In my curiosity as to the legality of such a build, I wanted some questions answered yesterday afternoon... Sooooo...
Yesterday I called the ATF, asked to be transferred to their Firearms Technology Branch, and after MUCH time on hold, I got to ask them what would be the LEGAL parameters for a homebuilder to make their own Semi-Automatic 12 Guage Shotgun, and I was quite disappointed with their response.
I told them I would like to design & build my own semi-automatic shotgun, but I wanted them to list the Maximum & Minimum criteria my firearm would have to meet in order to be a LEGAL firearm...
I asked them about 'ammunition capacity' in general, I didn't want to REFER to belt-feed devices as I was hoping they wouldn't elaborate on that... But they did.
Basically, IF it's a firearm, with a caliber over 0.500" Caliber, such as a 12 Guage Shotgun, it would need to meet the ATF's classification of "SPORTING PURPOSES"... Which they define as an INCREDIBLY NARROW criteria for the firearm's design!
More on 'Sporting Purposes' criteria can be read here:
Even though this describes the 'Sporting Purposes' evaluation with regard to IMPORTATION of FOREIGN FIREARMS such as 'Assault Rifles' and their ability to accept LARGE CAPACITY AMMUNITION FEEDING DEVICES (LCAFD) (Mags, Belts, Drums, Strips, etc.), I was informed that this SAME evaluation criteria is applied to the 'Sporting Purposes' review of 'suitability and adaptability' with regards to ALL SHOTGUNS with bore over .500" - as this is the criteria they use to determine whether or not a Shotgun is to be classified as a SHOTGUN, or as a DESTRUCTIVE DEVICE which is an NFA Controlled Weapon.
Basically, this is what I was told (Paraphrasing):
'If you're going to manufacture a Semi-Automatic Shotgun, it must feed from either a Tube or Box magazine, have a Minimum Barrel Length of 18" and a Minimum overall length of 26" (IIRC, it was 26" or 30"), and it must (in order to meet 'Sporting Purposes') be PRACTICAL TO CARRY and USE in a 'Traditional' Sporting event such as Hunting or Trap/Clay shooting, must have a maximum weight between 7 to 10 pounds, maximum of 3" width, and a 4" Depth (height of receiver & grip? I think?).'
A Belt, or Drum feed would be considered a LCAFD - therefore a 'Military/Police' purpose and thus (again, if over .500 bore) it's a Destructive Device. An NFA Regulated weapon, subject to $200 tax, etc...
Even if it's within that weight margin, and on the high side - closer to 10 lbs - they would likely consider it 'Unsuitable' for sporting purposes as its weight is 'Unwieldy' for clay or other shooting... This alone pretty much Nixes the ability to make a belt-fed 12 Guage or even a 20 Guage shotgun. The weight will be tough to meet in a belt-fed gun.
Then there's the ban on LCAFD's on firearms with a bore diameter over .50 caliber... SO, it's illegal to make any 12 guage or 20 guage with a belt feed device, or a mag capacity over 8 rounds. A Belt, or Drum feed would be considered a LCAFD - therefore a 'Military/Police' purpose and thus a Destructive Device).
So, in THEORY, if you want to make a BELT FED gun, in order for it to be legal, it must be either a Handgun, Rifle, or Shotgun Cartridge/Shell with a bore diameter of .500" or Less! Or else it would be considered a 'Destructive Device' and thus a regulated NFA weapon subject to $200 tax!
THUS IN THEORY, a .410 Bore Belt-Fed would be perfectly legal, but a 12 or 20 guage would be BAD JuJu!!!
Either way, the advice over the phone was given as general guidelines for the design of a 12 guage shotgun, and I was informed that even meeting those criteria, it might NOT be considered 'Suitable or Adaptable' to 'Sporting Purposes' if they think it appears to be made more for 'Police/Military' purposes such as 3-gun competitions, or other 'practical' shooting competitions, etc... And once completed it would have to be submitted for testing & evaluation by the BATFE Firearms Technology Branch technicians before it would be an 'approved' design.
If not submitted for testing & approval, if caught with a firearm, even if it meets the criteria, it might still be considered 'non sporting' due to an 'evil' appearance of such features as a pistol grip, folding buttstock, etc., and since it's not an 'approved' firearm they may still try to get you for an NFA violation.
Tread lightly if you think such a build would be a 'good idea'...
IMHO, this pretty much cuts the whole 'shotgun' thing out as I'm not convinced the .410 is even work making into a belt-feed gun, and I'd suggest re-focusing your energy on something a little more worthwhile like perhaps a .45-70 Belt-Fed gun or something...
Edited to add:
As I quickly chatted with the guy there, it sounds like the ONLY REASON the Saiga-12 was and is still imported is because it was designed from the start with a lower capacity (5rd?) box magazine, the neutered 'traditional' buttstock & grip, and it's box-stock form DOES meet the 'Sporting Purposes' tests - but they STILL HATE IT, but couldn't find justification for the banning of its importation. Once owners/companies here in the US came out with shotgun mags with higher 8rd capacities (the maximum allowed according to this guy).
Apparently the guy KNEW that people are converting them in the US to a form that definitely does NOT meet 'sporting purposes' with the pistol grips, folding stocks, etc., but there's not much they can do about that... HOWEVER, it DOES seem the guy is unaware that there are 10-round magazines now available for the Saiga-12 - which also don't exactly meet the 'Sporting Purposes' test - and thus might be made illegal / banned as soon as they get enough political backing to do more about these converted Saiga-12's!!!
Basically what it sounds like is this: If you want a Saiga-12, and 10 Round Magazines in the 'Not-So-Sporterized' Format, you'd best get off your ARSE and BUY ONE BEFORE THE ELECTION!!!
The Saiga-12 and their 10rd mags may soon go the way of the DoDo Bird, the Sriker/Street Sweeper, and the SPAS-12 Shotguns!