Crosman vantage NP - how to get nominal velocity?

[beatle]

So I have Vantage NP (model 5-30020) cal .177. Nominal velocity stated in specifications is 1000-1200 fps. Because of local regulations (Croatia) I decided to check real velocity and measured 430-450 fps. I decided to check if there is a hole in the piston and of course - there it is. So I cut a thread in that hole and fit a screw in. Now velocity is 620 - 630 fps.
I wonder if there is weaker power plant in my rifle or there is another power limiter except piston hole? How can I get to at least 800 fps?
Any help is much appriciated.

[mafatone]

You can change the Nitro piston, they do wear out and the piston seal. Crosman will ship you the parts

[Roadworthy]

Your velocity does sound extremely low.  How did you check your velocity?  If you used a chronometer do you know it is accurate?.  Using it under fluorescent lighting will give invalid results.  They actually flicker at an extremely high rate and the chronometer "sees" the flicker and gets confused.  Be sure you are shooting level across the chronometer.  Any angle relative to the sensors will get a lower than actual reading.  Is it set for meters per second?

Be sure your pellets are not too heavy.  Try for something in the vicinity of eight to nine grains for a weight.

Your problem can indeed be a weak Nitro Piston.  The fit of the piston and seal in the compression tube can also be a factor.  If the fit is too tight it will slow the travel of the piston when the gun fires, resulting in low pressure.  If it's too loose air can bypass it, resulting in low pressure.  It should take about two pounds of force (one kilogram) to move the piston in the compression tube.

Check your breech seal.  Place a tissue over the breech before you fire the gun.  If the tissue blows your breech seal is leaking.  It should be shimmed or replaced.

[beatle]

I used this method to measure velocity: cds.cern.ch/record/922655/files/0601102.pdf
I fired few shots with 3 different pellets at target 38 m (41.5 yards) away.
Results with piston hole (energy and velocity):

Browning wadcutter 0,48g pellet
4,34 J      441,35 fps
4,30 J      439,49 fps

Binco magnum 0,54g pellet
4,85 J      439,95 fps
5,12 J      451,92 fps

Gamo expander 0,49g pellet
4,27   J           433,07 fps
4,78   J           458,57 fps

Results without piston hole:

Browning wadcutter 0,48g pellet
8,56 J      619,48 fps
8,85 J      630,13 fps
8,75 J      626,33 fps

Have anyone compared results of this measurment method and chronometer?