A gimmick gun is often presented as a reflection of the smarts of the character, a notion which may stupefy the modern viewer or the fan of US westerns. Whereas a gimmick gun would be really unprincipled for a hero in an american western, the "hehe, fooled you with my preparedness!" type of thinking is instead a sign of high sophistication in the Italian west.
Here are some examples. Click on the images to make them bigger!
Django's portable gatling gun (Django, 1966)
The scene where this gun is used pretty much set the standard for all subsequent "hidden gun turns tables on enemies" instances in Italian westerns.
The fact that the gun is carried in a coffin is a particularly Spaghetti touch, a cool idea less successfully varied in numerous films.
The belt appears not to need any feeding. Also, the barrels do not rotate, in fact the barrels are not laid out in a circular pattern at all and thus the front part resembles a Mitrailleuse.
Compare this to the more realistic depiction of the Hotchkiss machine gun in Bullet for the General (1966), oiling the clips and all.
Sabata's trick gun (Sabata, 1969)
Sabata films are a treasure-trove of stupid guns, acrobatics and dynamite. This is a very James Bond-inspired, comic-book style film and perhaps the best in this sub-genre.
Lee Van Cleef's Sabata is equipped to the brim with tricks. For instance, he has an extra-long barreled rifle and a bag he uses as a shield and for setting gun-traps. Nearly all characters rely on tricks here, but Sabata out-wits them all.
Most inspiring is Sabata's tiny, seemingly four-barreled gun. It is strongly implied that the small size of the gun permits Sabata to draw much faster than his enemies. But the gun holds a secret. Should you be so silly to assume that Sabata can fire only four shots... a small panel flips out from the butt of the gun, revealing additional gun barrels! Sucks to be you!
Banjo's banjo-rifle (Sabata, 1969)
Banjo (William Berger) has a banjo-gun that is semi-realistic in the "hidden gun" category. The banjo simply holds a small rifle. It is less credible that the banjo would function as a good musical instrument, though? Not too sure where the empty shells are supposed to come out from.
The rifle resembles a Mare's Leg (a largely fictitious, "sawed-off Winchester"), but clearly the barrel has not been cut.
Trivia: the tune that Banjo plays is a melody from 3:10 to Yuma (1957). Why? Nobody knows.
Indio Black's harmonica gun (Adios Sabata/Indio Black, 1970)
Yul Brynner stars in this colorful and comic-book style Western. He brandishes a manually worked multi-chambered weapon that I initially thought was a completely made up concept. However harmonica guns were once an alternative to revolvers.
In dimensions it also resembles the Mare's Leg but it's not a cut Winchester. The last chamber usually holds a cigar, which Yul then coolly lights after killing all the enemies.
It's worth mentioning the same film also sports the "Flamenco of Death", where the dancer finishes the dance by launching a deadly stone from a dedicated stone-holder on the toe of his boot.
Doc Holliday's gun (Day of Anger, 1967)
Here the gun is not that gimmicky per se, but the film purports that the legendary gunslinger's gun embodies Holliday's gun skills, inherited through past use.
There's also a sort of tournament duel on horseback with muskets, loaded on the go...
...one might say the film is about guns somehow...
Yeah, that's pretty safe to say.
7-barreled rifle (Hanging for Django, 1969)
Again we have William Berger, whose character this time holds something that might be a kind of a nock gun, although it doesn't make much sense to carry one around.
Machine gun (Return of Ringo, 1965)
What is that they are wheeling in... Another Maxim-wannabe or a Gatling? It's hard to see from this blurry image.
No, it's another composite of different ideas. At least here they acknowledge that the machine gun is a heavy thing and needs two people to operate it. But the barrel arrangement is more reminiscent of "organ guns", volley guns and other proto-machinegun evolutionary dead end designs.
Well, that's it for now. I'll update the list when I get more screenshots.