Shooting at head/eye will deal damage, instead of healing.
Shooting at torso/groin will do nothing with non-crit shot, or heals low ammount of HP with critical.
Shooting at legs heals low ammoun of HP and normal at critical.
Shooting at hands heals always normal ammount of HP.
Well, so if you shoot somebody to hand by stimpack, penetrating his hand, flesh, maybe bones, nerve, it will not harm him, just because you shoot to his hand. He will not lose his gun if you hit your hand nerve? You cant damage his artery?
For christ, if someone is applicating stimpack, he has to penetrate
CAREFULY his
BLOOD VESSEL with
ACCURATE penetration depth. It is not easy to do this even if you are non moving and you need little experience with this. It is very hard, if you are traveling, i.e. in ambulance car, but it is IMPOSSIBLE to do on long distances.
I.e.: if you shooting tranquillizing dart to some wild animal, you have to aim for flesh, because the active drug gets into the blood circulation from flesh.. But the effect is not instantaneous, but you have to wait for a while. And there is always possibility that you will kill that animal, or criple him.
So i think that this idea has not nothing common with real world but but also nothing with Fallout.