I suggest a simple fix. Ok so we can't use the launchers while prone I get it. But if switching TO prone with a launcher, simply have our trooper go prone while laying the tube on the ground. He's still holding onto the grip but just lays the weapon flat like you would with a rifle. He can still lug it around while prone, maybe more slowly or something, but you should still be able to simply hit the deck. If you're for a large launcher, say a javalin maybe drop it and hit the deck next to it and then place your hand on it as you hit the ground? Just drag it along if you have to move? At this point you could even just leave it there and bug out if you really need to. If you stand up, you get up into position THEN the weapon follows after you as you lift it from the ground.