My stock hop up unit broke because I was an idiot when trying to replace the bucking. So now I plan to upgrade to one of the hop up units from MAXX, the only problem is I haven’t found a single bit of info on the two hop up units I have to choose from. They’re basically the same, but I imagine the different between the two is one works and one doesn’t. I believe I need the M4 variant, but if someone could confirm this for me I would he very grateful. I believe one is for HPA and one if for GBB, but I have no clue.