PLANO RUSTRICTOR FIELD BOX GRAY MEDIUM Get the best protection for .50 caliber ammo with a manageable footprint that's easy to store. The Rustrictor Field/Ammo Box Medium is sized after Plano's original .50 cal ammo box with a slightly slimmer profile. Rustrictor technology surrounds items inside to block corrosion or rust from developing. The elements are no match for the exterior, sealed shut with a brass-bailed latch and water-resistant O-ring seal. Also doubles as a great storage box for tools, electronics and more.