Frizzle, you got me interested, please elaborate more.
So if you're looking for a pocket cannon with no stock, that is, a pistol, you're stuck buying off the MA roster from a dealer or using one of the other methods.
There aren't any pocket cannons on the MA roster really other than the S&W 500 which is a pistol caliber.
If you get a virgin reciever (never built as a rifle, otherwise you're not making a pistol but an unregistered SBR) designed
like a rifle you can build a pistol from it perfectly legally; the problem is that if it's "full-semi-auto" and has a detachable magazine, you have to worry about the Assault Weapon Ban and its rules on pistols. If you build bolt, lever, or slide action, there's no restriction on weight, having a magazine outside the grip, or having a shroud on the barrel because it's exempt from the AWB.
Remington used to make a single shot bolt action pistol in rifle calibers called the XP100:
Some people have modernized the idea somewhat:
I'm thinking you build a modern XP with a magazine and some fancy new tricks.
Spend $200-$400 on a virgin Remington 700 reciever from Brownells or similar, there's aftermarket options too, and build a pistol with an internal magazine or a detachable box magazine (using the Magpul mag well and trigger guard maybe?). I say 700 because there are a lot of great actions, but the 700 is easy to work with and has a lot of neat aftermarket parts available cheap. Get a Choate stock, or a thumbhole, anything with a pistol grip and just cut off the 'stock' part and shorten the forend a bit. Or use a factory stock and bolt a pistol grip on there. Grab a cheap used barrel, cut it down, crown it, and you're off to the races.
The other option is to scratch-build an AR10 pistol with a fixed mag, or buy a fixed mag lower from one of the MA dealers who makes them and build it up. If it comes from the factory (or your workshop) with a fixed mag it is not an "assault weapon" by law or Mad Maura's definition... and Mean Arms makes a speedloader/clip system that works with AR10s in .308 nicely. Keep in mind that AR10 builds are a little more involved than AR15 builds; there are 2 totally different sets of specs, and parts from different MFGs are far less likely to line up even if built to the same spec.