well the internals are g36 knock offs. indide yours when you slide your forgrip off, the entire barrel assembly is held in place via a small pin in between two tabs on the front of the upper reciever (if this isnt there this could be part of the problem). slide that out and the entire barrel assy comes out. the hopup, inner barrel, body rail and outer barrel is all included in the assy. after done, inspect your hop up, if found defective, look on
www.uncompany.com for a new G36 hop up (tm has a metal wieght integrated into the barrel assy and theyre all interchangable but you can get just the hop up), pm Jugglez and he can get it for your (or any retailer but i deal through Jugglez) or look in the classifieds a remember shjnitmc was parting his out and as i said all the parts are interchangable between the brands/ makes