Why are you sticking with Arturo anyways? You can also pick up weapons from defeated enemies and get ammo from there. You could also build a weapons vendor and place it in the settlements and get ammo there.
There is also cricket roaming around the commonwealth with ammo. The junk vendors might also have ammo some times. Goodneighbor has 2 stores that sell ammo. There are other places you can get the ammo from. It usually shows up above level 25 and becomes an 'almost rare or uncommon ammo' in stores.
2mm EC ammo is a leveled ammunition, rare and requires you to be a certain level.
