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The basic physics of airsoft

The basic physics of airsoft

The biggest contribution to a good performance in an airsoft game obviously is tactics and team play as most experienced players would probably agree, yet there are still other means to maximize your chances of success on the battlefield. Amongst various forms of tactical equipment and accessories to give you the “unfair advantage” the biggest factor here is the range and accuracy of your gun. And quite nothing is as surrounded by myths and theories as the range of an airsoft gun. Therefore I would like to present some basic facts around the physics of airsoft and maybe bust some myths or at least share some interesting information.

Velocity
The first factor that comes to mind when talking about range (besides Hopup) is the muzzle velocity, mostly calculated in feet per second (fps). A higher fps value equals to more range, however this correlation is not linear as the drag force, and therefore the deceleration, increases squared compared to the velocity. This means that the gain of range of e.g. an increase of the velocity from 300 to 350 fps is greater than what can be gained by an additional increase of another 50 fps and so on.

Kinetic Energy
Relevant for safety however is not the velocity but the kinetic energy of the BB, which is indicated in Joule and is calculated as followed:

Ek = ½ m v²         (m = mass, v = velocity)

Therefore velocity limits only make sense when related to a specific BB weight, which at most game sites and events is usually 0.2 g, unless stated otherwise. So a limit of 100 m/s (about 328 fps) e.g. translates to 1 Joule of muzzle energy. If heavier BBs are used in a gun with a set spring rate, the velocity drops accordingly and the muzzle energy stays constant. Therefore you can calculate your maximum velocity for various BB weights at a given energy limit. This can be done even more easily with a premade calculator like the one from Begadi:

http://www.begadishop.de/calculator/begadi-calculator.htm

There is also an android version available, if you want to always have it at hand in your mobile phone:

http://www.androidzoom.com/android_applications/tools/airsoft-fps-calc-basic_qvzx.html

Impact Energy
To go more into detail about safety, what actually matters here is of course not the muzzle energy but the impact energy. Especially when playing with higher muzzle energies organizers often use minimum engagement rules, restricting the distance within that a player may be shot at to a certain minimum in order to compensate the high muzzle energies with range. However these distances are rarely calculated exactly to result in a set maximum of impact energy but are mostly rather a rule of thumb. The tricky part about these rules is that the correct calculation of a reasonable minimal shooting distance would actually need to factor in the used BB weight, as greater masses decelerate slower. This means that even though a heavier BB will travel slower (given equal muzzle energy), it will maintain its speed longer and therefore will deliver the same impact energy at a further distance than a lighter BB would be able to. Furthermore this also means that a heavier BB will have more range, contrary to the widespread conception that a heavier BB will experience more gravitational force and therefore hit the ground sooner, the decline of deceleration will actually overcompensate gravity. The gain of range might be minimal but at least one can assume that a heavier BB will never lose any range. For more details have a look at the Minimum Safe Engagement Distance Calculator from BBBastard:

http://msed.bbbastard.com/index.php?option=com_content&view=article&id=53&Itemid=57

Hopup
Probably the biggest contribution to range has the hopup system, consisting of a rubber sleeve (bucking) around the inner barrel and a rubber nub on top of it that is pushed down onto the sleeve by an adjustable arm of the hopup chamber. This leads to a part of the bucking protruding into a hole of the inner barrel, touching the BB as it passes by and delivering a backward spin to it. The physical principle that makes this mechanism help increasing the range is called Magnus effect and is also used in golf and various ball sports.

Magnus Force Effect Hopup Spin Physics


What happens here is that the spin creates a velocity difference between the two sides of the BB surface parallel to the flight path and spinning axis, against the adjacent air layers. At the one side that moves faster against the air layer, a higher pressure is generated, forcing the BB to the side of lower pressure. So if the backspin axis is exactly perpendicular to the direction of the gravitational force, the Magnus effect will compensate gravity to some degree, depending upon the amount of backspin. If too much backspin is applied, the BB will fly upwards resulting in even more range but at a ballistic trajectory. If the spinning axis is not exactly perpendicular to the force of gravity, the BB will drift to the left or right. This is why the engagement angle of the hopup rubber is so important. To improve the stability and angle of engagement there are various designs of both rubber sleeves and nubs available. However it is difficult to calculate the single one perfect design and it rather comes down to the trial-and-error method for each individual combination of barrel and hopup chamber.

