Ammo is really cheap at some vendors. You can buy 12 gauge, 10mm, .44 and 5mm for just 1 cap.
So i noticed that apparently some players buy off all the ammo they can find and turn it in for the bulk order jobs.
If you buy 500 10mm bullets for 500 caps at the gun shop in the Hub and walk just across the street and turn it in, you get 500 caps rewards + Exp + random items, including a gun and ammo (i did it once and got a 10mm SMG + 120 10mm ammo)
That´s the reason why this kind of ammo is almost always out in the shops.
I can´t imagine that this is working as intended.