Well we have heard of armor piercing ammo.
Maybe things like incendiary or explosive?
If all the videos of people playing pre-release are even remotely accurate, the only alternate ammo type is armor-piercing; it is a soldier ability which allows soldier rounds to completely negate kevlar (10% damage reduction ability for soldiers or buff from engineer as of the pre-release version) but it can't be given to other players. So alternate ammo would not improve your damage, just prevent it from being reduced. Sadly, no explosive or incendiary unless their hiding it really well.
Purely speculative, but I would imagine that sniping will be effective, particularly for the defending side on some maps; not in the sense that you're killing people left and right, but rather you would be causing enough damage to individual players with your initial shot that they are forced to retreat/hide or risking dieing to your allies or a follow up shot or two. It will probably be more of a fire support deal than some lone hunter hiding with a ghillie suit in the grass or on top of some hill a mile away.