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Aerodynamics: airsoft BB travel down a long metal barrel

  1. Mar 29, 2012 #1
    Hello PF,

    I come to you from the airsofting community with a question that has plagued much of the community for years. I hope to get some feedback from the physics community to help direct or debunk some of the "theories" floating around this issue.

    So, airsoft is a sport where participants essentially operate mock replica firearms and participate in re-enactments, war games, scenarios, etc. The replicas, while varied in their sources of power, they all end up doing one thing, shooting a spherical BB down a long tubular barrel, in the hopes of "hitting" or "tagging" an enemy player, thus eliminating them from the game.

    For reference, here is a link to GIF of how an electric powered airsoft gun works. The gray "tube" on the right side of the GIF is the beginning of the barrel. http://en.wikipedia.org/wiki/File:AEG.gif

    Typical airsoft BB mass's are 0.20g, 0.25g, 0.28g, 0.30g. Sniper rifles tend to use larger mass BB's up to around 0.40g. The difference in mass is that, while a lower mass BB will travel faster, it is more prone to environmental effects such as wind blowing it off course or brush blocking its flight path. Heavier mass BB's travel slower and have a sightly smaller range, but typically have a much more stable flight trajectory.

    Typical airsoft BB velocities are standardized at 350 feet per second with a 0.20g BB, 410 feet per second with a 0.20 gram BB, and 550 feet per second with a 0.20g BB. Most players replicas are tested or "Chronographed" using 0.20g BB's, in order to be sure their replicas are not overly powered. They will then switch to a larger mass BB for the duration of the game.

    A typical airsoft BB's diameter is 5.95 mm +/-.01 mm (assuming a high quality of BB manufacturing). Barrels come in varying bore widths. Normal bore barrels are around 6.05 mm. Tight Bore Barrels (TBB) are typically 6.03 mm, but can go as low as 6.01 mm, or even 6.00 mm in diameter. Recently, Wide Bore Barrels (WBB) have begun to surface, some as large as 6.23 mm, and custom barrels can have an even larger bore.

    Barrel lengths tend to vary greatly between replicas, anywhere from 200 mm - 510mm or more.

    The last facet of understanding (or so I hope) is that airsoft replicas also have a piece of hardware mounted on/around the barrel known as the "hop-up". The hop up essentially presses a small piece of rubber down onto the top of the barrel so that as the BB passes by, it will impact this rubber. The rubber bucking then induces a back-spin on the BB, causing it to cut into the air a bit and increase its potential range by staying aloft for a longer period of time due to its altered trajectory "hopping up" the shot.

    The question is: What barrel bore size is best for optimal range, velocity, and precision?

    Current thinking is that there is "too tight" of a barrel. 6.03 mm is generally accepted to be the optimal TBB size. 6.01 mm barrel's are generally stated to be too tight around the BB, inducing extra friction and impact on the BB inside of the barrel, causing it to lose velocity and also creating unwanted random "spin" so that when the BB exits the barrel it will curve off away from its intended target. Recently, wider bore barrels have been experimented with, and are surfacing on the market. The manufacturers claim that the wider bore creates a much larger air cushion around the spherical BB projectile as it rides down the barrel, thus keeping it away from the walls and creating a much more stable BB flight time.

    Here is an example of one such manufacturers claim:
    http://orga-airsoft.com/is-wider-better/ [Broken]

    The material and quality of the barrel also comes into play, but for the sake of the problem we should assume that it is highly polished brass. We should also assume that the player has maintained his/her barrel and that it isn't scratched internally.

    The velocity of the BB is not much of an issue because there are ways of increasing the energy behind the BB, thus compensating for any velocity loss (or vice versa if a tighter bore barrel increases your velocity). Range and precision seem to be the key issues. The farther the range is, the more BB's tend to start flying all over the place! The optimal situation would be that the BB's would all travel in a nice straight line, eventually all falling/dropping to the ground around/in the smallest space possible. Maintaining consistency between shots (having them all land in relatively the same spot) and also maintaining some sort of effective range is the problem, and when it comes to barrels there is currently no clear solution.

    Nobody has shown any actual science behind the choice of barrel. I was hoping that the PF community could shed some light on this matter.

    Please let me know if you require further information. I am new to these forums, and would be happy to flesh out the question a bit more. Thanks!
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Mar 30, 2012 #2
    Questions like this often involve such fine nuance that an answer arrived at theoreticly is often wrong. The best way to answer this question would be with a carefully designed experiment.
  4. Mar 30, 2012 #3


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    Science Advisor

    If I understand it correctly, power isn't an issue. If you have too much friction, just up the pressure a bit. If you are calibrating for maximum allowed speed anyways, seems like it's not an issue. Besides, you are losing just as much, if not more, on a wider barrel.

    Random spin is a problem. For that, wider barrel + hop-up seems like a decent solution. However, what would be far better is rifling. Back-spin creates lift, which helps carry the BB a bit further at shallow angle, but without back-spin, you can always aim a bit higher, and get the same range. Stability of flight with back-spin, however, suffers, compared to traditional spin imparted by rifling.

    It seems to me that if you have excess of pressure, you can rifle the barrel, causing the air stream that pushes the BB to rotate, imparting that rotation to the projectile. You'd still be losing some air, like with a wider barrel, but you'd be getting much better stability and quite possible an improved range.

    These are just thoughts based on basic aerodynamics. mrspeedybob is right. This is the sort of thing you'll only know for sure with testing. But if you happen to have a tight barrel and tools to rifle it, I'd give it a shot.
  5. Mar 30, 2012 #4


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    Gold Member

    The BBs are not all the same - ie the spherical tolerance is one thing you have to keep in mind. I doubt that are manufactured to the same tolerance as what you might find with the balls of a bearing to keep the cost of shot down.

    No matter how much engineering you do to the barrel, the shot itself is responsible for some of the distribution centered around a target, as each shot will behave differently once having exited the barrel. Spherical shot is not the best for long range and accuracy.

    That aspect of round shot was found out long time ago, and shot moved from being circular to elongated and pointed and barrels rifled to bring more stability to the projectile.

    Experimental results would be the best to determine shot distance and accuracy with variations on barrel length, bore and pressure for your situation.
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