How an airsoft gun really works - the physics

[ThunderCactus]

So I'll break this down into components, and explain the importance's and misconceptions of each part.

Muzzle energy is like saying fps, but more accurate. Instead of comparing one fps to another, I'll just state muzzle energy in joules since it's more comprehensible.
1.48j is 400fps on a .20
1.6j is 420fps on a .20
2.04j is 470fps on a .20

"Joule creep" is explained under barrels, and is the reason we should NOT chrono with .20g BBs


Starting with the 'mechbox'!
Your mechbox, polarstar fusion engine, whatever a daytonagun has, GBBR valves and whatnot, supplies a burst of compressed air to shoot the BB.
No matter what type of platform you are using, they all do the same thing with varying amounts of pressure, volume, and expansion rate, generally to achieve the same muzzle energy.

The main importance of a mechbox, is that is seals as perfectly as possible.
So the maximum amount of compressed air available is sent down the barrel, and also because leaks cause turbulent air flow and can affect accuracy.

Few things to note
1) It doesn't matter what's in your mechbox. If it's a stock crossman piece of crap mechbox and it compresses perfectly, it'll shoot as well as a $600 mechbox.
2) Once your mechbox is compressing perfectly, all other upgrade parts are in their simply to increase rate of fire or durability.
3) No matter what a polarstar user tells you, their gun does not apply any different physics to a BB. They use the same method of compressed air to shoot a BB, they simply have more control over the air volume and air pressure. If they had more pressure, or better compression behind a BB, it would result in higher fps.
4) Exception to the above rule, polarstars always win on longer barrels. The limitation of an AEG is it's set cylinder air volume and inability to maintain a constant pressure behind the BB on a long barrel. *but in order to maintain the same pressure behind a BB in a long barrel as you would in a short barrel, the muzzle energy would be MUCH higher.
5) Due to ported cylinders, the piston accelerates faster before starting compression, resulting in higher initial pressure.
6) If an HPA gun is shooting 1.6j at 90psi at an air volume that produces no joule creep, then an AEG with the same barrel shooting 1.6j, also producing no joule creep, is also averaging the same average air pressure inside.


The hop chamber & hop rubber
The most critical part in getting range and accuracy is the hop rubber. It's also the least expensive performance upgrade!
At ranges of 200+ft, the slightest little defects in the hop rubber will be apparent.

The hopup works by imparting backspin on a BB as it gets shot past the hop rubber. This backspin induces what's called the magnus effect.
The magnus effect is described here
The slower a projectile is moving, the more effect the lift will have. This is why when you slightly overhop a BB, there is a hump late in the flight path instead of it leaving the barrel at an upward angle.

For tuning, if your BB flies straight, then ends it's path by flaying off in a random direction, you need to use heavier ammo. Technically you do want it to fall short. At the end of a straight path of every BB, the forward velocity will become so slow that the magnus effect will take over completely, and depending how much the backspin has degraded, the BB is now very likely wobbling due to changing air conditions applying friction at differing amounts on the BB, and so combing the wobble and magnus effect that increases as forward velocity decreases: it'll veer off in a random direction. You want to balance between gravity and the magnus effect; Ideally, your BB should fly as far as possible, but succumb to the force of gravity before the BB flays out randomly. Using a BB that's too heavy will simply shorten your range.

Rhops and flat hops provide a longer, flat contact patch for the BB instead of a single round nub or fins. The longer contact area applies less pressure over a longer area to provide the same amount of backspin. Between Rhop and flat hop, flat hop tends to wear out faster, but the Rhop's performance is entirely based on the tech's ability to form the patch accurately.



