5mm round

 is a type of ammunition found in the Fallout Series.

Background
A high velocity round designed for assault rifles. Ammunition boxes in Fallout 3 and Fallout: New Vegas read "5mm LINKED M134, 1 TRACER M17 4 API M8, FUNC LOT POD80h003L041." In Fallout 4 and Fallout 76, the ammunition boxes read "5"MM, Mil, 7.62 x 51 MM ACT, (A165), FUNC, (ST. A.CL.) 13S455." 