I believe anywhere within 1km is enuff to kill the AMX-13 for
the 20-30mm family of armor piercing rounds. As for .50 cals...
Preferably within 500m.
Besides, the time u take to fire ONE 75mm round, is enuff for most
AFVs to fire a whole volley of their 20mm. Besides, do u think the
AMX-13 can acquire faster than modern IFVs? Lets not compare with
neighbouring vehicles for now. Which do you think will survive?
Between a Bionix and an AMX-13 firing on the move over rugged
terrain. Chances are, the 75mm will miss, while the BX will plough
enuff 25mm slugs thru ur engine block.
If the armor protection of the vehicle is not able to ensure the
survivability of its crew, then u can also throw those "Human
operator factors" out the window.