Nails are generally categorized by their gauge, which is the size of their thickness. This thickness is defined by the diameter of the nail's shaft, rather than the typically wider head.

Question: Concerning a .410-gauge shotgun. If the barrel is stamped 3" Can you shoot 2 1/2-inch ammo, or can you only use 3 inch ? - Roger Harmon The Gun Guy: Yes. You can shoot 2 1/2 inch or 3 ...