What is the point of armor piercing ammo?

I spent a good amount of time unlocking armor-piercing rounds for the first lmg only to find out they tickle "light vehicles". 200 rounds might get you one part damaged and by the time you do that you're almost certainly already dead. This games TTK on everything seems a bit out of whack. Give me a hardcore AOW playlist so I can stop suffering. OR make portal count towards exp again because I am suffering more than I ever had. Maybe lmgs are just not a viable option (usually arent) but damn is it one of my favorites right next to shotguns. I want to play portal but want actual attachments for my weapons.