Gun gives me new FPS every other day! Help!

[Angus1]

The gun in question is a cybergun Scar-L

 

I bought it new from the shop firing around 349FPS

A few months later (with no mods) it was randomly firing at 420(Ish) fps.

I gave it back to the shop to sort and they said they installed an M95 spring and was chrono'd at 320fps.

I took it to a game a  few months later and it chrono'd in at 260fps.

I began taking it apart, put in loads of different springs and got the following results. 

M95: 260 FPS
M105: 380 FPS  (new bucking installed)

M95(2nd go): 290-325 FPS - varied
M100: 412 FPS

 

In what I hoped was my final attempt, I replaced a dodgy piston that wasn't holding air, put the m95 spring back in, tested and got a steady 290 fps, which is less than. The old piston that didn't hold any air as even that would get 325fps...

 

My current theory is that i didn't seat the gearbox exactly perfect so the seal between the barrel and gearbox isn't ideal......???

 

HELP!!

 

[JVacation]

@Angus1 Did you check if the new piston has exactly the same amount of teeth? Could be that there are 1 or 2 teeth short. 

 

What was compression like in the gearbox? Was the air seal good with the whole compression set together and the nozzle sitting in the tappet plate on the end of the cylinder head.

 

 

[Rogerborg]

Ahoy, and welcome, and oof.  That does sound like random air seal issues.  There are loads of things that it could be, I'm sure you've checked some of them:

 

Piston-to-cylinder seal.  This is really down to the o-ring around the piston, I generally replace and/or stretch them, then lube with a little silicone grease.

 

I'd check that the ports in the piston head aren't clogged with anything, but then you replaced the piston.

 

Then the cylinder head o-rings: again, replace, stretch and/or lube.

 

Nozzle: lube, and one possibility is that it's wobbly on the cylinder head. Pressure from the BBs can skew it and cause issues.  If it's loose, you may be able to expand the cylinder head tube slightly by very carefully tapping a tapered swage into it - see Negative Airsoft's YouTube channel for this.

 

Nozzle to bucking: is the hop unit being pushed back against the gearbox with a spring?  Is the barrel secure in the hop, without forwards/backwards play?  C-clips can be very vague.

[Cr0-Magnon]

420fps on 0.12g's

349fps on 0.20g's

290fps on 0.28g's

 

....mystery solved? Haha, I know, probably not....but wouldn't that be funny?

[Rogerborg]

1 hour ago, Cr0-Magnon said:

420fps on 0.12g's

349fps on 0.20g's

290fps on 0.28g's

 

....mystery solved? Haha, I know, probably not....but wouldn't that be funny?

 

You know, I have seen that on Facebook groups.  OP sounds like he knows what he's doing, although granted I could go off on a rant about thinking and talking in Joules rather than fps.