There are a lot of different techniques regarding the manipulation of the AR platform. There are quite a few just dealing with the scenario where you may not have the use of one of your hands or arm. William Petty of @Centrifugetraining has some interesting options for weapon manipulation around law enforcement vehicles. In this example, he uses a thumb trigger press to shoot his AR.
However, it is so much more than just using your thumb to press the trigger. The scenario above looks like his support arm is down. Yet there a threat on the left side of the vehicle. William uses his working hand, his right hand, to grasp the mag well and not the pistol grip. Then William shoulders the rifle on his left shoulder. He then uses his forearm as a barricade stop while simultaneously pressing forward with his left shoulder. This captures the rifle and frees up his thumb to be able to press the trigger rearward.
You could use a barricade stop on the side of your gun, bring the gun to your left shoulder while keeping your right hand on fire control. That means you will have to keep barricade stops on either side of your rifle. One aspect I like about William Petty’s method is that it does not require any special equipment. Just a little practice and training this will work for almost any AR style firearm.
This technique is just one way to skin a cat. There are other methods to manipulate an AR when your support hand or arm stops working. William Petty’s method offers control of your firearm without much effort and it does not require any special hardware or accessories. His thumb trigger press is just one component of his technique and should be used in context. You can follow Centrifuge Training on their Instagram account.