User blog comment:Gamer.Matt/Military Terms and Definitions/@comment-3479148-20120407203054/@comment-770363-20120409163845

Mike is just the NATO phoenetic alphabet word for the letter "M". The reason they do this is because "M" can sound like "N" sometimes, especially when you're being shot at/panicked/have a bad signal. So forty mike-mike means 40mm, as in a 40mm grenade.