Two tiers, first must be researched before the second.
Magnum Rounds - 5 supply and 1 alien alloy. Gives primary weapon +1 damage and 1 point of armor piercing. However every shot has a cumulative 15% chance to cause the weapon to jam. Clearing the jam is a costly action (takes entire turn). Hope you're not on a timed mission (hint: you are).
Super Magnum Rounds - 5 supply, 1 alien alloy and 1 elerium dust. Gives primary weapon +2 damage and 2 points of armor piercing. However every shot has a cumulative 15 chance to break the weapon. Weapon cannot be used for the remainder of the mission. Will be repaired once you get back to the Avenger. Hope you brought a backup pistol.