The BB
Heavier ammo is always better.
A very common misconception is that heavy ammo travels slower. Although initially true, comparing a .20 at 420fps against a .30 at 340fps, after about 100ft or so, the .30 will actually be traveling FASTER than the .20
Heavy projectiles lose their inertia less quickly than lighter projectiles. This is also the main reason why you shoot targets 2km away with big heavy rounds, like .338 lapua, instead of 5.56x45.
Of course having a heavy round also helps fight windage and deflection due to foliage.
Because heavier objects lose their inertia less quickly, a heavy round also loses it's backspin less quickly, which means more consistent range and accuracy.
So running the heaviest ammo possible is always best for range and accuracy.
When you get to a BB that's TOO heavy, you'll notice it falls short. For example you change from .25s to .36s in a stock gun and lose 60ft of range.
This is because the hop rubber can't impart enough backspin on the BB to lift it to the same range, OR the BB is running out of forward momentum.
My classic example is a 300fps stock GBB pistol versus a 320fps stock marui aeg. You put .30s in both and the pistol will get better range every time.
Now there's two reasons for it; first and most likely, the rubber just can't apply enough pressure to get the friction to impart the proper backspin without jamming the AEG. The second, is that your gun just can't supply enough air PRESSURE to push the BB past the rubber, due to air leaks, running an unported cylinder, etc. So you lose too much muzzle energy just trying to get the BB past the hop rubber.
Upgraded hop rubbers can help greatly, but flat hops and Rhop is best. Due to imparting less pressure over a longer area, you can shoot a heavy BB with less resistance from the hopup, reducing fps loss from hopping heavier rounds, and reducing inconsistency.
Muzzle energy does factor in here, as heavier rounds lose their inertia less quickly, it's more beneficial to have higher muzzle energy on heavy rounds since they will travel further. BUT there are plateau's to the performance of each weight.
For example the difference between 1.6j and 2.04j (50fps) using .30g BBs is around 20-30ft. However using .36g BBs at 2.04j gets you an extra ~50-75ft of range.
So match your muzzle energy to your BB, run the heaviest BB you can without losing range, run light ammo only in CQB.


The Barrel!!
Okay so LOTS of misconception on what a barrel does, how it does it, what kind of barrel to use, and how much effect a barrel REALLY has on the BB.

So this is the theoretical explanation of how a barrel works;
Compressed air shoots the BB down the barrel, and the BB is centered in the middle of the barrel as it's moving by a ring cushion of compressed air between it and the barrel wall, meanwhile the decompressing air behind the BB is propelling it forward. Some air bleeds past the BB and barrel wall as the BB travels down the barrel, which is more noticeable in widebore barrels.
One thing that's critical to understand, is that once a BB has become stabilized (centered) in a barrel, it can only ever become DE-stabilized from that point. So for example if a BB stabilizes itself after 1.5" of barrel, and you have a 20" barrel, you run the risk of actually LOSING accuracy because now it has to travel another 18.5" without being affected by barrel fouling.


Barrel length
There's an optimal point where a BB is stabilized. Air pressure, volume, laminar flow, BB weight and hop rubber all have an affect on that. Safe to say it's different with every gun, but it's proven over a variety of platforms to be in the <3" range. The longer your barrel, the less air pressure is behind the BB for a longer period of time, the more time it may take to stabilize. Important to note; although the BB centers itself in a very short distance, the remaining distance is necessary to accelerate it up to speed. But if you can consistently get it up to 400fps in a 10" barrel, then there's no advantage to using double that length to accomplish the same thing.
GBBR's have higher initial pressure, making the BB stabilize in a very short distance. WE PDW's and kriss vectors and gas MP7s tend to get very impressive range and accuracy despite their short barrels.
With PTW's (which are internally the same as AEG's), we've noted better accuracy with 9" and 12" (220-300) barrels as opposed to 14.5" and 20" (again, less room for something to go wrong).
The case and point of the barrel length not making any difference in accuracy is in pistols. If you ever see anyone with a good pistol, try bench testing the accuracy on it. Brace the pistol and see it's grouping at range with .25s, .28s, or .30s. I've seen pistols with tiny 3" groupings at 160ft.
The reason many people think the short barrels on pistols are inaccurate are because of the people shooting them. It's VERY difficult to be accurate with a pistol, but I assure you that at least 90% of the poor accuracy at range is due to the user's accuracy, not the pistol's.

