I seen this in FO3 but have yet to see it in NV. I tried to figure it out in the GECK, but am unable to do so.

Unless there's another way to do it that I've not heard of, you go to each weapon, bring up it's form and check the box that says "NPCs use Ammo"

It might seem unfair but remember you're running around with hundreds of rounds for every gun you use, while NPC's have maybe 10-15.

You'd have to give all NPC's more ammo if they are going to fight you. And if you do give NPC's more ammo, it will serve to make the game easier.

If you are talking about custom Ammo then as mentioned you need to go to the data for that custom ammo and select the NPC use. What maybe better is changing the form ID of an existing ammo then duplicating/create a new form from that ammo. That way it will definitely force the NPC to use that ammo.

You can try this out http://www.newvegasnexus.com/downloads/file.php?id=35462
There's normal ammo and more ammo versions so you should be covered for MadMax rare guns/Normal amount of guns type of plays.

Running out of ammo in vanilla F3/NV?
Ammo in these games is like everywhere.