Well, in the case of your MP5K, a non-ported cylinder is too much even for a ~300mm barrel. You're not getting the max efficiency out of your spring. You should be using 3/4 (actually around 300mm) or 4/5 (360mm plus) port, according to Arnies.

but I get exactly the same results with this cylinder, is the port size wrong?
- the barrel is around 30 cm long-

http://i225.photobucket.com/albums/d...6/92c8717c.jpg

That looks like a 1/2 port for stock MP5K length barrel. It would likely give you too little air.

ok thx,
also I forgot to mention the piston head is ventilated.

Dude, if a 400 fps spring shoots 400 fps in both your guns, and the other spring only shoots 250,

IT'S THE SPRING!

Get a new one!

I have two brand new SP100 springs and they both give 250 fps with those mechboxes.
and I want 350 fps max, it's not like can just shove a 400 fps spring in them and be done.

I don't think its the springs.

If the 400fps spring is working at the intended fps and consistent, it's most likely your other springs.

+1 women and booze to the rythym section first.:cool:

tonight I try a custom spring, we'll see :)


of course :)

ok so tonight I tried with a SP120+ spring, and I get anywhere from 337 to 406 FPS.

I'm pretty sure it's not the spring, there's something about inconstant compression somewhere.

I question the junction piston/piston head.
I will test a piston head with bearings this week...

apparently I can get a high fps with a strong spring, but I get maximum 250 fps with a SP100, and as low as 190 sometimes.

can I get barrel suck even with a ventilated piston head?

Barrel suck doesn't exist.

ok so what then?

okk so I found the problem on the MP5k but I don't know how to solve it:

AEG ready to shoot :the nozzle is set on the hop sleeve, the is BB in the chamber-part of the barrel (just before the hop-up notch)
the thing is that at that point you can blow air inside the nozzle and it just flows around the BB through the front end of the barrel.

if I cap the front end, seal is good.

Air isn't leaking from anywhere I can tell, but still half of the power will be wasted because it just flows around the BB to the front of the barrel.
It feels like a super un-tight barrel.
what could I possibly do?

question: is it normal that the bb sits after the hop-up sleeve's rim?

BB should be inside the hopup rubber before it fires. The nozzle pushes up against the rubber [as it pushes the BB into the rubber] and makes a seal, then fires. Maybe try a new rubber?