Inconsistent FPS

[simonh]

I have an ICS CXP-APE, I've had it for a good while now and having recently got back into airsoft started using it again after it was sitting for 3-4 years.

It was shooting low so I put a new hop rubber in it and it was back up to around 250 ish fps. trying to bring that up I put a m110 spring in it and got it to 330 fps - too much for most sites I play at. Dropped it down to a m95 spring and back down to 250, and then tried m90, m100 and still couldn't get it shooting better than 280.

I then put a 11.1v lipo in it to see if it need a bit more power and immediately stripped the teeth off the piston. So I put a new metal toothed piston and new piston head in it and went back to the original m90 spring and it was shooting at around 280 consistently. I put a tightbore Barrel on it (from AK2M4)and no change in FPS I then went back to the m95 and fps went down, so again changed to the m100. 

I now get anything between 250 and 310 but with no consistency at all, I get good compression on the gearbox, I can't see any issues with the standard hop unit (they are prone to cracking apparently)  so now I am a bit lost for ideas, the bottom end of the gearbox is all standard and recently serviced as it is easy to do on a split gearbox everything seems to rotate nicely and smoothly. 

So now a bit lost for ideas other than changing the barrel back to standard or trying a different hop rubber, what I really want from this gun is consistent power and good accuracy - any ideas about where to look next?

 

[Fatboy40]

You've missed a key point out from your post... what weight BB's are you using?

(Just saying 250 FPS, with no context, doesn't help)

 

[simonh]

fair point! .25s

 

[Colin Allen]

If it is inconsistent, check the seal between the nozzle and the hop rubber.  After making sure that the hop chamber is empty, turn the gun upside down and place a small piece of paper over the feed tube and pull the trigger.  If the paper does an impression of a Russian tank turret, your nozzle is not engaging properly with the hop rubber lips.

What is the barrel length and what type of cylinder is fitted?

 

[simonh]

Cylinder is the original non vented that came with the gun, barrel length is 300mm. I’ll give the paper test a try

 

[Asomodai]

What mags are you using? Could be mid cap syndrome. Do the FPS issues happen on different mags? Do the FPS issues happen mostly when the mags are full?

 

[MrTea]

What hop rubber did you put in it? Try going back to the stock one if you have it.

I've always used Maple Leaf Macaron (yellow) bucking on just about every AEG I own (with the omega nub) and had zero issues with it. As a super off-chance suggestion, are you running mags with strong springs such as EPMs? Try running a standard mid-cap and see what happens.

 

[simonh]

I'm running some 200 round nuprol mid cap mags, they run fine in other guns I own so I had discounted those as an issue, not noticed if the differences are apparent right the way through the mag or just at the start or end - worth thinking about though actually.

Thinking about it when I first changed the spring I chrono'd it with the original high cap mag that came with the gun and it was inconsistent with that mag, but I put it down to it being a new spring and needing bedding in a bit.

Can't recall what the bucking was offhand, but the original was very crispy so went in the bin, I do have an MR bucking and nub in the parts tub that I could give a try. 

 

[Fatboy40]

Cylinder is the original non vented that came with the gun, barrel length is 300mm. I’ll give the paper test a try

I very much doubt that this is related to your problems but using a "full" cylinder with a 300mm length inner barrel will mean that your RIF is waaaaaaaay over volumed (the volume of air pushed out by the piston.

Google "airsoft over volume" as you may want to potentially consider changing it.

 

[simonh]

I opened the gun to check this and it is actually a vented cylinder, but is still the standard one.

 

[simonh]

What mags are you using? Could be mid cap syndrome. Do the FPS issues happen on different mags? Do the FPS issues happen mostly when the mags are full?

Give this man a biscuit, put my old windy mags in tonight and not a shot dropped below 300.