How far do you want to diverge from the norm, ie sidelock and boxlock?
If you can live with one of those and simply want to finish the gun to your desired degree of finish and stock dimensions then a donor gun is probably the easiest way. I tried this with several medium grade shotguns and it works out fine, you end up with a fitted hand finished shotgun in a reasonable time but are stuck with the engraving and aesthetics of the donor.
Italian rules, and the same probably applies to other countries members of the CIP, mandate that a gun must be serialised as soon as barrels and action are fitted together. And because from that point on it is legally a gun, it must be submitted for proof before sale. Proving an unfinished gun is a hassle at the Proof House in Gardone and this is the reason most makers do not want to get involved with barreled actions. If you feel able to fit barrels to an action then you might be able to find the individual bits in machined form.
If something totally different is the goal then you are on you own. My experience is that no maker wants to get involved with one offs or new designs. Not to mention that sooner or later you will have to get into CAD and plastic prototyping to test function etc and these add to the cost of the project. A current project cost seveal thousand dollars to bring to the plastic prototype phase and will cost lots more to become steel and walnut.
A good place to see how others build doubles is nitroexpress.com. It is mostly rifles but you get an idea of the process.
