Low FPS: custom gen8 build

So, Im working on this build for a while now. As a beginner there is alot to take in and time consumption to finally be able to complete the assembly. However the first time i went to chrono, the blaster shoots only 198 to 204 fps consistently with milkies. Can anyone suggest what else i can do to rectify this? Below is the details of my internals:

  1. gen8 nylon gearbox (i believe it was the same used in skd m4ss)
  2. metal 18:1 gearset, shimmed
  3. 6mm ball bearings
  4. chi hai blue motor
  5. metal tooth piston (plunger) with green o-ring
  6. 75% ported cylinder
  7. stock cylinder head with faucet rubber washer to correct AOE, wraped 3 times on top of the o-ring with white teflon tape to make it sit tighter
  8. metal air nozzle with o-ring inside
  9. 18cm inner barrel (i think) with inner diameter of ~7.5mm
  10. 1.2mm spring
  11. tested with 11.1v battery

The compression (tested with air nozzle at approximately half way point) seems to be ok, so could the air leak frm the air nozzle where it comes into contact into the t-piece at forward position? I was using the gen8 t-piece instead of the nylon t-piece that comes with the gearbox

The 2 main things that affect the velocity is, mechanically, the spring and otherwise, the fit of the gel ball to the inside of the barrel…

If you have a strong spring and balls that are too tight or loose in the barrel, the fps will be down… if you have the right size gels, a too soft spring… you will get low velocity. It sounds like you have the seals and things under control, so my advice would be checking the gels fit your barrel (ie: just a bee’s ball smaller than the barrel id) and try a bit more spring once you know your gels are the correct size for your barrel.

Look at Guido’s gel comparison video… he explains it well :+1:

To me… a gel blaster is all about the spring, the cylinder/piston seal, the seal of the T piece to nozzle and barrel… a straight, smooth barrel (inside) and the gels fit the barrel… these bits need to be all perfect… and if they are, the blaster should be accurate enough and good fps… the rest of the gel blaster is merely about operating the piston (the motor and drive) … something cool to look at and comfortable to hold and shoot.

So, imo, check your balls and go up a spring thickness or 2… say 1.3mm or 1.4mm

Oh, another possibility is a weak battery (not fully charged)

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First up welcome brother to the GBF it’s always good to see new faces. Your not wrong when it comes to the amount of info you try to take in as you gain more interest in gelsoft. Answer to your question. What amp is your trigger switch, where your gels fresh or old as its best to keep submerged in distilled water until your going to use them. Also what battery are you running as the ones on eBay with the black and white checkers print wrapped around the batteries arent always tru to their labels. Lastly without reading FPS does your blaster should like it’s internally firing right

Hi guys, thank you for the response.

Additional info here:
-trigger switch is rated 10amp
-battery is 11.1v Bosli-po Arena version, 25C
-no idea how old was the milkies as i borrowed frm a friend lol, but about the same performance using std 7.4mm orange
-the spring used was the 1.2mm one that comes with the gearbox

So, I noticed that the cylinder was actually vibrating and moves when shooting lol. There is a little gap (sorry I didn’t take any photo) between the end of the cylinder to the housing (gearbox shell). Measured that the cylinder I am using (7.22cm) is shorter than the original brass cylinder (7.26cm) Took everything off, and I did another compression test. My cylinder head is so so loose that even the simple test move it abit and air escape!!

What I did next:
-Replaced with a double o ring cylinder head, which sit very tight into the cylinder
-Find that AOE also need correction, so did the same as I did in previous setup
-New problem: Now the air nozzle is not air-tight. Turns out the new cylinder head’ nozzle is a tad smaller in outer diameter size (will update the size measured the next time I open the gearbox), even the o ring inside the air nozzle doesn’t help with the compression
-replaced with a 1.3mm uneven spring
-Testing: Somehow, the cylinder now doesnt move (as yet) so that is a good sign. The tight cylinder head does help to keep it in place. However I have yet to chrono, but not much difference to the previous setup is observed. I think the poor airseal of the air nozzle is to blame
-Ordered a new air nozzle with double o ring (which I actually wish I didn’t) and hope it seal perfectly. I could have gone to a local sealing retailer and ask for a correct size o ring, which I would do anyway, after I measure the OD of the cylinder head nozzle


