[Dawnbringer]

You will want 6mm brushings. You want them as the plastic ones will start to melt/deform when used with an upgraded spring. For which spring, it depends on what fps you are looking for. Check the spring guide (on the site, not in the mech box.) Most places wont let you go higher than 400 fps with .2s so I would say 120 max with 110 to be on the safe side. Make sure that there is room for the larger battery. What I would recommend is just getting a systema FTK for what ever spring you decide on. This way you know the internals are good and if anything breaks you will have your old internals as spare so you don't miss a game waiting for new parts.