Vietnam War US 25mm -
The 25 mm caliber is one of the standard sizes of cannon and autocannon ammunition for NATO forces. The round itself has a length of approximately 223 mm (8.6 inches).

The 25 mm round can be used in both an anti-materiel and anti-personnel fashion. When operating in an infantry mode, a 25 mm weapon armed with HE rounds can effectively kill large numbers of opposing troops either in the open or in light fortifications. When operating in an anti-materiel mode, a 25 mm weapon armed with AP rounds can disable many aircraft and vehicles, including some main battle tanks.