Shot-to-Shot variation w/ pressure

[MEHavey]

I was chronographing velocities-vs- pressure-remaining for the Cyclone where I found JSB Exacts to follow a *very* gentle curve over 48 Med-Pwr shots/200-160 bar (while the Crosman Pointeds showed almost *no* variation for the first 24 shots)

But then I ran across this reference in an (Air Arms S410) instruction manual for refilling from a SCUBA tank:

~~~~~~~~~
FILLING TO HIGHER PRESSURES WILL NOT INCREASE
POWER AND MAY PREVENT OPERATION COMPLETELY OR
CAUSE CYLINDER DAMAGE.
FILLING TO LOWER PRESSURES WILL NOT REDUCE POWER
ONLY THE NUMBER OF SHOTS.
~~~~~~~~~

It was that last line that threw me since it intimated that there was a regulator within the rifle which effectively buffered the effects of internal gun pressure.


What's the real story?

[DanoInTx]

There is no regulator.  What they mean is that if you overfill the gun you wont get more velocity, but you may induce valve lock which will slow the gun down.  The last part is a bit untrue, but what I think they mean is if you fill lower than full, but in the middle of the sweet spot you will get the same power, but fewer shots.....the whole paragraph is a bit wishywashie in my opinion because there is a whole bunch more to finding a proper sweet spot on a PCP than just "Don't overfill it wont work, and underfill for less shots".  

I personally do a full rated fill, say 2900psi.
Chronograph a long string and watch for the bell shaped curve.
Decide which of the shots in the middle are at my acceptable power range.
Fill again.
Shoot over the chrony till I get to the first shot within my acceptable power range, and document that pressure.
keep shooting while counting shots till the power drops below my acceptable range
look at end pressure

Now I know my starting optimum fill pressure, I know how many good shots I'll get before I refill, and I'll know what pressure to refill at.

hope this helps,