I've asked this question here before, but never got any useful
answers.
Some even insists that the AMX-13 had no room for a .50 to be
mounted. I refuted this wild guess by posting a pic of a foreign
AMX-13 with a turret roof-mounted .50 - with room to spare.
We may call the AMX-13 a light tank but when you actually climb
onto it, you realise how big it is externally.
The only satisfactory answer I got from one post was that the
AMX-13 never operate alone, and will always be supported by other
IFV like Bionix or Ultra. And these support vehicles will have
plenty of CIS 50, 25mm cannons, SAMs etc. What the AMX-13 3-man
crew should concentrate on is using its 75mm gun on targets that
require this firepower.
Still, I am pro putting the CIS 50 on tanks. There is already a
GPMG mounted co-axial to the 75mm. Unless the reason is that there
are no more room to store enough ammo for the .50.
Most of the time the stated purpose of the .50 on the turret roof
is air defense. This is when SHTF and you're alone, you have
something to shoot at enemy helicopters. Or if you're out of 75mm
ammo and you see a enemy IFV...The CIS 50 can fire Saboted Light
Armour Piercing rounds which would be useful against lighter
vehicles.
But this is all moot as we are now getting Leo 2s. So it will be
interesting to see if there will be a CIS 50 on the Leo turret
roof.