It is perfectly legal. It simply has to fire shotgun shells. So, you put a 2-3/4" shell in there and fire it, and it is legal.

Matt is right- Shotguns are exempt whether smooth or rifled.The rifled
concept is 120 years old. Nothing new.However long you make chamber it just
has to fire 12ga shells of any shorter length. You can't neck a 30mm
down to .729" 12ga caliber and call it a 12 ga. Cases can be paper, brass,
steel, plastic, aluminum. Barrels can be any length, If under 18" taxed.
AS for NEF I use a little muscle to latch it tight. The Ultra action is
an alloy with heat treatment for good strength. Firing our long
brass case with much heavier loads than we do in plastic,
actually gives less thrust against the breach that heavy loaded shorter
plastic does. Reason; upon firing the brass grips the chamber, and holds part
of the thrust, up to the yield strength of the brass case, which means it
takes up to about 7000 lbs thrust. Plastic only takes about a third of that.
I never had any locking or thrust problems with NEF. Even the heavy loaded
600gr slugs in 3.5" plastic cases mentioned above. Ed

Here is the Enfield I reamed out to feed 12 gauge and BMG
size cases. I added two rear bolt locking lugs, one the huge bolt
handle base, another added that locks in rear bridge. The front lugs
still contact on their top 40 %. This is concept from earlier shown
Mauser shotgun conversions, where they were made from 98
Mausers, giving a bolt action 12ga.This one is chambered for
my belted 700H 3.25" case, as a load developement test gun.
Loads up to 825 gr at 2500. Ed

The three From Hell cartridges----
My 28GA FH, 2nd Our 12GA FH, can
you guess the 3rd?. What do readers think. Ed

Thirdthe 8GA From Hell, a real cartridge. 4.3 inch long
brass case. 8 ga shorter plastic case, plan on in same chamber.
Looking now for guns or actions and barrels. I make it
from 20mm Suzy brass(20mm Hispano Suiza), by putting
on a 8ga size rim and reshaping like we did 12GA FH....Ed

Here is pic of a bolt action bmg upper that will
do our 700HE and 12GA FH. Looks strong and
priced a $1000 now, Just change barrel, decent price
compared to others.Ed

8ga, we gonna do a couple, first one a test
gun in an Enfield with rear locking lugs.
All locking is in the rear, it is not exactly like
my 700HE one, all lugs seats gone in the
front like a Mauser shotgun.
May have a way to get barrels 'fast'---
fast being like 2 weeks, from the time I
have money to do it. And only about
150 bucks each.......Am working on it.
In pic you see long case sticking out of
the action, a couple 8ga plastic cases, a
couple of the 8ga kiln slugs, 3 oz and
hardened. Am looking for a slug built like the
Federal 10 ga one that has rounded nose and
hollow base, that will fit bores tight.
The barrels I am working deal on will be smoothbore.Ed

Finally my Long Tom slug gun.....Ed

We're going to test smoothbore slugs in 87
like these Brenneke KOs with our special
powder loads to get velocity up. Many guys are
getting accuracy with these in smooth and
rifled barrels, and I going to see if they
will stay together and be stable with speed added.Ed

I just dont get it. WHAT THE HELL ARE YOU KILLING WITH THAT!!!!! Sure aint deer.....