Extra ammo will double the number of magazines you start the round with. So, normally the Beretta gives you 3 magazines (one loaded plus two extra). With the extra ammo item you would get five magazines (one loaded plus four extras). Extra ammo does not affect the number of knives you start with, nor the number of hand grenades (it does effect the number of H&K 69 grenade launcher grenades). This piece of equipment is useful if you use a weapon not used a lot by other players (meaning you won't find a lot of extra ammo to pick up from dead players). It is also useful for snipers, who may remain hidden for a long time and need extra ammo to keep firing.