So Standard Mfg. is making a 6 barreled pepperbox-ish revolver-ish thing that fires two barrels simultaneously.
https://www.gunsamerica.com/blog/six...hot-show-2017/

Apparently it is NOT and NFA item because ATF allows for more than one projectile to be fired with one pull of the trigger if they are from separate barrels. Apparently it is referred to as volley fire.

Well, back around the turn of the 20th century someone built a revolver that held 18 rounds and IIRC could fire 3 rounds form 3 barrels simultaneously. It even had moon clips!
http://newatlas.com/triple-barrel-re...urfaces/22523/

Hey, it could happen!! Maybe-ish.

You know, come to think of it, where ATF's volley fire exemption would really come into its own and possibly be practical would be if something like the metal storm became available.
http://www.thefirearmblog.com/blog/2...storm-may-may/