Another important factor regarding the hopup rubber is the hardness and moisture. Of course a wet rubber creates less friction and therefore is inefficient in generating spin, however a totally dry one will wear out, become porous and eventually rip. Therefore a slight bit of silicone oil can be applied to the outside of the bucking, leaving the contact area dry enough, yet still may prolong the lifespan of the rubber. The hardness should correlate with the muzzle velocity of the gun, meaning harder rubbers for faster BBs and vice versa. In general a softer rubber is able to apply more spin, however the faster the BBs are passing by, the faster the rubber will wear out, whereas a harder one is more robust and will last longer. Furthermore a faster BB will receive more spin by its speed alone anyway, making a softer bucking unnecessary.

Barrel length
The barrel length is mostly associated with accuracy, as a longer barrel will stabilize the projectile longer and therefore creates a straighter trajectory. While this is basically true, this effect is most probably a bit overrated in the realm of airsoft. Much more important is the length in relation to the volume of air that is compressed in the cylinder. If the air volume is too high for a certain barrel length, the BB will still be accelerated shortly after it left the barrel, which possibly destabilizes the flight path. If the air volume is too low, the BBs can’t be fully accelerated, wasting potential velocity. Therefore various cylinder designs are available, bore-up ones with a bigger volume as well as those with holes at different areas, reducing the utilized volumes to match the respective barrel length.

Another difference the length can make is the muzzle velocity. The longer the barrel, the later the BB will leave the barrel and the longer it will be accelerated, provided the air volume is sufficient to use the length. Another tricky behavior here is again the difference that various BB weights can make to the muzzle energy. If a heavier and a lighter BB is used in the same barrel, the heavier one is travelling slower and therefore stays longer in the barrel, giving it more time to accelerate (again, provided the air volume is sufficient). This means that a heavier BB, although its muzzle velocity is still lower compared to the lighter one, will have relatively more muzzle velocity than it should have according to the velocity conversion explained in the beginning of the article. Thus it has effectively more muzzle energy than the lighter BB fired from the same gun. This is something that should be considered while checking fps limits for a game. This effect is even more common with gas guns, as the amount of propellant per shot in those guns is most often much higher that than needed in stock configuration, meaning that nearly every increase in barrel length and/or BB weight will result in more muzzle energy.

Barrel diameter

Barrel Diameter Air Cushion Tight Bore


The barrel diameter has quite similar effects. A tighter bore results in less leaked air, meaning that more air is utilized to accelerate the BB, thus it creates a higher velocity. The effects on accuracy however are quite debated. One could assume that a tight barrel will stabilize the BB better than a loose one, however much more relevant is the regularity of the inner surface. Also a certain amount of air cushion is needed to prevent the BB from touching the barrel surface, which would destabilize the trajectory as well as the amount and angle of the backspin, created with the hopup.  So too tight is as bad as too loose.

BB attributes
Pretty much everything has been said about the weight, however there are a few other attributes of BBs that are relevant to their performance, most obviously the roundness. The diameter should suit the used barrel, but the most important part is again the regularity of the surface, the same as it is with the barrel. That includes the smoothness of the surface, although a rough surface will create more friction at the hopup, making more backspin possible, the smoother surface will have less air turbulences and therefore a more stable trajectory.

The very last point of the article is the BB hardness. The hardness does absolutely nothing to the flight characteristics, however it does change the amount of energy transferred to the target, making this a matter of safety. A soft BB, e.g. one out of plastic or starch, will deform upon impact and the amount of energy that is converted to deformation of the projectile will not be transferred to the target. A hard BB though, like those out of aluminum or glass (some of the new BBs are advertized as being out of silica (SiO2) which is basically the same, as ordinary glass is mainly out of silica; in nature silica can be found in the form of quartz), is able to transfer its complete kinetic energy to the target, leading to even more deformation of the target, or in case of the target being as hard as the BB, to breaking of the target. An impact on soft skin will not make any difference, as it is softer than even a plastic BB and thus will be deformed without taking much damage anyway. Problems will arise when hitting glass, metal or hard plastic surfaces, in general objects with similar hardness. These include facial bones that are only covered with a thin layer of skin, full-face masks, safety goggles as well as windscreens of cars. There have been reports of all kind of damages to those objects quite within the velocity limits that are commonly applied while using very hard BBs. Many organizers have thus banned these kinds of BBs from their games, though you might watch out for these possible consequences even when no explicit rules are applied.

 

This was my quick overview of the most basic facts about range, accuracy and safety. I hope you enjoyed it and maybe even got something out of it.

Now this was only scratching the surface though, if you are really interested in the physics of airsoft and want to go as deep as it gets, I recommend studying the Airsoft Trajectory Project at BBBastard. They provide you with all the formulas necessary and did also practical experiments, comparing them with their theoretical results and presenting them in clear, detailed diagrams:

http://msed.bbbastard.com/index.php?option=com_content&view=article&id=54&Itemid=59

 
Article written by Vader