A picture would help a lot. Sounds like it could be an MK-85, Wolverine, Bighorn or Black Knight.

Does the nipple look like a #11 or musket cap like these;

http://www.knightrifles.com/catalog.aspx?catid=redhotnipple


Or does the breech plug look like this;

http://www.knightrifles.com/productdetail.aspx?id=900021


Instruction manuals can be found here;

http://www.knightrifles.com/instructionmanual/default