Question: what is the 25 mm?
Answer: The 25 mm caliber is a specific size of cannon or autocannon ammunition. It has also been recently used for the Barrett XM109 anti-material rifle. Such ammunition includes the NATO-standard 25×137mm and 25×184mm rounds, Soviet 25x218mmSR, as well as the World War II-era French-designed 25×163mm and 25×193.5mmR rounds.