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Airsoft Physics

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As a newbie to airsoft, I've done the usual and trawled through endless threads trying to figure out the best way to make my new toy 'better', and been bamboozled by the varying, contrary and sometimes downright contradictory opinions on what works and what doesn't. And it all seems to come down to the fact that there are large gaps in understanding of what actually happens when you pull the trigger. So I've spent a little time thinking about the mechanics of launching a small ball through the air with some back spin to take advantage of the Magnus Effect, the results of which are noted below. These are pure thought experiments with no evidence to back them up, (yet, but I'm looking for proper science to back some of them up) mixed with practical experience of what works in spring piston air rifles.

 

1

Any spring rifle will be subject to recoil before the projectile has even started moving in the barrel. In a spring air rifle this is so pronounced that if you don't support the rifle in a neutral manner that is identical every time then the effect this has on the recoil will affect the point of impact (POA). Look up 'Artillery hold' for more information. This makes a good argument for trying to ensure that all the different (critical) parts of the assembly are held securely and immovably together.

 

2

If you have a 6.05mm barrel and a 5.97mm BB to give the accepted 'best' clearance of 0.08mm, then almost 3% of the cross sectional area of the barrel is effectively open to gas/ air blow by. So we have to accept that it is not a perfectly sealed system and all the time the BB is being accelerated up the barrel, gas will be passing it. It has to, as the only way the BB can accelerate is if there is more pressure behind it than in front of it. Now, I don't know if the Magnus Effect works in a closed tube, but if we assume that the BB is travelling along the top surface of the barrel (which seems to be supported by observation of wear in barrels) and that it already has backspin imparted by the hop, then that air passing between the BB and the lower surface of the barrel is going to produce a low pressure area, possibly enough to separate the BB from the upper surface of the barrel. What happens then is any ones guess, but I'm going to suggest that as air is able to pass all round the BB and the low pressure area on just one side is replaced by low pressure pulling in all directions equally, the Magnus effect takes over and reattaches the BB to the ceiling of the barrel. In effect the BB 'skips' along the barrel ceiling until it exits. This could also have an effect on side to side variation. In a close fitting barrel the BB would only have to deviate a fraction from the axis of the barrel and as it reattaches to the ceiling its periphery won't touch perpendicular to the axis of spin. It will contact the 'curve' of the ceiling to one side or the other. If however you're using a 6.23 barrel the geometry is is the BBs favour. Imagine rolling a tennis ball down a length of gutter compared to doing the same thing with a marble.

 

3

When the BB is first loaded into, lets call it the chamber, there is a lump of rubber in the way stopping it simply rolling out. So it's going to take a certain amount of pressure built up behind the BB to get it moving. Except it's a far from perfect air seal. Reduce the clearance in the chamber to 0.03mm (ie 6.00mm on a 5.97mm BB) and the gap reduces to less than 1% of the cross sectional area, which means the piston is going to have to travel less distance to create the pressure to give the BB it's initial shove. And if you could accelerate the piston faster to the point where the BB starts to move and give the BB a higher acceleration over the hop...more backspin.

And while we're talking about the hop, there are a dozen plus ways to try an centre the BB under the hop or the hop over the BB. H nubs, M nubs, concave nubs, double arm hop ups, Miracle Barrels etc. Or you could just, maybe, reduce the bore to a tighter clearance at the hop? Eliminate variance in the initial position of the BB and all those issues go away. This incidentally is something I'm going to try, and it would retro fit into pretty much anything.

 

Anyway, that'll do for now. Please feel free to shoot me down in flames, add something, point and laugh, whatever. I'll be interested to hear the thoughts of others more experienced than me. :)

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I've just looked up an online airflow calculator, and that 3% gap on a 6mm diameter bore with 120psi behind it will leak about 900cc of air a second. And I worked that out twice because it seemed so high! And your air cylinder compresses maybe 50cc....

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If a good airseal in cylinder along with a tight bore wouldn't that create a stronger push allowing for more range and a higher muzzle fps? The lower pressure being forced out of the barrel. Then once bb leaves the muzzle it's purely atmospheric conditions and friction that effect it.

