He keeps a loaded three-fifty in his drawer for protection.

three-fifty 是口语中对 .357 Magnum（点三五七马格南）手枪的简称，three指"3"，fifty指"50"，合起来表示0.357英寸口径的手枪子弹规格。这种命名方式来源于子弹直径的英寸计量单位（0.357英寸）。