I recently picked up a JG mp5-a4 RIS for £40, it was compliantly taken apart and in bits, as a project. Tappit plate was broken when I looked at all the bits over and that was it. luckily I had gotten a free gearbox with my other jg mp5 though it being a CA gearbox it just wouldn't fit in the JG body, being too fat, so I took the tapit plate from that and put it in the rebuilt JG gearbox and it fitted fine. I was having great fun putting it together and spraying over the ugly blue two-tone, until it came to test fire it... Grind clunk... the piston jammed and the pinion gear was grinding, at this time I didn't know it was but I learnt in the end what the sound was. So I took it apart and put it together a few times as the spring popped out dislodging everything, which was very frustrating.

So after about 16 attempts and jams I managed to get the gears to sit in there bushings correctly, reseat the trigger as its spring kept on popping out. But it was still making the grinding noise and it was chronoing at 364-368 fps, then it jammed again, so I looked at the piston and discovered that the CA piston was the wrong shape and was getting stuck on the guide rails. I changed it with the original JG piston and spring and put it all back together. This time though I had the idea to look inside the motor well of my other JG mp5 to see what was different, I was missing  this little coin shaped piece of metal that pushed the motor further up into the gearbox. So I went to the shed and found a nut the same size and thickness and put it in the motor plate and screwed it closed, and tested the gun I was so relieved when it sounded exactly like the other gun and the grinding had stopped. Chronoed it and it was coming in at 295-303fps more than enough for the engagement distance I play at.

Now all I need to finish it off is a front sling mount @ £10.21, a CA rear sight @ £15.32, 10 midcap mags @ £35 so I'm looking at over £60 in total with post from www.firesupport.co.uk. Though I'm still hoping for the guy who sold me the gun will send me the rear sight but he hasn't replied so that probably won't happen....
Can't wait to try it out in a game and see how it performs I'm tempted to pick up a tactical torch for £25 but I want the midcaps first, mostly because I don't do CQB anyway and my local site is expensive to play at.
All in all lots of lesions learnt and lots of money saved...
...
Meddy out