ooooh good observation I will see that tonight thanks a lot dontask!

I had a 350 FPS loss on my VSR-10 because the cylinder was dry... Spray some silicon lube in there!!

Your fps problems are probably related to the cylinder head and nozzle. I had an aug set to me from another gunsmith who had the same problem and could not get it working. The compression on the cylinder head and piston were great. Compression including the nozzle was great aswell but for some reason the mixture of parts did not work with the stock tm hopup unit. So what I ended up doing was swapping the upgraded parts back to stock tm and the fps was where it was supposed to be. I would suggest narrowing down your problem by leaving the tm cyilnder head and nozzle in one of the aug boxes and testing it.

Don't forget to check your piston! It could very possibly be a stripped piston as well.
But its probably the shimming because it's kind of rare for three mechboxes to simultaneously strip their pistons at the same time lol.

update:
the piston is not stripped.
the short barrel got a bb stuck in it twice when I tested hence the krr krr.

I reshimmed the gearbox and got 249 fps with the long barrel (46 cm)
battery is recharging now...(9.6v 2000 mah)

Cleaned the barrel at all?

the barrel is clean, it was stuck in the bucking.
I re-assembled the hop-up, tests tomorrow.

Sooo, new tests brought some clarification:

I justed tested another mechbox in the AUG with both short and long barrels, and I get the expected amount of fps (350-ish)
So it's definitely a mechbox related problem.

I changed the spring and guide back to stock on the faulty mechbox and it's still stubbornly giving me 250-ish fps.
apart from that it's working like a charm, and I'm not getting any barrel issue anymore.

I tried every part combination I could.
I won't exceed 250 fps, ever, even with different springs. :(
I fail.
this gearbox is cursed.
btw systema bore-up kits are crap, don't buy them.
EDIT: actually they're not, the O-ring just needs to be stretched.

Like I said, gimme gimme, fixy fixy! I've worked on my AUG a bunch.

Or get thyself to a local doctor.

sorry guys, I fix my stuff myself, that's the whole point :)

problem solved !
-not enough grease on piston heads and nozzles (between nozzle and cylinder head)
-systema O-rings need to be stretched a little bit before use.
Using size-up o-ring doesn't work :D

thanks to all!
special thanks to Amos and Crunchmeister for the good advice :)

ASC wins.

http://media.ebaumsworld.com/mediaFi...855/861035.jpg

No problem dude. Always glad to pass on what I've learned along the way. I had similar issues in my M15's mechbox when I swapped for the Systema silent piston & cylinder head sets about 6 months ago. I couldn't figure out why I was only shooting 320 fps with a Prometheus MS110SP spring. It was Illusion who replied to my thread telling me that Systema rubbers had to be stretched to work properly and put me on the right track. Now my gun shoots 385 pretty consistently with .2s, and I couldn't be happier with this gun.

Once I realized that, I started studying how the piston head / ports / o-ring and worked together in the cylinder to give good compression, it all made sense. It's a learning process that can be quite furstrating at times, but once an understanding of something hits home, it just seems so easy and obvious.

But anyway, glad to hear that you got you issues worked out.

I just think that bass players are superior beings.

Well, there's that too...

lol

updatez:
I still have 2 gearboxes that both use a SP100 spring with readings around 250 fps.
(both ver 3, one is AUG, the other is mp5k, both unported - cylinders, heads and nozzles are new.)

just to make sure I tried one with a 400 fps spring, and got my f-ing 400 fps right away.Switch back to SP100 and boo, 250 fps again.
since no spring I know of is rated for 250 fps, it can't be a spring labelling mistake.

I am guessing that this powerful spring is creating a wider spread of the O-ring, thus making it work properly with a 400 fps acceleration, and improperly with a 340 fps acceleration.
What I'm wondering is why, since both O-rings look healthy and are the right diameter...