2mm EC ammo is a leveled ammunition, rare and requires you to be a certain level.
What level are you?
It usually shows up above level 25 and becomes an "almost rare or uncommon ammo" in stores.
There are other places you can get the ammo from.
Goodneighbor has 2 stores that sell ammo.
The junk vendors might also have ammo some times.
There is also cricket roaming around the commonwealth with ammo.
Some factions vendors also sell ammo.
You could also build a weapons vendor and place it in the settlements and get ammo there.
You can also pick up weapons from defeated enemies and get ammo from there.
Why are you sticking with Arturo anyways?
- Is it really rare for him to have it?
It will be rare when you have not found the weapon using it yet or when you are not at the designated level for it to show up in vendors or enemies.
- Or does he have it only once in stock?
It will restock. I have bought ammo from Arturo several times and I have seen it restock twice. The first time I bought from him and just a couple of hours ago.
- Can vendors get a "Full inventory" so that they are unable to restock on new types of items? Or something else?
I have not tested this yet but I don't think it is not a full inventory its mostly what the game says they can sell to you at the time they restock and what level you are at. Sometimes it may just be random
Finding the Gauss in itself without using vendors and only out in the wasteland or from enemies will take a while. So the ammo will be a lot harder to find than most other items also.
At higher levels when I was probably above 40 or something the ammo was easier to find when buying from vendors. So I think it depends upon your level to find the 2mm EC ammo from a vendor.