HK fps issue with 11v battery

Hey guys. Can anyone tell me why my HK has consistent 330fps with a 7v battery but when I use an 11v the first and last shots of a burst are 330 but all shots in between are all over the place ranging between 100 and 360. I have a 14t piston with 1 and a half teeth removed, 18:1 gears and a strong return spring with 2 coils removed. Im using a 1.25 unequal spring. Cheers

Sucks when an upgrade degrades performance.
The rps would change. Could be airflow.

The air seal between piston to cylnder, head to nozzle and nozzle to t piece is good. I assumed because the fps is so consistent on 7v that there’s no issue with air seal. Why would the first and last shots be on point? Maybe a stronger spring or more coils taken off the return spring 🙅. I cant see any signs of pre engagement either. Surely a 1.25spring would get the piston home in time

Oh and the rps on 11v is around 26. 35000rpm red chi hai motor

It sounds like either the tappet plate isn’t getting fully forward, so the nozzle can’t seal, or the magazine is running harder thus stuffing more gels in and jamming the nozzle partially open, or some combination of the above.

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That sounds good. You probably don’t even need 11v.
I was referring more to air returning to the cylinder when the piston is retracted.
Could be way off the mark. Just thinking aloud. :sunglasses:

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You can try shortening the tappet tail a little at a time and see how it goes.
Make sure the tappet is being fully retracted and is going all the way forward first.

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Ok thanks guys. Do I shorten the tail so the sector gear doesn’t pick it up as quick? Do I shorten the the whole tail or just the part where the sector engages?

Shorten the bottom of the tail 1mm at at time, your rof is not extreme so you may not need much.


You remove the lead tooth on the sector gear to shortstroke.
It will reduce fps unless you upgrade the spring.
Is the cylinder ported?
What type of pistonhead are you currently using?

Thanks mate I’ll try that.

100% cylimder. Alloy piston head 8 holes

Sounds like you have it all. Tricky.
Vacuum issues can sometimes be fixed using pistonheads with larger slotted hole.
Anyway, I’ll stay tuned to see what solution you arrive at.

I took about 1mm off the tappet tail as @Rattler suggested which slightly improved the fps and consistency on 11v. Now ranging between 190 and 230fps. So now my question is do I keep trimming the tali until the fps gets up into the 300s and more consistent on 11v? Is there a point where trimming the tail will start to have a negative effect?

You can take more than 1mm, even 3mm is not a problem

Thanks mate :+1:

With the tappet route, stop before it starts chewing gels.