I use a rifle that started as a gen III HJ416 by JG. After injecting it with some money and labor it looks like shown below (sorry for the mess around). Changes were focused mainly on the visual side, but few technical issues also was altered.
Most important external change that I have done is having a metal body instead of ABS one. The body is by DBoy and has all original markings (as the ABS body by JG had). Front RIS is the original (JG) one, and this is a bit problematic, due to several issues - one is the color (the difference is not that dramatic, but still it is clearly different) and the second one is the height of the top rail. The rails mismatch by around 2..3 mm, so in order to mask that and also make the whole rifle more stiff I have mounted an RIS extender (or whatever it is called) on the joint and put the optics there. The sight is a classical closed red dot (reflex) sigh by King Arms. Also I have changed the pistol grip (DBoy) and the trigger guard (Magpul). Inside the front RIS I have installed a new, custom made RIS holder (an alumina cross, that is fixed to the barrel) and gas piston.
In the place of the odl (and crappy looking) crane stock I have installed an Magpul MOE stock in the same collor as the trigger guard (a khaki-like something). On the stock I have an magazine pouch that is utilized for holding the battery - an 8,4 V, 3000 mAh Traxxas NiMH. Having the battery, especially this heavy, on the stock feels much better than having it in the AN/PEQ replica. Also this accu is too big for the AN/PEQ replica. The only remaning problem are the cables, but this will be solved somehow soon (I hope).
Inside of the gearbox not much was changed. As the gen III JGs are quite good and reliable I opted only for changing the spring and piston for something more reliable. The old spring was quite compacted so I have installed an SP 130 from guarder. Also a new cylinder, cylinder head and piston was installed, all by SHS. I had to add some teflon on the cylinder head, though it was double sealed. As you can see the new cylinder is type 0. This was followed by installing an longer 6.04 mm inner barrel.
To hide the barrel I used an barrel extender and a silencer. The inner barrel ends around the middle of the silencer so it has some chance to silence to sound of the BB pellet going out from the barrel ;-). Although I do not care much about this. The silencer also had HK markings, but I do not know what kind of replica this is. The barrel extender is still unpainted, but I plan to do it in the next few days. After that I have to borrow a chronometer to check how good the SP130 spring is working and do I really have ~130 m/s BBs flying around.