In Short it by default increases clip size by and recipe cost by five, this is adjustable if you extract it and it contains a variable for boosting dmg this boost the cost by the increase. so if ya chage the boost to ten times and the damage to two times ya got a cost of fourty iron for base magazine and a cost of twenty steel and one hundred copper for piercing. Crafting time has not been fiddled with, may change later.
Also lowers the amount of magazines a inserter will put in a turret to 1.
Probably anything that also modifies the ammo cost or damage