Offhand, I'm thinking 13/3/2 for stats.

And just to throw in my own two cents, it's not only a Sawed Off Shotgun, it would also be a Singnature Weapon.

The twin-barreled, breech-loading hammer pistol that El Mariachi uses in (the best parts of) Desperado and Once Upon a Time in Mexico is, loosely, a shotgun, but it's not sawed-off at all. I'll have to go and check my copy of Flayderman's for specific numbers, but that style of gun was popular for a while around the turn of the century and was specifically manufactured to be that size and to be fired one-handed. Sawed-off shotguns generally still need both hands to use effectively and experience problems of balance. El Mariachi's gun, you'll notice, has sights at the end of the barrel and a complete, finished stock, so it wasn't cut off on either end. It's a smaller-gauge weapon than your average shotgun but could still take scattergun shells or good, old-fashioned solid shot.
That thing actually is a hand cannon, regardless of what Quentin Tarantino's characters say about piddly magazine-fed firearms or modern revolvers.

word,
Will Hindmarch
Feng Shui
Atlas Games

Ah, yes. Statting it. I haven't been able to check the manufacturer and such for the gun yet, but I'm willing to give it a go on the numbers.

Double-barrel, hammer-action hand cannon: 10/3/2

Strictly speaking, this particular gun just isn't going to cut the mustard against modern foes unless it's your Signature Weapon. It's such a pretty gun, though, that it'd damn well better be, right? Of course, in the sublime opening sequence to "Desperado" with its ridiculous flying bad guys and floating gun, we see El Mariachi pull off a terrific reveal of his concealed hand cannon.
I'll resist going into a critical tangent wherein I compare the folkloric (?!) sequence at the beginning of "Desperado" with, pretty much, all the gunplay in "Once Upon a Time in Mexico."

word,
Will
Feng Shui
Atlas

Oh.

Now I get it.