Should be fine federally.

At least one - the Connecticut Valley Arms Electra - was commercially sold and is a non-firearm.

Sounds like a volley gun black powder cannon, so I dont see any reason why that would be considered a DD or firearms.

Somewhat related to the form factor you might be after is this 40mm volley gun. The barrel assemblies were preloaded, then they slide onto the firing mechanism base for fast reloading.

https://www.reddit.com/r/MilitaryGfys/comments/sgxan2/20barrel_40mm_grenade_volley_gun_tested_by_the/

I like it.
Here's a random thought on a muzzle-loader of this type:
I'm not sure what sort of pressures this would see, or what muzzle velocity you'd get, but maybe the end goal could be accomplished with something closer to a large, very short barreled cannon vs a volley gun.

Would a thick walled section of tube (say, 6" or 8" tubing, 1/2" wall), 1" or 2" or 3" long function as a barrel for this?  
I'm not aware of any length to diameter requirements to be considered a barrel.
I'm envisioning a very large wad or shot cup (full diameter, with an overshot card glued or crimped in place) sitting on an evenly distributed layer of black powder in the bottom of the barrel.

Breech end would need to be welded to the back end of the barrel.  (Unless you wanted to get crazy and machine this from a solid)

If this "cannon" arrangement functioned, it might simplify the design and construction as well as allow for more traditional ignition methods.
eta:
weight to powder charge might be out of proportion with this vs the volley gun, I imagine you'd need to anchor it well so you don't lose it (or catch it) when you fire it.

I recall your breach loading cannon project.
I'm envisioning this thing more like a claymore in size and use.

Why weld the breach?  That comes with the design I was envisioning.  Cheap and easy to make with the tools I'm familiar with.

I'm thinking you start with 6" or 8" diameter, 1/2" wall tubing (or 3/8",  whatever it needs to be to survive the number of shots we want).  That's an off the shelf piece of steel tube.  Your local steel yard or online metals.  Cut to length on the bandsaw (or your supplier will for you).  

There, your barrel is done (unless you want to dress the edges)
At this point, you need to close the breach end of the barrel somehow.
Easiest way is to weld a piece of flat plate to the back end.  

Sure, you could machine something, but I can't think of a mechanical interface that's easier to make than just welding a flat plate to the breech end.

The thing is 6" or 8" diameter. I dont think improvised solutions or leather plugs will work.


Your firing mechanism can then attach to the back side of the plate with a few screws and fire through an ignition hole in the center.  (Electric, percussion, blank cartridge, or even flintlock)

That strikes me as far simpler to manufacture, and lighter, than a multibarrel volley gun arrangement, even without looking at ignition systems.

How are you planning to construct all the volley gun barrels?  You can machine from solid, but that's a lot of time and machining.  Start with a tube?  but then you need to close the breaches somehow.  Maybe smaller diameter barrels can get away with something improvised, but i don't think the 6" or 8" barrel could.  How are you joining the barrels?  Pallet banding might work, but the barrels will walk and slip relative to each other.

Or, here's another idea.  What's the legal status of a recoilless black powder firearm?
Do a 6" or 8" diameter tube barrel, firing mechanism on the side, load your shot  in the front and a mass-matched salt water counterweight into the back.  Then you dont need to seal the breach.  Your whole firearm gun would be a short length of tube with a firing mechanism grafted on the side.  (I'm thinking a big ole sewer pipe that's 4-6" long)

Eta:  all these ideas are a for a single barrel muzzle loader to approximate the claymore.  I know the volley gun can work, I just see that as way more complicated to manufacture than a single larger barreled gun.  (Again, assuming you can hit the required muzzle velocity and chamber pressures in a simple tube like this)

I know you want to replicate the fragmentation characteristics of the claymore, but I wouldn't write off a single tube design so quickly.
Frankly, without constructing and testing it (legit tests, witness plates/targets and whatnot), we're both speculating on the effectiveness (or lack thereof) for such a design.
It may be an 80% solution (or a 20% solution).

Quantity has a quality of it's own, and I believe the simplicity, size, and weight may go a long way, depending on your use case.  (This'll still be a hefty thing)
You could, for instance, place two of these together, 15-30° apart, and coverage might not be so bad.  (Or maybe a two-barrel version with a frame that holds them at whatever angle makes sense)
As you mentioned, the volley gun will probably have the size of a rolling suitcase.  At that size, you might legitimately want to add bicycle wheels and a handlebar and roll it around like a miniature field piece.

Also, the shot pattern of a large single barrel design can probably be tuned quite a bit with loading techniques and wad design.
A wad with overshot card will do one thing, an epoxied wafer of shot will do something else.
A giant flitecontrol wad would do yet another thing.
I'm not very familiar with black powder, but I imagine the different grades will burn differently and affect performance too.

You could also look at other techniques to shape a single barrel's shot output.  Perhaps a square barrel would work?  I'm not sure what sort of pressures black powder would generate in this configuration, but it might work.
Alternatively, giving the shot charge and powder charge a curve or bended shape may work, given the extreme diameter to length ratio of the barrel.  Such a charge might shape the shot column into something along the lines of the claymore, wider than it is tall.  
Maybe cut the barrel back on the left and right sides and could squeeze the top and bottom "flaps" in to have it act along the lines of a duckbill shotgun choke.  
Lots of ways to approach it.

All that said, we know you can perfectly replicate the claymore with a volley gun, it's just going to be a tradeoff of size, weight, and complexity vs performance.  
(At the extreme end, a volley gun with 700 barrels could replicate it perfectly, at a weight and complexity penalty)

I'm not sure if flechettes would work in this application.
I don't have any good cites, but my understanding has always been that flechette performance is typically underwhelming and extremely inconsistent when driven by a relatively low velocity, low pressure gun, like a shotgun.
I think the volley gun barrels would perform closer to shotgun barrels than anything else.

Something to consider, I don't know how consistent black powder ignites and burns compared with smokeless, but if the timing is not perfect or close to it, the barrels might fire slightly out of time and turn or move the gun, and throwing off the nice claymore pattern.
Ignition may need to be very controlled, or the device may need to be heavy enough that recoil doesn't move it much before all barrels fire.

All that said, I do want video of you taking a sounder of hogs with whatever you make.

What you want is to do is duplicate a canister load coming out of a m1841 mountain howitzer.  About 4.62 id tube, 32 inches long at least 1/2 thick walls and do a chamber
plug to fit an 8oz bp load. Mount it anyway you want but the recoil will be massive.