So, the joke from Fallout 3, and early on with my friend in New Vegas is that your companions must have magic companion ammo. That's why they never run out.
While watching my friend play, a companion of his is killed. After combat he loots his corpse, and on his body is labeled "Magic Companion Ammo." Seriously. Thank you Obsidian. We were killing over laughing. Well played.
I don't wanna sound like an ass but I can't say I believe this unless it only works on a certain companion...
Yeah, yeah "Pics or it didn't happen." It was boone who died. If you have boone, make a separate save file, get him killed and loot his body to see for yourself. We play on the 360. It's what ever their base ammo is. If you give them different guns and ammo, they use that up like normal. It's their base gun and ammo. We only saw it after they died, not in their normal inventory.