder card or plastic wad, shot, then overshot card. With the overpowder card and overshot card you need a 12 ga jag to push them down. The larger surface area lets it go down flat onto the powder or shot. You will have to use loose powder as there are no pellets big enough to use (diameter). When loading a shotgun, you want to use an equal volume of powder and shot. For Triple Seven reduce that by 15% since it's a little more powerful than black powder. Generally this will give you a good pattern but usually a little more shot will pattern best. Traditions makes a shotgun powder measure that reads in ounces. You may also have to play with wads to get a good pattern. Shotguns have more variables and are harder to get set right. Once you've got the right setup for what your hunting write it down so you don't forget. I used to have a TK-2000, but it was a little more hassle than I wanted.
You should contact Knight for specific info on the shotgun in question.
http://www.knightrifles.com/ContactUs/default
