Why do some PCP's get same velocity with different psi?

[DougT]

I know the answer is the design of the internals but some seem to be drastically different.  (.22 cal)  The RWS/Hammrli 850 seems to need about 1400psi to get 850 fps while the B-50, Disco and QB's seem to use closer or below 1000psi.  Why are they that different?  

I am looking for a .22 PCP/HPA conversion that can shoot a 14.X grain pellet at 850 fps for at least 100 shots.  Using a regulator/22 cu.in tank and a gun that can do that at 1000psi should work.

[TCups]

No expert, but I think it goes something like this:  at high pressure, the valve that opens to fire the pellet snaps shut very quickly from the back pressure of the compression tube.  At lower pressures, the valve stays open longer.  In essence, the same volume of compressed air is ultimately put behind the pellet, and in either case, this occurs in a short enough interval of time that the end result in terms of muzzle velocity is the same.

[DougT]

I understand that a certain volume of air is required to move a certain weight of pellet a certain speed.

A further question.  What is the lowest psi in each of the PCP or HPA converted guns to get mid 800 fps with a 14.X grain pellet?  Disco, QB-78/79, Marauder, B-50/51, 850?

[ribbonstone]

How long the valve stays open.

A short quick blast of 1K air won't push a pellet as fast as a long blast of 1K air (but the short blast will use a lot less air, giving a longer shot count).

What decides dwell time is a combination of the strength of the strike to the valve and how strong the resistance to opening (caused by the air pressure pushing back and the valve return spring). Some valves are built for short opening and some for long.

So on one end of the system, you've go the striker/hammer and the spring tht drives it  giving the force to open the valve.  A heavier striker and or stronger spring making for more force being transmitted to the valve swtem...which opens it farther.  Also, in order for the valve to close, it has to push that hammer back.On the other end of the system, have the force of the valve stem return spring and the air pressure holding the valve closed.


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Not the only way to lessen or gain speed.  Th transfer port can limit the amount of air that actually gets to the barrel as well.  Can also limit the valve stems travel.

One gun that comes to mind is the Sumatra rifles.  Their power adjuster does NOT change the hammer tension at all, the force pushing the striker stays the same at low power or at high power.  The force holding the valve closed stays the same on high or low power.  What changes is how much of the valve stem is allowed to be moved by the striker.

the power adjuster blocks to various levels the travel of the valve stem.  So at the same pressure with the same striker weight/spring tension,  could limit the valve stem travel to produce 600fps...or could set it to produce 1000fps.

[DougT]

So, air rifles like the Disco and B-50's valves stay open longer, thus letting a longer rush of lower pressure air into the chamber/barrel.  This should be the same "quantity" of air as the 850 uses at 1400psi, even though it uses a shorter duration rush of air.  

I would think for a given tank size at a given fill pressure, a Disco should be able to use the remaining pressure from when the 1400psi regulator is not a factor down to it's lowest effective pressure (1000psi?), equalling more shots per fill.

I'm still deciding which rifle I want to use as a starting place.  I need great accuracy, good power, reliability and good shot count.

[ribbonstone]

Average PCP will find a pressure range where the forces kind of counterbalance (the sweet spot).  Using a Disco. sized air tube as an example, it might have a run of 20 shots where the vel. doesn't vary more than 20 feet per second, say from 1800PSI to 1100PSI.

So over that 700 psi range, the forces counterbalance. As the air pressure decreases, the  valve's dwell time increases, and the result is about the same vel. over that pressure range.  At fill pressures higher than 1800 psi, the force of the air pressure is too much greater than the force of the striker, giving a very short valve dwell,and vel. is lower.  At some even higher pressure, just like a too-hot co2 gun, the force of the striker is totally overcome by the force of the gas pressure, and the valve is locked up releasing no gas at all when fired.  At pressures lower than 1100 psi, the valve is at max. opening (longest dwell time).

With a regulated HPA supply of air (lets say produced by a paintball type regulated HPA tank) is that the pressure the valve needs to open against never changes, at least until the pressure in the regulated tank reaches the regulator pressure...then, like you mentioned, it becomes a low pressure PCP and has to try to self regulate.

Problem for both PCP and HPA guns is this: the valve opens a lot faster than it closes.  To close, the valve stem has to push the striker back, being resisted by both the weight of the striker and the spring tension that powered it.  OF the two, it's the length of the opening movement of the valve that does the most for the pellet, that longer slower closing stroke is mostly wasting air.  One of the reasons that as power increases, gas use really-really increases.  At some point, no matter how much energy is put into the striker, vel. doesn't increase but gas use does.

I try not to let the air tank get to regulator pressure.  On an 850psi out put tank, will stop shooting by 1000-950psi.  On a 1200psi out put HPA tank, will try to stop shooting by 1300PSI.  There is an area right around the regulator's set out put, where the regulators seem to regulate inconsistently, and velocity gets erratic.