Barrel length effects on accuracy - Breakdown and recommendations

[Pseudotectonic]

Does barrel length affect accuracy? Yes? No?

 

How? And how much?

 

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Let's do a simple thought experiment. Using maths, and a very typical situation. I have a variable barrel that goes from 50 mm to 550 mm. And I have an imaginary target at 30 metres. I want to find out the different ways the difference in barrel length can affect the grouping diameter.

 

For the purpose of this calculation, we can eliminate the factors of air volume and hop up. Because for every build these would have been tuned accordingly to work with each barrel length. They will all have been tuned to output the same joule with the same BB and with the same hop up performance.

 

Therefore this calculation deals only with the geometrical effects of a longer or shorter barrel.

 

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Diagram A: Spread introduced by barrel BB travel

 

Assume the barrel is a stock 6.05 mm barrel. The BB has a typical 5.95 mm diameter. Therefore the variation is 0.1 mm.

Assuming the BB travels in more or less a straight line, this 0.1 mm variation at the barrel exit will give us the worst case BB trajectory.

 

The final spread diameter at 30 metres is therefore a simple triangle calculation.

 

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Diagram B: Spread introduced by inner barrel wobble inside outer barrel

 

Due to the construction of airsoft, the inner barrel is normally not fixed inside the outer barrel, but simply rests inside with a little clearance space around. During the firing cycle the gun will definitely be subject to vibrations of all sorts, making the inner barrel essentially wobble inside the space it occupies.

 

Assuming this clearance is 0.4 mm (measured from an AEG) we can similarly calculate the spread it creates at 30 metres.

 

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Diagram C: Spread introduced by difference in ergonomics

 

A longer barrel will allow you to hold it steady at two further apart locations, because of the longer handguard. This can be fully exploited to create a more stable barrel.

 

Assuming your most steady hand will involuntarily sway within a 2 mm range, we can again extrapolate its effects on the grouping using the same calculation.

 

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Factor D: Wind

 

There is no diagram for this because wind is not affected by barrel length. We assume a moderate wind affecting our BB at 30 metres to introduce a spread of 50 mm.

 

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Results:

 

 

In the order of significance, the barrel length can influence the accuracy through:

 

• - Ergonomics
• - Inner barrel wobble
• - BB travel inside barrel

 

I will discuss each below.

 

The biggest advantage of a longer barrel has surprisingly nothing to do with the actual ballistics of the BB, but the effect of simply having a longer stick to hold onto gives you a better control of the aim. For shorter barrels below 200 mm, the ergonomic disadvantages will affect your accuracy a lot for long range engagements, although the shorter builds are better for CQB where accuracy is less important than manoeuvrability. The biggest thing you can do to exploit the advantages of a longer barrel in terms of accuracy will be to eliminate hand sway as much as possible while making use of the longer form factor. That means using bipods or resting your gun at something solid as much as possible, and so on. Again it remains true that it is 99% the user and 1% the gun.

 

Even with worst case geometrics, the inner barrel wobble and BB travel inside barrel does not really introduce much spread once you go above 200 mm barrel length, where it generally becomes less of a problem than actual wind blowing your BB. Above say 300 mm, the effect will become so small it becomes virtually no difference in most practical situations.

 

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Recommendations:

 

• - Below 200 mm is strictly for CQB. The BB, the barrel, and the gun itself will all suffer from stability issues for making accurate shots.
• - While barrel length affects accuracy in pure mathematics, the ballistic effects will have become virtually irrelevant above 300 mm.
• - Longer barrels (longer guns / sniper rifles) can potentially have better groupings, with the right skills, and probably with the right bipod.
• - The best method to improve accuracy is improving largely your skills, not your gun.

 

Conclusions:

 

Nothing is new here, but I guess you can't really snipe with an MP5K. But you can potentially snipe with an MP5.

[mrfoxhound]

30m is med for bolt guns running high power, 30m for everything else is lowest  range , using 30m to test barrel length accuracy, is insignificant, all air muskets are able to hit a target at 30m ,everything is needed to get accuracy, correct cylinder volume ,efficient air seal, air nozzle engagement, tappet return, magazine spring tension everything needs to be taken into account before you get to hop rubber ,nub and then barrel ,and ammo weight , you cant just remove them variables to come to a conclusion .    

[Fatboy40]

9 hours ago, Pseudotectonic said:

Recommendations:

 

• - Below 200 mm is strictly for CQB. The BB, the barrel, and the gun itself will all suffer from stability issues for making accurate shots.