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A BB isn't a bullet or an air rifle pellet. If it doesn't have clearance to 'roll' down the barrel it'll just rattle around until it finds its way out. You hear good things about the 6.23mm bore barrels for accuracy, but i'd imagine it takes a lot of air to actually accelerate a BB up one. A BASR is limited to 500fps and that's fairly easily achieved. It's just that I don't think it's being done efficiently or as consistently as it could. So tight in the chamber to give good positioning under the hop (hop is everything,almost) and a really good first shove and backspin, then a more open bore to guide and finish accelerating the BB. And once the BB has reached the desired velocity any extra barrel is worse than useless.

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I know 6.00mm seems like it'll be a problem with jams and dirt etc. but don't forget its only where the hop is, so the flexibility of the hop material should sort that out.

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Fps loss when going from 6.05 to 6.23 is about 55fps so you do have to increase the amount of pressure supplied which is why orga barrels work best with gas or hpa.

 

There is a lot of discussion on whether the bb runs along the top of the barrel of whether it sits more central because of the high air pressure sitting around all sides like a cushion. Not sure it's ever been proved conclusively what actually occurs in barrel.

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Just from observation is why I say this. My logic tells me hop off the bb tends to roll along bottoms or barrel the pressure pushing it forwards a distance till gravity and air pressure start winning.

 

Hop on puts a back spin on the bb. Which would make it raise in the barrel as it travels. Which creates the lift as it travels using the air pressure difference between top and bottom edges.

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You'll have high pressure air behind the BB, but any air passing the BB will be at a lower pressure. It's being accelerated though a gap so pressure will drop as velocity increases (relative to the air around it). Bernoullis Law.

I've heard tale that some users are certain the BB travels along the ceiling, but as you say there doesn't seem to be any hard evidence for this. I'd be interested in finding out!

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I'll keep my reply short, and valid to the OP's points

 

1) it's negligible if anything, as the pistons are very light weight

 

2)tighter tolerance = more chance of a barrel jam (from dust/dirt/etc etc). Bb's are not always to tolerance and can jam a gun (had it happen to me in the past). Yeah you are correct the bb can skip along the top of the barrel, some times this is desired though as it can help stabilise the bb (look up the "long range barrel mod" on the sniper forums).

 

3) this is the reason tight bore barrels give a fps increase.

 

4)Possible but bb tolerances are not always that good, also bb's are not garenteed to be consistent in there makeup, meaning there could be an air pocket in them (cheaper bb brands) that could upset the weight balance/spin off the bb. But the main reason I would imagine is due to it being easier to produce a consistent barrel diameter then one with a step.

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I'll keep my reply short, and valid to the OP's points

 

1) it's negligible if anything, as the pistons are very light weight

 

Absolutely. The magnitude of the forces versus the all up weight of the gun is far less than in an air rifle, but they're still there and if a critical component can move unpredictably it's not going to help accuracy.

 

2)tighter tolerance = more chance of a barrel jam (from dust/dirt/etc etc). Bb's are not always to tolerance and can jam a gun (had it happen to me in the past). Yeah you are correct the bb can skip along the top of the barrel, some times this is desired though as it can help stabilise the bb (look up the "long range barrel mod" on the sniper forums).

 

This is why I propose to only 'choke' the area under the hop. There seems to be an accepted value of 0.08mm clearance on diameter for a good balance between accuracy and muzzle velocity, but I haven't been able to unearth much empirical evidence for this.

 

3) this is the reason tight bore barrels give a fps increase.

 

Tight bore barrels seem to be a very popular upgrade, certainly the manufacturers promote them quite heavily, but fps doesn't make consistency or accuracy. Finding direct comparisons between a 'standard' barrel and a TBB is proving difficult, but this could well be down to the fact that all the components have to be optimised around each other. Just chucking a TBB barrel on a standard rifle is probably not going to show it in its best light.

 

4)Possible but bb tolerances are not always that good, also bb's are not garenteed to be consistent in there makeup, meaning there could be an air pocket in them (cheaper bb brands) that could upset the weight balance/spin off the bb. But the main reason I would imagine is due to it being easier to produce a consistent barrel diameter then one with a step.

 

I think the same applies to airsoft as it does to air rifles. If you're trying to achieve consistency and accuracy you don't put the cheapest ammo through your gun. Field target shooters will select, weigh and wash pellets in order to achieve the best result. I'll be taking a micrometer to some different brands of BBs at some point and I'll share what I find.

