To do it properly you would need to make the breach plug fill the area of the chamber, that way the ball or bullet would not fall into the chamber area on top of the powder charge and rattle around. H&R did make a breach plug for it's muzzel loaders that sealed with an O ring, they worked great until a few people shot themselves in the head with the breach plug when they opened the rifle too soon after a hang fire. After that H&R went to a threaded plug. I would say the press in O ring sealed type would work in a 45-70. I don't know about the twist rates etc, but it should function. Just cut a slot in the breach plug for the ejector to fit in and then machine the back of the plug for the primer nipple. Larry