BBs and FPS

Discussion in 'General Airsoft Discussion' started by stxrus, Dec 15, 2012.

  1. stxrus

    stxrus New Member

    482
    0
    Cotton Valley
    can't find any info on this, so ... if a gun fires at 400 FPS with .20s what would the FPS be with .25s, .28s, .3s etc? is there a formula for this?
     
  2. Knief

    Knief Moderator Staff Member

    13,850
    3
    Ann Arbor
    Yes, there is a formula for it, but but takes a few unit conversions. I've worked up a spread sheet to spit out values if you want me to emailnit to you. For now, you can check out this reference guide I made for Michigan Airsoft rules (350fps, 410 fps, and 550fps).

    [​IMG]

    And this handy converter on Arnie's:

    http://arniesairsoft.co.uk/?filnavn=/articles/fps_limits/fps_calc.htm
     
    Last edited: Dec 15, 2012

  3. LegacyAirsoft

    LegacyAirsoft You can say I tech a little...

    6,146
    8
    Massachusetts
    There is also an app on the iPhone App Store (if you have apple) that allows you to keep track of all your guns, gear, etc. and it also has an fps calculator w/unit conversions. I think it's just called airsoft app
     
  4. SilentVectorX

    SilentVectorX New Member

    597
    1
    San Angelo
    Of course, the conversions the previous two posts showed only work if the muzzle energy stays the same.

    Piston weight and cylinder/barrel volume ratio can affect the actual output with different BB weights.
     
  5. Knief

    Knief Moderator Staff Member

    13,850
    3
    Ann Arbor
    Yes, that's always the caveat. That said, for most guns, especially AEGs, your muzzle energy stays near enough the same to make little difference. That why we allow players to chrono with one weight bb (usually .20s or .25s) and then use whatever they want on the field. P* are an entirely different animal because of the way they work, and the safest bet for them is to chrono them with the bb weight that the players plans on using or the heaviest bb weight around. That's not a concern for 99% of players.
     
  6. SilentVectorX

    SilentVectorX New Member

    597
    1
    San Angelo
    I think Knief has more of an event admin's perspective, while I have more of an AEG technician's perspective. Usually my concern is that I may have to experiment a bit with piston weights and cylinder/barrel ratios to nail down a precise muzzle energy with a specific BB weight during a build or upgrade.
     
  7. McGee

    McGee New Member

    464
    0
    San Francisco
    I don't understand - if the P* shoots under 400 fps with .20 bb, wouldn't that mean the velocity (air pressure, essentially) is low enough to prevent problems even when they use heavier BB?

    How is it different between a polar star and an AEG?
    (I need to know because I'm a field admin).
     
  8. Knief

    Knief Moderator Staff Member

    13,850
    3
    Ann Arbor
    You can do what's called "over voluming" in a P*, which as I understand it, essentially allows for a bigger burst of air to drive the bb. In simple terms, it means the air pushes the bb harder for longer. The effect is more pronounced on a heavier bb because it starts out slower and thus stays in the barrel longer. That allows that bigger burst of air to impart more force and thus greater energy onto it. To be clear, it's not that the pressure released is higher, it's that more air comes out. Normal AEGs don't do this because of the way the air is driven. The quickness that the air leaves the nozzle is determined by the stroke of the piston, which takes some time. This is also why putting a longer barrel into an AEG doesn't up the velocity significantly, like it would with a real rifle. Normal GBB rifles don't have a similar effect because the amount of gas released is determined by a mechanical valve which is (at least as much as any airsoft company chooses to) optimized to get a bb down the barrel and not much more, preserving as much gas in the reservoir as possible. That is to say, GBB rifles aren't over volumed the way that you can tune a P* to be over volumed.
     
  9. McGee

    McGee New Member

    464
    0
    San Francisco
    That's basically turning up the gas, while keeping the same nozzle, right?

    Since the published range of the blue nozzle includes FPS above 400 fps with .20g, Polar Stars will always be the source of complaints, deserved or not...
     
  10. stxrus

    stxrus New Member

    482
    0
    Cotton Valley
    normally i would just say "thank you" but the 20 character rule dictates i must type stuff that is not actually revelant to the answer.

    agan, thaks guys and especially Knief
     
  11. AirsoftimusPrime

    AirsoftimusPrime New Member

    2,482
    0
    Satellite Beach
    THIS.

    Polarstars, for example, will not apply to the conversions because of overvoluming.

    For example- my P* at one point fired 400 FPS with .2s. On the same settings it shot 370FPS with a .3. That's 1.5J to 1.9J on the same settings- it takes 450FPS on a .2 to get to 1.9J. This means in the hands of an idiot who decides to spend $1k+ on something he doesn't understand, it's dangerous. However, a bore-up cylinder will do the same thing at a less extreme level.
     
    Last edited: Dec 16, 2012