I was more referring to what effect it would have in multiplayer.

I'm not really sure what you mean, multiplayer is the exact same as single player.

I mean, what the effect over armor piercing will be, except for increased damage. There probably will be other bullet types, I'm wondering which ones, what they'll do and what effect it will have on playstyles.

I accidentally used the word "multiplayer", because I'm used to saying that

The IRL ammo types I can think of are standard, Hollow Point, Armor Piercing, Full Metal Jacket, Total Metal Jacket, Incendiary, and accuracy rounds. Most of these types of ammo are just designed to do extra damage to certain types of target. Because there are no armor types that I'm aware of, I'm not really sure what other ammo types would do. Perhaps they are geared towards bodytypes, HP for lights, AP for heavy, something like that. There might be Incendiary rounds that add fire or explosions to your ammo. Maybe there are certain types of ammo that do a debuff, causing people to move slower or something. Could maybe add to gun stats, such as a round intended to be low recoil or high accuracy.

It also occurs to me that there might only be +damage ammo, and AP just a one stop on the path of upgrading the basic ammo supply skill, such as the turret skill can be upgraded a number of times.

Interesting theory. A bullet that drains small amounts of ability pips or debuff would be a really nice addition. Could be overpowered though.

Don't hate on Steam maaan.
I can certantly see how for some people it just makes things complicated, but I love it. Automatic updates, friends, in-game browser, achievements and other cool stuff.

Wrong topic much? ^^