[quote]Will they provide just a damage boost[/quote]Yes, in that they allow your gun to fire, without which your damage output is zero.[quote]or unique effects that add (even more) variety to gameplay?[/quote]All evidence points to your weapon having special effects, not the ammo. [quote]When firing Heavy Ammo, will there be a downside to balance this increase in firepower[/quote]Your heavy weapon is meant to be a powerful weapon, it's balanced by the scarcity of heavy ammo. [quote]or will the differences be minimal?[/quote]We don't know specific weapon damage stats. [quote]How will be able to tell when someone is using a different ammo type?[/quote]There are only three types. It will be clear based on the weapon.