Fluctuations in FPS

[DNewman]

Hey everyone, I'm relatively new to air soft forums and therefore am not sure if this is in the correct place although here is my problem.

I have a Magpul G&P M4. Recently my gearbox has been properly shimmed and had the piston head and o ring replaced. lately on shooting my AEG I will be shooting at 170 FPS several times and then my FPS will rise up to around 315 FPS. This is more noticeable when using full auto. The first 5/6 shots could be low FPS but the rest are around the 315 mark (which is what I used to shoot at consistently).

Does anyone know what might have caused this? I've tried a leak check on the hopup by placing the paper over the hop up feed hole.

Many thanks!!

DNew.

[Simon77]

Re: Fluctuations in FPS

Hmmm strange one, I had something similar a while back and found that the cylinder had dried out just enough so that the piston didn't cycle fully before re engaging so low fps, (didn't travel the full length giving the full stroke before the gears pulled it back for the next shot) solved it by regressing the cylinder and orings in the piston and all sweet again.

When it was serviced they did lube the new parts enough, and relube the inside of the cylinder enough???
Only thing really I can think of as anything to do with the hop area your still get higher (more consistent) fps but crap range.

[DNewman]

Re: Fluctuations in FPS

Hmmm strange one, I had something similar a while back and found that the cylinder had dried out just enough so that the piston didn't cycle fully before re engaging so low fps, (didn't travel the full length giving the full stroke before the gears pulled it back for the next shot) solved it by regressing the cylinder and orings in the piston and all sweet again.

When it was serviced they did lube the new parts enough, and relube the inside of the cylinder enough???
Only thing really I can think of as anything to do with the hop area your still get higher (more consistent) fps but crap range.

Fantastic, thanks for that Simon. What I'll try then is to re-grease the piston and cylinder and re-lube the o-rings. What grease/lube is best to use for the o-ring?

Interesting to known that it's not my hop up unit though.

Thanks!!

[Shizbazki]

Re: Fluctuations in FPS

Hi

I would recommend

• Get your own Chronograph, i assume you already have one? If not i would recommend any Chronograph by XCoreTech the X3200 has been a staple of many Airsoft sites.
• Using the same weight BBs be it .2g or .25g etc all the time when testing your gun
• Turn the hop up fully off and leave it that way when testing the FPS of a gun
• Clean your Barrel
• Check the air nozzle seal first before any disassembly
• To do this get a piece of paper like A4, cut a small piece of it measuring about 1 x 1cm
• Ensure you gun is full unloaded, power it up by connecting the battery
• Without a magazine turn you gun upside down
• carefully place the piece of paper over the hole where the BBs goes into the gun
• shoot a few shots in semi
• if the paper flies in the air then its the Air nozzle at fault, all you will have to do is reseat the air nozzle correctly
• if the piece of paper stays in place or barely moving because of vibrations you have a good air nozzle seal and move on to the next part below
• Open the gun down to the Gearbox (herein called GB)
• Ensure the Spring has been decompressed, use a paperclip and make a small hook on one end to fit, insert it into the hole where the motor goes and use the hook to pull the anti reversal latch a few times, that should decompress the spring
• Open the GB being careful not to disturb the shims so that you can put it back together correctly
• Remove the cylinder and piston
• Use a non petroleum based grease on the o-ring on the piston and smear some on the inside of the cylinder
• Whilst your here inspect the greasing on the gears, if need be add some lithium based grease on the gears
• Put it all back together again
• test a few shots.

In terms of what grease to use, everyone has their firm favourites but as a good starting point:
For the o-rings use a thick silicone based grease such as Abbey Silicone Grease:
http://www.zerooneairsoft.com/produc...oducts_id=2390
I still use the above as i have loads left and find it gives me a very good airseal for the piston, my G&G TR4-18 (a HK 416) fires a very consistent 340FPS with lows of 338 and highs of 345FPS.

For the gears use a lithium based grease which also has a thick consistency such as Abbey Gun Grease LT2
http://www.zerooneairsoft.com/produc...oducts_id=5448
Personally i found this a bit too thick for my liking but you may like it, i normally use white lithium grease as it has a slightly thinner consistency and ups my ROF from around 17RPS to 22RPS

[Simon77]

Re: Fluctuations in FPS

Yep, just like that lol.

For grease I use the g&p grease set which has the cylinder oil/grease, gear grease (lithium based) and bearing oil/grease. These seem to give good consistency for the GB.

It does sound like something is either not refitted right I.e the nozzle or that it's dry in places causing the inconsistent fps. Hmmm

[DNewman]

Re: Fluctuations in FPS

Hi all!

A massive thank you to you all for taking your time to answer. I've done the paper test and that's fine, so i'll make my next step stripping the GB down and lubing up the o-rings and the cylinder. See how this effects the FPS.

Thanks again!

Dan