If you have two 1.48j guns, one with a 20" barrel and one with a 9" barrel, the setup on the 9" barrel pushes that BB out with a lot higher pressure, and therefore stabilizes that BB in the center of the barrel much faster. Whereas the 20" barrel has lower pressure to center the BB, it runs a greater risk of bouncing around. So with higher FPS guns you can get away with running a longer barrel since you'll maintain higher pressures to center the BB, but ultimately shorter barrels are better overall. They also reduce the risk of inaccuracy due to fouling; the crap that builds up in the barrel.

The long barrel = more range myth comes about from two things; when people would upgrade their barrels they'd do the hop rubber at the same time and falsely attribute their massive range and accuracy gains to their barrel instead of the thing actually doing all the work; the hop rubber. And second, in real firearms a longer barrel often means a higher muzzle velocity. More velocity means more energy, more energy means more range (need long barrels to accelerate those heavy ass rounds). Lengthening the barrel would normally allow us more length to accelerate a BB to increase the muzzle energy (and therefore range), but in airsoft we have a muzzle energy LIMIT (which is relatively speaking, quite low). And due to the limit, longer barrels become increasingly unnecessary, especially since the longer length doesn't contribute at all to accuracy. So if you're getting 1.6j on a 200mm barrel, then having a 650mm barrel at 1.6j won't make any difference.


Magnus effect IN the barrel (myth)
Okay so someone started an idea that the BB rolls along the top of the barrel as it's being pushed through due to the magnus effect, and that somehow widebore barrels help this happen. The concept is that this is beneficial in that it makes the backspin more consistent by dragging it to the same speed as the BB. The concept is sound, except for the fact this would invariably create fouling along the path of the BB very quickly and ultimately reduce accuracy. And some modded barrels DO force this to happen (they're bent downwards). But this does not happen in normal, straight barrels.
The magnus effect creates an airfoil (like a plane's wing) by moving air around itself in the direction of spin. Because the air can't go underneath and ahead of the BB in flight, it goes straight down, creating the airfoil. The reason this airfoil cannot be formed inside a barrel is because the BB would have to be pulling the air in front of it (atmospheric pressure) and push it into a high pressure zone (all that compressed air pushing it forward). Because of the high pressure pushing the BB and also bleeding forward past it, it's impossible for it to form the airfoil.
The magnus effect also reduces in strength as the velocity increases. Which is also why, when you drastically overhop a high fps gun, the BB only curves up many meters after the BB leaves the barrel.


Joule creep
Part to do with length. Joule creep is when you chrono at 1.34j with a .20g BB, then magically chrono at 1.52j with a .28g
Way back when everything was AEGs, joule creep was pretty much non existent. Between most people running stock guns, and upgrading to the low-volume end of cylinders for their barrels, AEG's almost always lost muzzle energy by using heavier ammo.
It's specifically GBBRs, polarstars, and bolt action rifles that are most commonly susceptible to it. Although AEG's can be made to joule creep a ridiculous amount as well.
It's caused by two things:
1) Having very high pressure behind the BB. A .20g is very easy to get moving, it gets pushed out no problem, the air finishes depressurizing freely once the BB has left the barrel. But a .28g is heavier, takes more effort to get moving, more pressure builds up behind it, and ultimately it ends up accelerating faster than the .20. This effect is most common in short barrelled GBBs such as pistols.
2) Having too much air volume being pushed down a long barrel. Instead of a short burst of 120PSI gradually decompressing and losing pressure as the BB goes down the barrel, the extra air volume maintains that same pressure all the way down the barrel, resulting in higher fps, for the same reason as the first method. This is the more common joule creep apparent in most GBBRs and HPA setups.

