I discovered the same thing about the plastic straight line ones when I tried to use them in some of my inlines with scopes. Also the cheap ones I got on sale have weak springs and tend to drop the top hats. Plus the plastic ones are too fat and the bottom part sits to high to get a top hat on easy. You have to monkey around to much.

I am wondering if you could not modify a dog bone capper to make it fit musket caps? If you look at the red plastic dog bone cappers, the musket cap sits in the same place a 209 primer would, but you'd have to drill out the back side, just a hair smaller so it would hold the top hat more secure and bring it closer to the face of the capper. I really think it would work. But since I seldom shoot musket caps except on my Black Mountain Magnum and my Staghorn... I have never converted one.