When bashing an opponent, you apply damage and also shock. Shock is applied if the following is true (StunDamageEffect.OnApply):
damage done > target initial health - damage to hit body part
So, let's say that an Arthron has 300 health and I apply 225 damage with my heavy.
If I hit the torso (120 HP, 30 armor): 195 > 300 - 120 -> target stunned
If I hit an arm (70 HP, 20 armor): 205 < 300 - 70 -> target not stunned
If it is a desired behavior, it's really a RNG design since we can't choose the hit body port. And seeing the cost of taking return fire if it is not stunned ...
I'd suggest that the formula be changed to: damage done > target max health / 2
or: damage done > 10 x target final WP (and facing the enemy would reliably trigger a bash to the head)