Coax does not kill shoot you. I have used it on tanks as well while waiting for the main round to reload and you can see the damage on the other tank in the form of little chunks flying off. Will the .50 cal kill another tank, no, but it will an M3, jeep, and blow tires in the M8.
A couple corrections here.
The coax gun DOES killshoot you, but it doesn't have the ability to cause damage to a tank in armored areas. So, shooting a friendly tank in the armor nets 0 damage and doesn't harm you, but spraying the vulnerable parts of his track just might remove your own.
The chunks flying off an opposing tank don't necessarily mean you are damaging it. If you have a tiny hit sprite, you are dealing absolutely zero damage (that goes for ANY round in the game, including main gun shells). If you have a larger hit sprite, sometimes accompanied by gray smoke, you are dealing damage. However, the only point that an MG can generally deal said damage is on the tracks and it takes quite a bit of damage to destroy one of them.
The .50 cal will kill another tank, if fired into the dashboard from inside said tank.