At the 2014 SHOT show, the helpful folks at Sage Ordnance had a lot of interesting large-bore firearms. Although launchers aren't necessarily firearms in all cases, Sage doesn't do smoothbore launchers and IIRC the rifled barrels require registration as firearms (destructive device for the 40mm version as well). The even more interesting weapon was Sage's BML-37 box mag launcher, but the Deuce is a far less "over the top" launcher for standard law enforcement applications. The Deuce is an over-under chambered in 37mm or 40mm, and the first trigger always fires the top barrel, and the second pull fires the lower. Why an over-under? Well, Sage had pulled statistics on law enforcement use of 37mm devices, and determined that the average use of these devices required 1.4 rounds. So if you have a two-shot weapon, there are very few occasions where you'll actually require a third. Also worth noticing is that the Deuce was in the 2014 Robocop remake. It was the "really big gun" which the crooks thought they'd be able to take out Robocop with. The "Deuce" logo is clearly visible on the side of the weapon in a few shots during that gun fight.
And if you really want to fire four shots without reloading, just carry two of them.
As always, if you click on an image which interests you, it'll load a larger version.
Left side view. Notice how it's using an AR-15 buttstock? IIRC, the rear of the weapon is threaded for a standard buffer tube. Unfortunately it lacks the recess underneath, so you couldn't attach a Magpul PRS to it and expect it to stay locked into position. But most folks will want a collapsible buttstock anyway. Depending on recoil, a Mako GL-Shock stock might be a good fit.
Right side view. Notice that the break-open release is ambidextrous? We thought it was a nice touch.
I chose to operate the release from this side. It breaks open readily.
Part of me looks at this and thinks, "Ya know, those Double Tap pistols are looking kinda small now..." But seriously, notice the cutouts on the left and right side of the barrels for quick extraction of spent cartridges. Also, the hook directly under the picatinny rail is spring loaded - it's what latches the action shut when you snap it back into place.
This is how far apart it'll hinge for loading. With the action angled this far apart, the breech face doesn't get in the way of the cartridges you're inserting.