When you first get your companion, the guns they have usually have unlimited ammo.

But for some reason, a shot gun and demolition expert companion of my now would not use any shot guns unless I gave her the ammo for it. This is even the case when I given back the one she uses with unlimited from the start.

Bug or are there a way around it?

Only their default weapon gets unlimited ammunition. This is not a bug. If you want them to use weapons other than their default, the weapon you want them to use needs to have a higher DPS than their default, and, you need to give them ammunition.

Companions have "http://fallout.wikia.com/wiki/Magical_Companion_Ammo", that gives unlimited ammunition for their default weapons. Other guns they use require ammo normally.

Thanks guns for the replies.

Would I need to give back the default weapon by type? Or if I had sold the one they were carrying, that's it? If this is the case, would I not be able to upgrade their default weapon then?

As a side note to companion weapons... I was happy to see my companions use the weapons I gave them... I've been using Veronica and was excited to watch her switch out her power fist for the Super Sledge I handed her... UNTIL... I turn around after a battle and she's gone back to her fist... I check her inventory- NO SUPER SLEDGE.... WTH? So I equip her with Nephi's Driver, no problem, till we are battling Cazadores and I watch her take a blow that knocks the weapon out of her hand... Instead of retrieving it, she just re-equips her fist... So that's how I lost the Sledge... Now I have another thing to watch with my companions: Are they still following me? If yes, are they still carrying the weapon I gave them?

You'd need to give back the specific weapon you took from them, and even then, removing it from their inventory tends to break it so it won't shoot at all. It's not a bug, as the companion weapons don't even show up in their inventory without modding them.

I made that mistake, too. To fix it, first you need to remove the changes you made to the weapons, or the mod that did it for you. Then install this mod- http://www.newvegasnexus.com/downloads/file.php?id=37109. Give each of them a couple rounds of upgraded ammo corresponding to their weapon types, and after they burn through it, they should go back to being unlimited. I don't know how it worked, but it did.

If you aren't on a PC, it shouldn't have been possible to take the weapons in the first place. No clue how to fix it either.

Damn that's bs. I hope companions can't lose guns that way either :/.