M4 won't fire when buffer tube is fully screwed on

[Rossios]

Ok well this has been a problem that I have also had with a previous M4 project but ended up turning it into a stock-less CQB M4. Now I have started another M4 project and I spent all day yesterday building it up using a G&G SRL as the base gun. I tested to see if the gearbox cycled with just the pistol grip screwed on and nothing else, and the gun cycled flawlessy, when I then put the gearbox into the lower reciever and screwed on the buffer tube, same thing happened again, gun wouldn't cycle and just makes a whirring noise, if I loosen the buffer tube just enough the gun will cycle properly but after 5 or so shots the screw loosens and the buffer tube is half hanging off. I've taken the gun apart and rebuilt it 3 times now and everytime it does the same thing, any suggestions?

[sj_asc]

Re: M4 won't fire when buffer tube is fully screwed on

Check that the stock bolt isn't pulling the spring guide off center, or even the gearboxes position in the lower receiver. I would suggest fitting the pins and motor grip first before the stock bolt.
Also bare in mind if you assembled the gearbox outside the gun with the motor grip that the grip position can sometimes be affected by the lower receiver, particularly if you have switched brands or are mixing and matching.
The grip is mostly responsible for motor alignment and is an aid to what is essentially a bearing for the motor shaft and pinion gear. If the grip is not correctly aligned with the lower then you can end up with a lot of noise and or complete seizure. If it functions with a lot of noise then you will be accelelerating wear and have poor efficiency.

In short, the motor grip and alignment is equally as important as shimming the other gears, any hard work done on the gears in the box will be wasted if the meshing between the bevel and pinion is not ideal.