A single-shot pistol made from a short length of steel pipe, a nail for a firing pin, and a rubber-band-powered striker mechanism. The Nail Biter fires a single .22 or .25 caliber round from a barrel that may or may not be straight, through a chamber that may or may not seal properly, using a firing pin that may or may not strike the primer centered. The weapon represents the absolute minimum viable firearm — the least amount of material and engineering required to make a thing that shoots a bullet.

Nail Biters are the most common weapon in the Shelf, produced in quantities that no tracking system can quantify. The materials cost less than Φ5, the assembly requires no tools beyond a file and pliers, and the instructions have been passed through the community for generations. Every teenager in the Shelf knows how to build one. Most of them have built one. The quality varies from functional to suicidal — some Nail Biters will fire reliably for fifty rounds, while others will blow apart on the first shot.

The weapon's single-round capacity is its defining tactical limitation and its cultural significance. A Nail Biter gives you one shot. One chance to solve whatever problem requires a bullet. This makes every trigger pull a deliberate choice — you can't spray and pray with a weapon that needs to be reloaded by unscrewing the barrel and inserting another round. Nail Biter violence in the Shelf is almost always intentional, personal, and close-range.