My Hexagon PPSh is playing up again! I put in a magazine and attempted to shoot it, the weapon cycles fine (ie the whizz of the gears) but no BB's come out. I've tried both my mags, one of which I know works, and I've even put a single bb in the hole and tried to fire it (upside down) but nothing comes out at all. The other problem is that the fire selector, no matter what position, will always do semi and then sometimes it randomly changes to full auto if you hold the trigger down. Anyone have any ideas?
Could be your tappet plate or tappet plate spring. Does the air nozzle move back and forward when you fire the gun? If not does it seem loose when you pull or push it, if so I'd strongly suspect your tappet plate spring is broken.
Not sure about the semi and full auto problem
No I can see the air nozzle moving back and forth when it shoots.. :/
Can you hear the piston hitting the front of the cylinder?
If not, then it could be that the piston head has come off.
We had exactly the same problem and there are a couple on the forum and on other forums across the globe with this. It is probably the gears are Knackered. They appear to be wrongly shimmed and then the middle gear won`t connect properly because it is worn away. We have just had to buy a new set of gears from HK and fit them. All back together now working (Apart from fire selector which is on Fully auto all the time as I spent ages trying to mae it work then gave up)
Good luck
Contact me if you need some info on gears if that is the problem as you have to buy a full set as you cannot get the individual gear 
Ah right cheers for all the help guys 
when you put your mags in, and take them out do the BB's full out? If so it doesn't sound like a feeding problem. If you do a lot of semi you could of burnt out your trigger. Other than that sounds like on of the gearbox compoents has failed. Wont really know until you crack it open. I have had the same problem as you and thats when i broke the spring on my famas in half after about 100k rounds. I didnt mind replacing that, it had done its time 
Here's the thing, if your nozzle is moving, then your gears are working. If you hear the gun firing, then your piston is fine. If bbs are falling out when you pull out the mag, then the mags are fine. Check if your hop chamber is properly aligned. It can easily misalign or even break every time you open up the receiver. Check the contents of your hop chamber if everything's ok. About the selector, most probably, your selector lever is misaligned with the selector plate. Check if the metal selector lever arm engages the plastic selector arm on the left side of the gear box.
Some tell tale clues:
1) high pitched whirring sound , gearbox not firing= gears are spinning but not engaging the piston: piston or gear problem
2)motor overheating, gearbox locking up = overshimmed
3) gearbox firing but no bbs come out = magazine / hop/ or barrel problem.
I think it's most likely to be the last one because everything else seems fine with it
