While inside the H.E.L.M. you can fire your grenade launcher during a slide. I have not tested this on mouse and keyboard, only controller. Right at the beginning on the slide, press the fire key or trigger to shoot the launcher. I got this to work most consistently by pressing the button to slide (b) and immediately following up with pressing my right trigger. I tested it with breach loaded and heavy grenade launchers and both worked. I had a friend also test it and he got the same result. I also tested it with other weapons such as shotguns, rocket launchers, and auto rifles and saw no effect however this could be due to bad luck and timing. Due to the fact that most other areas that don't allow you to fire weapons, such as the Exo Strangers campsite, also lower them to an almost stowed state, I did not test in any other area.