Step 10: Trigger rod
I used a a 1/8 thick by 3/4 inch wide aluminum bar. Iron or steel bar will work too. So will a round rod of some sort. Do what ever is best for you.
Picture #1: Measure from the center of the XM PB4 grenade valve (the silver circle) to the top of the control box. Transfer that measurement to one end of the trigger rod. Bend the trigger rod 90 degrees or more. shape and bend the tip so that sliding the trigger rod forward will depress the grenade valve.
Picture #2: Cut the long end of the trigger rod/bar to match the lenght of the control box. Depending on the material used for the trigger bar/rod find a away to secure it to the control box. I used a flat bar so I used "L" brackets to hold the bar captive.
Picture #3: Attach something (screw, rod, d-ring, etc) to the bar so that you can grasp it and push the bar forward which will depress the back of the XM PB4 40 mm grenade shell.
Make sure that when you Extend the launcher the something above (screw, rod, d-ring, etc) has clearance to slide backwards unhindered. When the launcher is collapsed, you want something to hinder the something above from moving forward as a "safety".
Picture #4: *Optional*
Install a spring to prevent the trigger from accidentally pressing the valve.
Pictures #5-10 are to help visualize the whole mechanism.