The idea I have is to separate the hop up window from the barrel. Have a carefully made chamber with the bore required and hop up window shape of choice which is fitted into the hop up. The barrel is then attached to this. So I get to experiment with different chamber diameters, hop ups and barrels without having to start over from scratch every time. And if it works it's a mod that would retro fit to a variety of different systems.

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The hop up chamber as you call it doesn't just provide back spin to the bb its also part of the loading system and holds the bucking air seal lip in place so the nozzle mates correctly. Its all made to line up perfectly.

Everything inside an airsoft gun has to sit just perfectly or it doesn't feed and fire correctly. This is why so many people struggle with aftermarket parts as they are not always 100% compatible with every gun.

About 75% of my Sigs internals are custom made and trust me if anything is even slightly off it throws a major wobbly.

I understand where you are coming from but I honestly don't think that it is really feasible. A unit like you are talking about would require it to be larger than a normal hop unit and most airsoft guns are pretty tight on space as is.

The system works because it becomes basically a solid unit when assembled if the barrel is mounted after the hop window there is a massive risk of things going off true.

The design has been poured over by a lot of people whether amateurs or companies and while there are ways to improve it requires a complete redesign of everything and then you lose compatibility with everything else.

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Stop it. :)

Anything related airsoft physics and its opposite can and will be explained by very solid theories. The most notable example from the last few years is TBB vs wide bore.

Fluid dynamics is one of most complex scientific fields and the working of an airsoft gun is also insanely complex from this point of view. No one understands it fully. Hobbyists like us grab and try to explain parts of it just to find out that it doesn't work like that.

 

The only way to make something work is by experimenting. And even that could give inconsistent results.

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Stop it. :)

Anything related airsoft physics and its opposite can and will be explained by very solid theories. The most notable example from the last few years is TBB vs wide bore.

Fluid dynamics is one of most complex field and the working of an airsoft gun is also insanely complex from this point of view. No one understands it fully. Hobbyists like us grab and try to explain parts of it just to find out that it doesn't work like that.

 

The only way to make something work is by experimenting. And even that could give inconsistent results.

Which is why I am hoping that getting a barrel and hop unit made from ALON isn't too far off. That coupled with a good high speed video camera will answer a few questions.

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ImTriggerHappy, thanks for your reply. I've just spent half an hour staring at the AA L96 hop up trying to think around the issues you've highlighted, and while it's not impossible it's certainly not as easy when viewed as an assembly as when you consider individual components! I think I have a couple of different approaches that would work, but one relies on the accuracy of a component I have little or no control over, and the other requires modifying a third party component. Certainly the second option would let me experiment with accurate and consistent placement of the BB at the start of the firing cycle. Just for my own curiosity I'd like to find out how important this is before I dismiss or expand the idea. It's just that in every system I look at the object seems to be locating the BB consistently when its smaller than the hole it's resting in!

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Samurai,

 

if you can point me in the direction of any scientific examination of what's going on inside our toys I'd be grateful!

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It's just that in every system I look at the object seems to be locating the BB consistently when its smaller than the hole it's resting in!

http://www.google.co.uk/url?sa=t&source=web&cd=2&ved=0ahUKEwian6rczr_PAhWoLsAKHZ9TCpcQo7QBCB0wAQ&url=http%3A%2F%2Fm.youtube.com%2Fwatch%3Fv%3DZC-r6Gg6jhg&usg=AFQjCNHlnVKICngTkKt8rG6EtaxmSYNA_A

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Airsoft Physics is a fascinating subject, especially as there aren't many papers specifically pertaining to what happens to a single spherical projectile in a smooth bore barrel.
Kudos on doing your research and approaching it an the ideal manner :)

I'll try to explain as much as I currently understand and have observed - though I'm definitely no physicist and my interest (and research time) is more focused on motors.

1) Basically you've stated Newton's 3rd Law here - For every action there is an equal and opposite action.
So you're totally correct!
Any slight wobble or deviation in the makeup of the gun can introduce inconsistencies that are amplified at range.

