Every time Robocop's helmet is removed, his chin guard just seems to... disappear. At first it's on, then we cut to another angle and it's not there anymore. This happens in every incarnation of the franchise, so it must be a deliberate choice. The question is... why did they decide to shoot those scenes like that?
You're probably just not supposed to notice. It's the same reason why, the one and only time we see Batman remove his cowl on-screen, the black stuff around his eyes suddenly disappears. In other words, I don't know either, and it bugs me too.
In Prime Directives, there is one scene where Robocop removes his helment in one shot, and the chin guard remains.
Perhaps it usually just falls off when it's not being supported by the helmet being screwed on, but they have that happen offscreen so that it doesn't ruin the drama of the moment. The first time he takes his helmet off especially, it kind of would have messed up the emotional impact of the scene if, after unscrewing the bolts, you saw the chinguard go toppling off, clank off his chest, and go bouncing across the floor as it landed.
Here's one that kinda annoys me. Robocop is, by directive or nature, a cop. So why is he putting so many pullets into the center-mass of everyone he shoots at? While I don't deny cops have the right to use lethal force, Robocop has outstanding targeting abilities and would be perfectly capable of putting a round into a criminals kneecap, taking them out of the fight without a kill count.
You can still fire a gun with a busted kneecap. The only way to reliably, totally take someone out of a fight using a gun is to use lethal force. Fleshwounds aren't, and if the wound isn't enough to at least send them into shock, they're going to be more than capable of firing back.
"Shoot to maim" is only used in potentially dangerous circumstances like the person is reaching for a gun. If the situation evolves into imminent danger that means they are an immediate threat (gun in hand) and you shoot center mass.'
Do also keep in mind that they wanted to use RoboCop to "deal" with the criminals in Detroit... Deal here meaning getting rid of them.
A gun is a lethal weapon. If you can shoot to maim, you weren't in enough danger and it can be classified as excessive force. If the person dies due to your maiming, you can be charged with manslaughter. If they can survive, you can be sued and will likely lose. They could also change their story. For most states, if you feel threatened enough to use a gun in self-defense, it is suggested you shoot to kill. In Robocop's case, he is acting in defense of others, which is still justified.
Everyone he shoots has a gun, is shooting at him, or is threatening to hurt someone else. These are all perfectly solid grounds for shooting to kill.
Victoria Police is recognized as one of the best police forces in the world. Their policy in a situation where use of deadly force is to be used (going by my knowledge of LAPD SWAT standards as an example, gun up, threatening officer or civilians or point a gun at an officer ordering compliance) they aim for the upper body and keep firing until the threat is stopped. This is because the first shot does not always do the job.