This is why chronoing on a .20g BB is highly inaccurate.
The two easiest ways to circumvent this problem are
a) Chrono on the ammo weight the player will be using and hope they don't lie about the weight.
b) Chrono everyone on heavier ammo, like a .30g BB. Although still not perfectly accurate since people typically use .25 to .36g ammo, it gives you a much better idea of what their gun is actually shooting in terms of muzzle energy. Now although joule creep can work backwards (A gun can potentially have higher muzzle energy on a .20 as opposed to a .30), higher muzzle energy is much less dangerous on lighter ammo than heavier ammo because it loses it's inertia more quickly.


Barrel bore
Let me start off by saying, the tanio koba twist barrel is a wide bore. If the rifles in it really did impart sideways rotation on a BB that already has backspin on it, the gun would only ever shoot in a very wide spiral cone pattern. The magnus effect works in every direction, but we only want it to be directly vertical.
The reason they can only be used at 330fps is because at higher fps, the BB is traveling so fast through the barrel that the air in the grooves simply can't keep up to it, they need to travel about 20% faster to maintain the same speed as the BB. So when there's no pressure in those grooves, it gets filled by the ring cushion of compressed air around the BB. No cushion around the BB means no stabilizing. So just use a smooth widebore, rifling doesn't work, and doesn't offer any advantage over a smooth bore.
There's lots of conjecture on this subject, but what I've found from actual field testing is that it really doesn't matter until the barrel starts to foul.
AEG's don't have the volume to use 6.23 barrels, so they'd lose performance there.
The idea with the wide bore is that a larger air cushion will somehow improve performance. Although it hasn't shown to HURT performance as long as you have the volume and pressure to use them, it's more susceptible to problems with turbulent air flow. But in the end should be less affected by barrel fouling.
The idea with tightbores, is to increase air pressure around the BB (forcing air from a large volume to a small volume increases it's pressure) and making it stabilize faster. We didn't get that memo, so lots of people tend to run 500mm 6.01 barrels in their sniper rifles lol
Let me make it perfectly clear, the super tight barrels are for SHORT barrels, like pistols. They lose less air around the BB, making compressive forces higher and more efficient use of the air as well. HOWEVER, they're far more prone to fouling and losing accuracy quickly.
Between the 6.23 and 6.03 in the few polarstars I've seen, I couldn't notice any real difference in accuracy or range, just gas consumption.
With AEG's, a good quality stock 6.08 can potentially be just as good as a prometheus 6.03.
With ptw's, I haven't noticed any difference between my 6.10 orga and 6.04 stock barrel on the same hop roller.
Basically just go tighter as you get shorter, try not to run anything smaller than a 6.03, AEG's shouldn't use the 6.23s, and make sure you have enough air volume if you use a 6.13
As for fouling, If you shoot more, get a widebore. If you shoot less, tightbore.


Barrel QUALITY
This makes ALL the difference in barrels. ALL OF IT.
I've seen my 420mm 6.03 vsr barrel shoot as well as a 6.10 363mm, a 6.23 509mm, and a 6.01 460. Basically as long as you have a quality bore and the right force behind the BB, all barrels will perform the same until they start fouling.
As far as I'm concerned, the highest quality barrels out there (for ptw and AEG) are the prometheus 6.03, PDI 6.05, orga 6.13, stock systema ptw 6.04 and orga 6.10 for ptw. They have the best bore quality, and pick whatever bore you need and length you want.
A madbull barrel is an improvement over a stock barrel, but don't kid yourself, it's aluminum, it scratches, it's not as good and not nearly as durable as the aforementioned barrels. The price is attractive, but one scratch is all it takes for you to lose your money.
Brass is good, stainless is better, stay away from aluminum.

*There's been a lot of hating on brass as a barrel material. The reason for this is because people refer to STOCK barrels as BRASS barrels. However the defects of the stock barrels are MANUFACTURING defects and have nothing to do with the actual material. Brass is just fine as a barrel material.


