Understand that I've not yet done this, but may in the future. A neat conversion that would be fairly easy would be a 22 WMR. The idea is to use a liner and secure it in each 410 barrel so that it allows existing firing pin to hit the rim and existing extractor to catch the rim. Counterboring at the breech to a larger diameter is likely required. At the same time, the liners would need to be parellel so that they would regulate. This would require the liners to be closer to the center at the breech and to the outside of the barrels at the muzzles. Support bushings at each end and a couple mid barrel would be necessary, and keep added weight to a minimum. The biggest issue for me is finding a 410 double I'm willing to modify. As we all know, nice ones are spendy.

Otto