When you find yourself in a situation that involves wearing a gasmask while trying to operate an AR style rifle, there are a whole new combination of factors to be considered.
The biggest concern for me comes from obstructed view. If my goggles or gasmask viewport fogs up, it isn’t completely out of the question for me to instinctually fire at a target if I’m using a handgun. But, I am a shooter who prefers using a handgun over a rifle. It has just always been my preference. With a rifle, unless I am really close, I need to have a fairly decent sight picture to be accurate.
One way to help un-obstruct your sight picture is to use the larger aperture of your open sight. As a side note, I would recommend using open, iron sights for shooting with a gas mask on, specifically because you might very well find yourself having to aim from an odd angle that would prevent a holographic or laser sight from being completely effective.
One final aspect of using your rifle with a gasmask concerns the physical layout of the mask. On the mask that I use specifically the canister screws into the mask on the left side. Therefore, a left handed shooter, or someone who is taking an off-hand shot, could very easily find himself, (or herself), at a disadvantage if they haven’t ever trained to shoot from the side where the canister connects.