Long barrels, short barrels, wide or tight bore, all have the potential to shoot exactly the same before fouling.
My kriss vector with it's teeny 6.05x5" barrel gets the same range and close to the same accuracy as my 6.04x9" and 6.10x16" ptws
Experience with my 6.03 VSR barrel was that fouling can start to affect accuracy after as few as 130 rounds, so clean your damn barrels after every game. Use windex. or alcohol. Acetone is best but don't let it touch your hop rubber!
I don't know how or why this started but it doesn't make the least bit of sense; people have said to clean your barrel with silicone oil.
The BB never touches the barrel wall, it does not need lubricant, lubricant will only help accumulate dust and foul the barrel faster.
Use windex, remove all the grease and residue from the barrel, you want it to be a clean, polished metal surface, free of any contaminants.

And lastly, there are very few physics principle that transfer over from real steel, so please stop comparing the two. Rifled rounds DO NOT WORK in airsoft.
Airsoft principles are solely based on stabilizing a projectile at very LOW speeds.
Real steel principles are solely based on stabilizing a round at very HIGH speeds. Nobody cares what a bullet does when it's no longer going fast enough to kill something. But we ONLY care about what a projectile does when it's not going fast enough to kill something. Hopup would work on a muzzle loader with ball rounds, but the ball would have to be travelling at sub-lethal speeds for it to have any effect.

[Dracheous]

You're just missing the notation of pixies and unicorn horns ground into powder to make boom sauce. Otherwise a great post with plenty of base information.


There are plenty of older stickies that have far more detailed information; but tend to be item specific. This does a nice job of cutting some fat and simplifying to those who are not looking for the science on their hop up and barrel builds from that what's his name guy :P ((Just kiddin' Illusion!))

[ThunderCactus]

well it IS the noob section lol

[Rusty Lugnuts]

the tanio koba twist barrel has rifling in it. maybe to get the bb centered better

[Jaelommiss]

Originally Posted by ThunderCactus

In practice, the BB bounces quite a bit inside due to the hop rubber starting the BB off on the wall of the barrel and the magnus effect messes with the air current around the BB, but it ultimately becomes centered in the middle of the barrel.

One thing that's critical to understand, is that once a BB has become stabilized (centered) in a barrel, it can only ever become DE-stabilized from that point. So for example if a BB stabilizes itself after 9" of barrel, and you have a 20" barrel, you run the risk of actually LOSING accuracy!

Is it possible to find out how long it takes for the BB to stabilize without testing dozens of barrels? It seems like this would be extremely important to know before you buy a new barrel.

[ThunderCactus]

Like I said, it depends heavily on your setup.
For testing, pick the length of barrel you want, you need .20s, .30s (or whatever weight you wanna use), one barrel with Rhop (since it doesn't make sense to have 8 Rhopped barrels), and a few ported cylinders of different ports (diff brands have diff volumes too, so you'd have to be real picky).
Change cylinders until you get joule creep with the heavier ammo, more creep is better. It means there's constant pressure behind the BB, which means it's getting optimal pressure for the length of the barrel. There IS a point where you'll lose fps, where the air won't be able to reach peak compression before the BB leaves the barrel.

Otherwise, you're talking about the minimal distance point of stabilization that's possible. That would require a few barrels and cylinders and BBs to do the same test above several times. You'd be tracking 20-30 rounds on each barrel and cylinder for range and accuracy until you find the point of diminishing returns.

The TK twist barrel has rifles to allow more air to center the BB in the barrel. Just like a widebore does. The actual rifling makes no difference in the range or accuracy what so ever, it's just a gimmick that happened to function like a widebore. If the rifling DID impart any sort of spin on the BB, you'd notice it, and your gun would be horribly inaccurate. You're also trying to impart spin on an object that's already spinning really fast in one direction, so like a gyro, the BB actually resists any other spin that the rifles are trying to put on it.

