Enebias: It depends. I never bought a 5.56 round, but when later I had access to powerful gear like the AK-97 and enemies started to get tough I needed to buy a few 7,62 ammo, definitley less common than the former. Anyway, the loot is also more "juicy", so I don't have to worry much about that kind of expense.
cw8: I always had to buy crapload of ammo. Think that's because I have 3 riflemen and I have to use burst fire sometimes, actually quite often at critical points. Went to LA with only 4900 scrap.
Hmm... I have five rifleman, so I suppose it makes sense that ammo has seemed scarce to me. The AR description during character creation made them seem like a solid default option. Everyone (except Vulture's Cry) has at least 3 points in AR. It's made a good team so far: We form a crescent in a strategic position, begin combat via sniper, and let them come to us. Rifles first, then pistols at close range. (Ace's Wrench for Angela at close range of course.) A bit tedious, but I haven't had to use many healing items (3 iirc).
Btw... the M16... Holy Firepower, Batman! Explored the Citadel, bought the M16, stocked up on some much-needed bullets.