2) You're correct in saying that there is no complete consensus as to what happens to the BB once it's past the hop system.
Some people think that the BB rolls or skips along the top and some people think that it doesn't, that it rides down the barrel in a cushion of air.
BB Bastard were planning on doing some tests with a high speed camera and transparent barrel but I don't believe that project was ever finished.

In my experience, the actual inner bore diameter of the barrel matters much less than virtually every other variable.
I know that doesn't help to explain what's going on but it may put it into perspective a little.

I have personally seen no noticeable difference in long (or short) range accuracy and consistency between super tight (5.98mm) and super wide (6.23mm) barrels.
I have seen accuracy and consistency improvements when moving from a Prometheus 6.03 barrel to an EdGI 6.03 barrel, the latter being custom made and internally lapped to a mirror finish - giving almost perfect consistency, straightness and concentricity.

The explanation of why wide-bore barrels work that makes the most sense to me, is that they allow more clearance around the BB so that any inconsistencies in the barrel or ammo will have a lesser chance of causing it to meet the side walls and experience non-ideal friction. They also help to regulate the extreme excesses of air generated by HPA and Gas systems, reducing the turbulence as the BB exits the barrel.
So they basically allow things to be of a lower quality :P

3) Have you tried or investigated the R-Hop system?
It's a small concave patch that sits inside the hop window and matches the internal and external shape of the barrel.
This means that a correctly installed patch will hold the BB centered in the barrel without the need to adjust the inner bore diameter.

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Hi Samwise,

 

Having seen the improvement that can be made to a £100 airsoft rifle with just a bit of thought and no money, I'm even more intrigued as to what is actually going on inside. I've just copied the list of what I've done from another thread. This relates to my Well MB 01...

 

Thanks. I've already been into the trigger and reshaped and polished, or at least as much as you can with the soft pot metal internals that come as standard. They really were bloody awful! Now it has a proper face and edge to break off rather than just smearing the moulding flash. It still has a fair amount of creep, but the let off is quite a bit more consistent.

Other things that have been done. Barrel cleaned and polished, and the sharp edges around the hop window taken off.

The standard bucking has been flat hopped with a pencil eraser nub. The one non factory part is an aluminium hop arm I made out of a bit of scrap I had laying around. Zero play and a lot stiffer. The grub screw on the hop adjuster has received a drop of soft thread lock to stop it moving from shot to shot. The bucking has been dental flossed, both to the barrel and at the nozzle end.

Masking tape barrel spacers and I've also used the tape to shim the barrel and hop assembly to the outer barrel so there's no play. The outer barrel has been straightened. I was on full left windage on the scope, and a quick check with a straight edge showed why. It must have received quite a whack at some point!

The piston head has been modified to stop the O ring flapping around. It's now a much more consistent fit. I also put some PTFE tape into the O ring groove to increase the base diameter. It's not enough to force the O ring against the cylinder wall but it holds the seal central when putting the piston into the cylinder and stops the seal getting pinched. I also spent five minutes getting rid of moulding flash so the spring seats nicely and doesn't bind.

The cylinder head has been PTFE taped, as has the outer barrel to receiver threaded joint as it isn't the tightest fit! PTFE tape on the muzzle insert stops that coming unscrewed. I also applied a small chamfer to the ID of the cylinder nozzle in the hope that it will help centre and seat the BBs a little more accurately. This seems to have reduced the frequency and severity of flyers over the unchamfered version.

I now have a perfect air seal (it was far from perfect before!) and the gun will happily hop a 0.4g BB (ASG 0.4g White, which are very good on consistency of diameter and weight.) At 27m (as far as I can shoot at home) the grouping has halved in size from 'out of the box', hitting a 3" target 8/10. My air rifle chrono doesn't seem to like BBs, but I've got one on the way so I'll check and post fps once it arrives.

But for the sum total of naff all Pounds Sterling and a few hours of my time my MB 01 is now accurate enough to be fun.

I'll post some pics of the work done once I figure it out. :)

There is a Promi 6.03 455mm barrel, AA hop up and Modify flat hop bucking waiting for the Mancraft bolt kit to make it worth while. Then I can start experimenting with pressures/ volumes/ efficiencies.

This...http://www.instructables.com/id/Mythbusting-Airsoft-Hopup-and-Barrel-Dynamics/step7/What-about-wide-bore-barrels-623mm-ID/ was very interesting.

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