Talk:Ammo Stock/@comment-18402226-20130930065600/@comment-92.85.41.8-20131219023817

It's because you're killing the same enemies over and over. The more common the enemy, the more common the 'common' mods they drop. Conversely, mods that drop from a rarely encountered enemy are quire rare (for example something that drops commonly from a Grineer Ballista, which only appears in a certain lvl range, would be more rare then an uncommon drop from a Charger).

Keep in mind that 'common' is only how often it drops from a particular enemy - not how often you'll see it drop in the game.