  1. included the measured length of the cylinder
  2. worth noted that the piston does slides fairly easily without coming off track with the gearbox closed
  3. fps measured at 220 with orange std gel balls, which is still way too low for a 1.3mm spring, whereas the cylinder swap to 80% ported
  4. a friend actually advise to test whether my piston is sitting too tight inside the gearbox, which is what I am gonna try the next time I disassemble my blaster apart.

Hi, you may be right. I have an 80% ported cylinder & original unported ones available to experiment on. I will post the updates here when I do.

Oh, I never knew consistent fps indicate good airseal. It was indeed very consistent around the 200fps mark. Do you think that this low fps is actually because of a bad spring? Apart frm the 1.3mm which I swap over, I don’t hv other spring I can compare it to.

But anyway, I am gonna run another chrono with fresh milkies when I got the chance

Accuracy is good (shoot straight) for 10 metres, maybe up to 15m, I didnt measure though. But after that it will drop. Puzzles me when a good gel blaster shoots 220 to 240 straight out of the box :joy::joy:

I would also be puzzled :thinking:
You said the barrel is 7.5mm id so you need the gels to be around 7.3mm to work well.

Well I just shove whatever gel balls (7 to 8mm?) I have at hands, sometimes even a mixture (different brand) of them around :joy::joy: but would it contribute to such low fps? Haha. Will keep you guys posted on the updates. Thanks a lot man, appreciate your insights

Agree that does seem excessive to be just the balls but yes keep us informed

OK, so here’s an update:
-after I reassemble with a 1.3mm spring and an 80% ported cylinder, went to a game and get my fps tested. It only got up to 220ish fps with clear 7.4mm std gel balls.
-disassembled the gearbox, leaving only the piston in & tightened up the gearbox. I found that the piston wasn’t actually moves freely without a little push, back & forth. (tested it before inside the gearbox although without tightening the gearbox screws and I can remember that it actually moved ok).

Now this is something worth noted, normally we would tightened up those screws to check on the shimming job, so similarly to test how a new piston is going to fit into a gearbox, you would want to tightened up the gearbox as well.

I have filed / sand some materials off the drain / rail (or whatever its called) of the piston, so now it actually moves freely with gravity (wasn’t a perfect fix but I think it will get the job done)

I also found that the rubber piece on the tip of the air nozzle is loose, so I glued it in place. Tested the compression and it does improves so so much.

Nice work bro glad you got it sorted and running better

One thing I don’t know if it might cause this issue but,
u are using 75% ported cylinder and spaced it out for aoe correction, you now have even less air volume, this could be why you are getting low fps than expected, maybe…

What ? Even though this is nearly a year old …
It’s only a 180mm barrel, even with 3mm of AOE correction it’s still way over 2:1 :rofl:
But good try :+1:

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What do you use to calculate VE? Do you use the on site VE calculator by Hamish? Another online calculator? Or just work it out by hand, calculating the volume of the cylinder and divide by volume of the barrel?

I just want to know the best way to calculate and remain consistent, because it seems different methods give slightly different answers.

Use either the one on here or the A/S one

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To use the A/S one I linked correctly you will notice it is 70mm standard which allows for cylinder head, so if you have an 80% 72mm cylinder it will actually be input as 55.6mm .
Or it you have a cylinder , just measure from where the port end to the end and subtract the 2mm

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sounds like it would be easier to just use the on site one? Does the A/S one give you anything extra?

when they say 70% or 80% for a cylinder…the port is not always at the 70% or 80% of 72mm though right? I assume it is always best to measure the port to the end of cylinder and put that into the calculator?

You can use the one here np, I like the other one because it gives me more freedom

Most of the % ones I have measured are pretty close.

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