Assuming your using the hex head breech plug,
This: http://www.knightrifles.com/productDetail.aspx?id=326403

You can also use a socket but with the plunger guns it's more hassle than it's worth. The tool is better. It's the way they're designed. They have a finger which holds the plunger open till you pull the trigger. With the tool it's all 1 piece.



If your not then you need the 2ended version: http://www.knightrifles.com/productDetail.aspx?id=326401

The bad thing about this tool isthat it's for #11 nipples only. You'd need yet another tool fora musket nipple.