Mutation Magazine is a special item for Mutation Mode, adding 50% more ammo for all weapons the players carry along. It works together with normal ammo mag, so players can buy and equip both to maximize their gun's ammo capacity in Mutation mode.
Because of the difference between Mutation Magazine and Ammo mag, you should consider carefully when picking either of them (In case you can't buy both). Weapons with 3 magazines (ex: MP5 with 30/90) can have more ammo with Mutation Mag, but weapons with only 1 magazines will benefit more with Ammo mag.
In CF Vietnam, it's possible to have 2 Mutation Magazine, therefore doubling players' ammo capability for all weapons in Mutation Mode. Players simply have to buy the Mutation Magazine in the Item Shop, then purchase the Special Mutation Package (Containing: Mutation Namecard, Mutation Armor, Mutation Magazine, Mutation Grenade, Assassin and Maiden). Afterward, a second Mutation Magazine will be added to their inventory (This one will not have Renew option).