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Thread: Gas piston affect on swept volume

  1. #1
    Sharp Shooter

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    Default Gas piston affect on swept volume

    Evening
    Did a search but didn't come up with anything.
    Does any one know how a gas piston affects swept volume compared to a spring? Like a formula or anything.
    Example: I have a air rile with 55cc swept volume and with a spring I get lets say 650 FPS with a 14.5 gr pellet. Is there way to calculate what possible FPS could be achieved with same pellets with a 450 newton, then a 750 newton and then a 1250 newton gas spring without having to assemble and test fire all three?
    I would assume that the gas spring would compress and push the air faster than a spring?
    Thanks
    Tim
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  2. #2
    Sharp Shooter
    The Professor

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    I am not aware of a calculation for what you are looking for.

    A gas ram does not affect the swept volume, that remains constant being the volume of the cylinder in front of the piston.

    A gas ram may compress the air quicker, but there comes a point where no matter how fast the air is compressed you will not achieve any extra velocity.

    The power that a spring (or gas ram) air rifle can generate is not only down to how strong the spring is, it is also controlled by the volume of air available to be compressed.

    Take by way of an example a BSA Meteor - the gun gives around 520 fps with its factory spring.

    Cramming a bigger stronger spring is not going to give much extra because there simply isn't enough air available to give extra velocity.

    All the extra spring will achieve is create more recoil and over stress the gun.
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  3. #3
    Sharp Shooter

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    I agree 100% with Dale. But I just want to add, in order to increase velocity, you need more air to be compressed to a higher pressure, so a stiffer/bigger spring is needed. Then also a larger transfer port to allow the larger air volume out quicker(not to fast either) and lastly, a longer barrel. This is because the compressed air volume needs to "act on the pellet" for a longer period of time to transfer as much of its energy as possible to the pellet to increase its velocity. It's pointless having a huge supply of highly compressed air and the pellet has left the barrel before it's transferred its energy to the pellet. So now you have a rifle with a large heavy piston, a very very stiff spring and a really long barrel, all this adds up to is a hugely inaccurate disaster because of the massive recoil and inherently long lock time because of the extra long barrel.

    The way I see it is that the manufacturers have done their math (the good ones at least) and have come up with guns that shoot the best for what the are and have been designed for.

    So if you want more power and velocity with good accuracy, get a PCP. If you wanna have fun, plinking, targets and such, get a Springer, just don't expect it so shoot as well or as powerful as a PCP, leave the math to the manufacturing boffs, just shoot and have stress free fun!!

    PS, I know this went way off topic, but i just threw it in anyway.

    Cheers
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  4. #4
    Sharp Shooter
    The Professor

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    Quote Originally Posted by madness View Post
    in order to increase velocity, you need more air to be compressed to a higher pressure, so a stiffer/bigger spring is needed. Then also a larger transfer port to allow the larger air volume out quicker(not to fast either) and lastly, a longer barrel.
    This is only partly correct.

    It is quite correct to say to increase velocity more air is required, however, you don't always need a bigger spring.

    Once again using the BSA Meteor as an example, the older models had a detachable piston head with an o-ring seal, the forward face of this head featured a radiused face which gave a measure of lost volume. I have machined these heads to a flat face that had two effects, it removed the lost volume and increased the overall swept volume of the cylinder. Using a factory standard spring this modification effected a useful increase in velocity and hence power without making the gun unpleasant or over stressing the design.

    Another gun to consider is the AA TX200, consider a Mk2 version with the shorter stroke piston, the standard factory spring will give an energy output of approximately 12 ft.lbs, now switch out the Mk2 piston for the long stroke Mk3 piston whilst retaining the Mk2 spring, the power will increase appreciably.

    Transfer port sizing can be a bit of a dark art, what seems obvious in theory does not always work in practice.

    As to power and accuracy with a spring rifle, if you want higher power then get a good rifle that is designed for the power - a Weihrauch HW80 is designed to be running at around 18-20 ft.lbs of muzzle energy and when correctly set up will be a sweet shooter at that power level.
    Last edited by Dale; 21-09-17 at 07:46.
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  5. #5
    Sharp Shooter

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    Prospero Internal Ballistics and Spring Calculator (Excel)

    May be what you need.
    Does not take into account transfer port size. As shown by others in the past, the transfer port does influence muzzle velocity contrary to common logic at times.
    Last edited by Jan Frederick; 23-09-17 at 14:00.
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