I am liking this plan!
My first preference would be to go with the Ackley Improved-type of set-up.This allows for use of existing bullets, cases, barrels and magazine dimensions, which would make the conversion nicely inexpensive, a neccessity since the market is pretty-much limited to Korny Kaliforny.
Additionally, the ability to run off-the-shelf BMG ammo is a major boost to viability of the concept. Remember, this is a PRK-only restriction, so it really doesn't concern the Federal-level BATFE at all. Cal-DOJ is who's in charge of this one, and they can only enforce the law as it's written, and when it comes time to establish what the caliber of a given gun is, it's what's stamped on the barrel that gets run through the paperwork, NOT what the chamber will actually accomodate.
There's precedent for this. There was a batch of those cheap derringers that come in a gazillion different configs but still cost uder $100. Most of 'em are .45 Colt/.410 shotshell, which can't be sold here in the PRK 'cuz they're sawed-off shotguns by our law. However, there was a batch of sxs 2-shots that were only marked (And paperworked.) .45 Colt. They were BORED with extra-long chambers, however. (Lots of free-bore is a good recoil reducer, you know.)
Nary a peep from Cal-DOJ about 'em. Never heard of any prosecuition, or even awareness of anything from them. The only contact they ever had with 'em was through the paperwork for the NICS and the PRK handgun registration provision, and all that said about these guns was that they were chambered for .45 Colt.
Evereyone knows how dangerous it is to put the wrong ammunition in a given gun stamped for something else. Nobody in their right mind would actually FIRE a .410 shotshell in one of these things just because it just happened to fit in the barrel, would they? Why, it's liable to blow up in yor hand, right? Just because it FITS doesn't mean it's SAFE...
...Even though it's safe in the same gun from the same maker when stamped with the appropriate caliber designation...
'Nuther point. The Federal-level "Over .50 caliber is classified as a Class III Destructive Device" provision
APPLIES ONLY TO HANDGUNS.
That's why it's OK to have rifled 12-guage barrels for shotguns. Long guns can be chambered for any old thing you care to ream the barrel for and have the
cajones to hold up against your shoulder and shoot. Look at the .577 Tyrannosaur, for instance. That caliber was designed to be the absolute BIGGEST case/bullet combination that could be reliably cycled through a large-ring Mauser reciever for the express purpose of being The Ultimate Dangerous Game Hunting Back-up Gun. I think the magazine of a rifle so chambered holds all of ONE additional round beyond what's in the chamber. (I might be wrong on this, and it'll hold 2. Not sure.)
Rifles in .577 Tyrannosaur are not classed as DD's. Niether are double guns chambered in .577, .600, or .700 Nitro Express. They might be expensive, but there's no additional hoops to jump through to get one.
They're LONG guns. At least until a criminal steals it and takes a hacksaw to the end of a $60,000 double gun to make it more "bad-a$$" and fit under a jacket so he can use it to hold-up the local "Stop & Rob" for $45 to get some drugs...