I have enjoyed my 4 bore muzzleloader so much that I am thinking about building a singleshot 4 bore cartridge rifle. Are there any falling block or break open type actions out there that are big enough? I'm not really interested in the big bolt actions.

Farquharson Kits:

http://www.actioncastings.us/

There was some discussion on this forum a couple of months ago about these kits.

I seem to remember that they said it was suitable for up to .500 3" rim diameter (approx. 3/4")....so I think the 4 bore would be out of the question.

Beware of those kits. There are no decent drawings to go along with them. The castings are no good without knowing where to put what size hole how deep.