[Rusty Lugnuts]

I don't believe the rifling is a gimmick, those twist barrels work amazing, other than that we are in total agreement

[Drakker]

HS5 actually came up with evidence that the BB rides at the top of the barrel, that is why a smooth and quality barrel is important, any defect at the top of the barrel will deflect the BB and it will bounce on the sides of the barrel, altering the BB's backspin and lowering accuracy. According to this hypothesis, the longer the barrel, the bigger the chances that the BB will encounter a defect along the way, which is why longer barrel tend to lower accuracy and shorter barrels tend to have the BB fly up higher before stabilizing on a straight line.

[ThunderCactus]

Originally Posted by Rusty Lugnuts

I don't believe the rifling is a gimmick, those twist barrels work amazing, other than that we are in total agreement

Work amazing as compared to what?
How do they work against a PDI 6.05 or orga 6.13?
I've seen them in action myself, I didn't think they did any better of a job than a prometheus 6.03

[DipTwit]

Cool writeup, very helpful stuff in there. Only niggly points to mention are:

You list 5 very TRUE statements, but label them "common misconceptions"... but a common MISconception is by definition something that people often *think* is true but *isn't*

You describe the magnus effect as being the result of faster air moving over the top of the bb (much like an airplane wing) but that's neither true, nor the magnus effect. (The *bottom* of the bb is moving forward faster relative to the air not the top. The magnus effect is due to lift generated by the air overtop of the bb being directed downward as it goes behind the bb, as mentioned in the wikipedia article:

It is an angular deflection in the fluid flow, aft of the body.
http://en.wikipedia.org/wiki/Magnus_effect

Still *great* to have this stuff written down, kudos!

[ThunderCactus]

my bad, fixed

[jordan7831]

Originally Posted by Rusty Lugnuts

I don't believe the rifling is a gimmick, those twist barrels work amazing, other than that we are in total agreement

I thought the twist barrels channel the air around the bb, but do not physically touch the bb itself.

[Ricochet]

Rifling in an airsoft is either counterintuitive or pointless. Some years ago they had a short fad as someone thought "hey, they work in real guns, why not in airsoft". The problem is two things, well more than two, but only two really matter.

First: The idea behind rifling is to create a spin on the projectile of clockwise or counterclockwise to help with stability in flight. BBs from an airsoft gun are given backspin by the hop-up unit to give them stability. Adding a second type of spin doesn't do anything but risk destabilizing the projectile once it leaves the barrel.

Second: Rifling a orb projectile is not going to promote stability. BBs are totally spherical, so they will want to fly agains their spin. The backspin of the hop-up is there to fight gravity, adding a sideways spin will make the BB want to fly sideways. That's why objects that are spun, such as bullets and footballs, have a point on them to help stabilize forward with a spin from rifling.

So in short, you "rifled airsoft barrel" that shoots straight, is having no effect beyond a normal barrel. If it did, your BBs would be screwing off on you, literally. The other theory is that due to the rifling inside the barrel you are helping to impede the air cushion because now it can escape past the BB through the rifling.

[Rusty Lugnuts]

Originally Posted by ThunderCactus

Work amazing as compared to what?
How do they work against a PDI 6.05 or orga 6.13?
I've seen them in action myself, I didn't think they did any better of a job than a prometheus 6.03

IF you look back in my posts you will see i sold my Orga barrel in the classifieds, further back i sold a gun with a madbull black python ver 2 barrel, further back an m4 with a ss tightbore ( i think it was Airsoft Surgeon but i don't remember for sure) anyway my point is i have played around with barrels a bit. So lets not think that i have no experience in that area. AND i never said or thought that the rifling spins the bb, i think it helps smooth the air escaping past the bb. you have your opinion and i have mine ( and a twist barrel . This is my last post on the subject

[DipTwit]

The only thing that "rifling" the air around the bb could possibly do is increase the air pressure of the air 'pocket' which, although it sounds like it might have a positive effect, probably would make a minuscule difference. Practically speaking, the direction of it should (as stated umpteen times) negatively affect the bb's spin.

My guess is that, aside from the wider bore, you're getting other properties that are so small they are pretty much snake oil - you *want* to believe in them, but you can't measure a difference in reality.

There's a lot of other snake oils being sold in Airsoft...