The PT-76-57 in war Thunder is a Light Tank with a 57mm autocannon that's got a laser range finder and a VERY fast reload. What makes the PT-76-57 special is that it fires an armor piercing high explosive. Where most autocannon tanks fire a solid projectile the PT only needs on good penetrating hit to cause colossal damage.
The 57mm autocannon also has INSANE zoom and when you're not zoomed in you have an excellent FOV. That makes it good for brawling as well as long range combat.
The mobility of the PT-76-57 is pretty good, too. The only thing letting it down is the poor armour, which isn't really an issue since everything else more than makes up for it.
Basically, imagine a ZSU-57-2 but you have one fully stabilized cannon! Insane! 🙂