Thought I'd pass this along for those who may not know it (I didn't): I was reading around someplace the other day, and found out that companions and other NPCs can in fact use ammunition types other than normal in FNV.
I'd tried this in the past, and never had it work. The trick, it turns out, is to not put any normal ammunition in their inventory. Even if the handloaded/specialty ammo does more damage, they'll always use the normal ammunition in preference until it's gone.
This is good news for low level characters, since it means you can outfit companions with "specials" for the 357 revolver. Also good for higher levels, too -- I outfitted Maeva today with Magnum buckshot for her riot gun, Natasha got Match ammo for her carbine (CaliberX added ammunition type), and the gunslinger got a mess of SWCs for her .44.
Been through some combat, and they all fired without issue and the ammunition counts decreased, so I can only assume it worked. This should prove excellent for helping companions bust DT; since now I can outfit them with AP ammo if needed. I'm oh so tempted to load the big demoness up with slugs exclusively and see how that goes in her riot gun...