Yeah, this game isn't meant to be a realistic shooter in any way. Of course there are players that would love more realistic gunplay, but this game is closer to arcade than tactical shooter.
The tac rifle is meant to be a tier 2 rifle (tier 0 is pipe, tier 1 is AK) and a stepping stone to the m60. I do wish they would introduce more ammo types, but as was already stated, I think due to the more arcade-y nature of the game, TFP decided to consolidate to make managing ammo easier.
EDIT: There are a number of mods out there that add weapons and ammo (quality varies), but I mostly stick to vanilla when it comes to the weapons.