KA M4 AEG losing power every shot
 

Hi,
I have a feeling this one is simple but I'm looking for second opinions. After a spring downgrade on a KA M4, I've been observing the following on chrono:

1st shot solid 330FPS
2nd 275FPS
3rd 220 FPS
4rd 170FPS
5th 120FPS
6th 97 FPS

The last shot seemed to skip. As if the gear went under the tappet plate and grabbed it on the way back.

and then the cycle restarts at 330ish and goes down again for a few shots.


I was suggested by a friend who has some experience with AEGs to replace piston o-rings.

Anyone else has an idea?

regrease and reshim perhaps ?

Each time the gear box cyles, your shot to shot should slightly vary but not keep on dropping FPS unless there is something slowing it down.

A spring downgrade should not cause those problems.

The other problem that can happen, is you may have placed one too many shims from a gear to another causing binding to increase as you shoot more.

I suggest you take the gear box a part and recheck everything before you wreck your gun.

Sounds like an airseal problem. It'll have to be opened up and the cylinder head and piston head checked.

Well i reopened the gearbox, closed it up again and it worked at a solid constant FPS. I think it was an issue with the tappet plate which wasn't secured down fully by a shim.

Thanks for your help guys. It will be helpful in the future.

I'm pretty sure it wasn't that, there are no shims for a tappet plate.

However you seem to have fixed your issue just by sheer luck.

over shimming the sector gear would cause the tappet to stick back or not open though