So today I took the gun apart, really easy actually, 2 push pins out, the whole upper receiver slides off with rails attached, hop looks completely different to ones I was looking at, so need a re-think on which to buy, barrel is 433mm, according to some websites its only 363mm, which is crap, obviously just going on a standard m4, and this is much bigger than my g&p, so I have ordered a new hop rubber and nub, a 455mm tightbore, went for the 6.01 as I have one in both my other guns, bit more maintenance but great results, will put these in first whilst trying to find a hop unit, also a 200mm silencer/barrel extension just to fill the gap between the rails and the outer barrel, will see what difference these make before deciding on whats next. Again cheers for all the info, its welcome anytime as I have only been airsofting for nearly a year and so much to learn