If you could not guess, it affects your ammunition. More to the point, it grants you a 70% increase in your ammunition for all ballistic weapons. That is over half. Two red runes will push this figure to 120%. That is more than double the ammunition! This is especially helpful for Commando's, who will spend the majority of their time engaged in ranged combat, or for anyone using the deadly but extremely low ammo capacity pistols. If you plan on doing a lot of ballistic combat you should drop points into here ASAP.