I have done demonstrations at a few clubs. What I do, is ask them to let me shoot one target with my Trapdoor and Krag. Then they can inspect it. If the rounds cause any damage, I will buy them a new target. There's never been any damage, and I've never bought a target yet. Then I offer my rifles to anyone who wants to try them. Thats usualy all it takes.
You can also remind them that the .56-50 Spencer is a SASS Legal round, with its 350 grain bullet. If their targets won't take a Trapdoor round, they won't take the Spencer round either.
Of course, the trapdoor has knocked a target over now and then . . .
Cody-Dixon style is also an excellent way to go. All you really need are two additional rifle targets per stage set further out, widely spaced. Most clubs have extra targets laying around, and you can always offer to buy a few more over time.