If you want to do some external upgrades for the Tokyo Marui M4A1 MWS GBB, especially the front-end, you will need this Barrel Guide from Orga Airsoft... "Currently exchanging the outer barrel from the proprietary Tokyo Marui GBB to an aftermarket outer barrel (Madbul, PTS Dytac) is not possible. By using our Barrel guide we have made it possible to use aftermarket rails.