As an early employee of one of the fastest-growing high-end AR-15 makers, Duncan has the perfect perch from which to observe the black rifle’s transition in shooting circles from a scary military oddity to the hottest item in the gun store. He — and everyone else I talked to — credit the gun’s flexibility for the surge in interest.

Users can change calibers by swapping out barrels, bolts, and magazines; they can add and remove accessories like Trijicon optics, Surefire flashlights, or Crimson Trace laser sights; they can swap out the rail system on the gun’s fore-end to accommodate more or fewer accessories; they can change grip styles and stock sizes to tailor the gun to fit their own body; they can even theme the gun with special paints and decals (zombie apocalypse themes are popular, but I’ve also seen Hello Kitty). And they can do all of this by either ordering new parts and accessories from online or local shops, or by taking parts from different guns in their collection and mixing and matching them to produce something completely new.

“I always tease that it’s like Legos for grown men,” Duncan elaborates, “because there’s plenty of guys that get one, two, six ARs. And they’re constantly tinkering with them — changing barrel lengths, changing optics, putting different sights on them. It’s the same reason that a guy gets into remote-controlled cars or fly tying. Because it’s a fun hobby, and it’s a distraction from reality sometimes.”