Underbarrel Pistol Mount (5e Variant Rule)
While notoriously crappy Zip .22 could be mounted on other weapons, it made little use. Naturally, many shooters decided to mount something stronger instead.
Under these rules, any one-handed firearm can be mounted to any firearm of same or bigger weight, or on any two-handed firearm, and used to make an attack as a Bonus action, with disadvantage.
With GM's permission, it's possible to mount two-handed firearm onto another two-handed firearm, though the main firearm must be same or higher weight than secondary firearm (e.g. underslung/underbarrel shotguns and grenade launchers, twin machine guns, double versions of certain guns, etc). In the Futuristic epoch, "cramming more firepower" went on to new heights with invention of commonplace Twin-Linked and Combi-Weapons; as such, it becomes possible to mount two-handed weapons onto other two-handed weapons, no matter how much sense it makes (no GM approval required, as such practice is standart in Futuristic epoch); "main firearm must be same or higher weight than secondary firearm" restriction still applies.
It is possible to link both guns in such a way, that they both fire from same trigger. In such case, guns fire simultaneously onto same target as single attack, without disadvantage. But, both guns must fire simultaneously onto same target; if main gun shoots in short burst, then secondary gun must shoot either same amount of shots or it's maximum burst size, whichever is greater (e.g. if main gun shot 3 times, and secondary gun has short burst 2 - secondary gun shoots 2 times). Switching between simultaneous fire and non-simultaneous fire is Bonus Action.
Attaching more than 1 secondary guns to a gun usually requires GM's permission. In Futuristic epoch, main gun with 1 secondary two-handed gun and about 1-2 secondary one-handed guns is within boundaries of "standard", but anything more than that requires GM's permission (even future tech has it's limits).
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