the recruitable NPC characters that use ranged weapons have "magic" ammo that never gets depleted when using their own weapon.
I find the trick to equipping them with other stuff is to only let them carry ammo for weapons that you want them to use.
in my current playthrough, I have Boone equipped with the AM rifle and 500 rounds of .50 cal, a machete gladius, and for a bit of variety I gave him a .44 revolver w/ ammo, the shishkebab, and a modded grenade rifle with about 20 rounds.
depending on the enemies and circumstances (range, what he's being attacked with, etc) Boone just switches to something that will be effective for that situation and I prefer it this way.
my last run thru I'd only let him use his rifle or the AM rifle and it left something to be desired when all of the combat devolved into the same chorus of:
:EDE combat music: zap BOOOM zap BOOOM BOOOM!
Now I get cinematic looking battles with nades getting lobbed over me into the fray and boone doing his little flaming Iaido routine on any of the melee chars that step up, and all told it's just as effective on top of being more entertaining.