Crazyquik, liners are not the way to go to build a double rifle from a shotgun, even in .22lr! with a 410 double, one can cut the barrels off just in front of the chambers, and square the muzzel end of each barrel butt with the face of the barrel. Bore out the old chambers to remove the steel all the way out to within the thread depth of the diameter of the "RIM" of the 410 shot shell, then thread. Then turn down the chamber end of the barrel blanks so they can be threaded into the old shot chamber area to flush with the chamber end of the old barrels. Once this is fitted to the barrel butts (which are now what is known as a mono-block)they are then removed and chambered! screwed back into the mono-block, and the regulation can be done in the normal manner.Of course the fireing pins have to be modified for the rim fire, and actually the 22 Hornet, or 218 Bee, is a far better idea, simply because it is center fire! After the regulation is complete, then cut can be milled in the chamber faces for the extractor, the ribs fitted, and final sights fitted.

There is actually a lot more to it than what I have just described, but that would take a book to explain! Once you get into it, it becomes painfully clear why "REAL" double rifles cost so much, and how much skill is really involved!