The cool thing about this DIY submachine gun is that while there is a 98% chance it will paper cut up your hands during assembly, there is a 0% chance it will blow your head off or inadvertently light up your mama's brand new media center upon completion. Well I mean, when that happened to me it was just a BB gun, but she still made me sleep in the backyard for a week. I can hardly imagine the consequences of butter fingering a Heckler & Koch MP7A1 in the living room.
The flat-packed paper submachine gun is not a project for the weekend crafter, but one for the seasoned model builder or black belt origami-ist. Made from thick white die-cut card, the finished MP7A1 sports all of its lethal counterpart's aesthetics, plus an extendible stock, a removable clip, and a folding front grip for bonus fun at cosplay nights and conventions.
Paper submachine gun construction time depends on skill level. I'd allow myself 3 days of round-the-clock work with a steady supply of iced coffee, Red Bull, and Little Debbie Swiss Cake Rolls, so maybe take that as a starting point for your personal estimation.