Well we tried changing the spring and have managed to break the gun, tried putting a M105 in it
It won't fire now, even with the old spring back in.
Its been left at the local site for the tech to look at it next week and fix it, they suspect that the mosfet is turning off to protect the gearbo as it may have stripped some teeth from the piston due to the spring being too strong
Lessons learned, and looks like I will be without it until I get back from ny hoilday at the end of the month
Luckily I have my other M4 I can use for next weeks outdoors game