finding an airleak?

[lurkingknight]

I have a project p90... I've replaced the trigger/wiring and installed a triggermaster mosfet.

The gearbox is supposed to have an sp120, but is shooting around 360fps. I tried an sp110 and it shoots about 20-30 less. The gun has a 247mm madbull barrel and guarder clear bucking/nub. Cylinder is ported. The spring chart says I should getting 430~ It's not at upgraded so I'm guessing 410-420 would be more accurate. The chart says an sp110 should be around 400. The 110 is currently in my m14 and it's shooting 407-410.

So I'm convinced I've got an airleak somewhere.Though I'm not quite sure how to diagnose it. The funny thing is that I was able to get another 15-20fps by rotating the port to the top of the gearbox where it's hidden. The bucking is properly installed but I'm not sure if it's leaking from there. How do I test the various spots it can leak?

Piston O ring
Cylinder head O rings
nozzle/tappet
hopup

[CDN_Stalker]

Start by checking the compression through the cylinder, then work your way outwards. Do you have a ported or non-ported piston head?

[HKGhost]

With such a short barrel, it will be harder to achieve the correct FPS from your springs. The bbs spends a much short time in the barrel so it can't accelerate up to speed. You'll need to go with a heavier spring like an M120-M130 to reach 400 FPS, where as a 363mm barrel will only need an M110. But do check for leaks like Stalker suggested.

[CDN_Stalker]

What you outlined is the entire reason shorter barreled guns are provided with a ported cylinder, to get them up to speed almost instantly, and to also roughly equalize the cylinder volume with the barrel volume compared to a longer barreled gun.

Honestly, in all my years of being a gun tech, the biggest pain in the arse and time waster for me was chasing down airleaks............

[HKGhost]

I am aware of matching cylinders to barrel lengths, but the OP got a different FPS just by rotating the cylinder, so I'm going to suspect that the cylinder isn't the correct one for his barrel. But at the same time, I have gotten lower FPS with a short barrel even with a matched cylinder. It's not much but noticeable.

[lurkingknight]

the sp120 is rated higher than a m120 spring, it should be closer to 400 than where it is now. ACtually it should be 400 on the dot as it's a broken in spring.

piston head is ported.