 

My son's backup AEG, a modified G&G Firehawk with a 160mm inner barrel (120mm is stock length for it), says otherwise. Shooting 0.32g, with a Max Model M4A-PRO + o-rings rather than a spring in front of the hop unit, it has a huge amount of range and is very accurate.

 

Looking at inner barrel length in isolation is not the whole picture.

 

(I've also a CYMA MP5K, running a Snipetor r-hop patch + Modify flat bucking + 0.32g BB's, and it's range and accuracy is also way better than you'd expect)

[Adolf Hamster]

It's an interesting dive, but there are some thoughts to consider.

 

First is straightness, keeping a half metre tube straight to within the diameter tolerances we have is unlikely when things are being produced as cheaply as possible. Not to mention that the inner barrel might be perfectly straight but the outer has other ideas. I have pondered that a barrel curved intentionally downwards to force the bb to ride on its top edge (thus adding rather than detracting from the hop spin) could theoretically give better stability. I have a vague foggy recollection the americans had some sort of miracle barrel with a groove in it to acheive this but that's got as much weight behind it as the tk twist.

 

Second is the concept of stabilisation ie the whole riding on a cushion rather than bouncing off the walls, which i've yet to see the kind of full nerd level fea type analysis to confirm if it's even a thing, let alone give an idea of where the sweet spot is in terms of bore/length and the effects of bb weight and energy level. I suspect if it is you'll find that for a given ammo/energy level, too short gives too little time for the transition from bouncing to drifting, and too long reduces the pressure to where it can't even initialise stability in the first place.

 

And third is the everything else that goes on in our systems, which is why my typical summation is that barrel length is mostly irrelevant as simply getting the rest of the pew good enough to even notice the change is a challenge in an of itself.

 

Which ultimately boils down to barrel length being a choice made on other factors, like the handling, aesthetics, and wether your chosen field/style of play grants a benefit or a penalty to running a longer gun (ie poking through hedges outdoors or handling in tight spaces indoors)

[GiantKiwi]

The only real effect of barrel length is volume relative to the volume of whatever is providing the propulsion.

 

The barrel length is ultimately irrelevant otherwise.

[Skara]

I was going to write a wall of text to respond and debunk, but...

 

If a long barrel is betterer than a short barrel, then explain the Mk23...

[Sewdhull]

It is easier to hold a lighter weapon steady than a heavier one, the addition of a shoulder stock to a pistol is a great bonus to accuracy. It's not about the barrel.

 

Barrel length and bore don't affect accuracy until they are at extremes. BBs don't rattle around the barrel on the way down, they are guided by equalising pressures around the BB, keeping it central and straightness and uniformity of bore will be the primary factors affecting accuracy.

 

There is a school of thought that wider bores allow a steadier passage of the BB down the barrel,  pressures will equalise regardless of the bore.. again within the extremes.

 

Clearly hop matters but is outside the scope of this thread.

[ak2m4]

this is just from my own research in my own particular case.  I've got hundreds of identical polished new barrels, polished to RA 0.5, all different lengths from 128 to 500 - same bore size.  Have tested about 400 so far and one clear observation is that the shorter ones are more accurate.  

[Muzzer]

2 minutes ago, ak2m4 said:

this is just from my own research in my own particular case.  I've got hundreds of identical polished new barrels, polished to RA 0.5, all different lengths from 128 to 500 - same bore size.  Have tested about 400 so far and one clear observation is that the shorter ones are more accurate.  

Say what! Did you mis type you mean longer more accurate, surely 

Edited September 25 by Muzzer

[Skara]

1 minute ago, ak2m4 said:

the shorter ones are more accurate

Probably because there is less "length" to go wrong compared to a longer one.

 

In the wall of text I deleted I mentioned how my 5" M4 is way more consistent than it was with a 10" barrel, essentially achieving the same, if not slightly more range.

And the 5" is just a cut down and recrowned Guarder barrel I had while the 10" is a prommy 6.05...

[ak2m4]

3 minutes ago, Skara said:

Probably because there is less "length" to go wrong compared to a longer one.

 

Yes it's highly possible.  I also found that having the last 3 or 4 inches polished to a high degree can make big improvement for any lengths.

Edited September 25 by ak2m4

[Impulse]

I think the only thing that really affects performance when it comes to barrels is correctly voluming your build with the BBs you plan on using, and in terms of gas guns making sure you have the correct barrel length to joule creep to the power you want. Too much air volume leads to inconsistencies in shots, as the excess air can push the BB off-centre when it leaves the barrel, but too little volume leads to lower power and the BB decelerating while it is still in the barrel.

 

Also, apparently barrel bore influences accuracy and power, with wider tending towards more accuracy and tighter tending towards more power, but I think the difference in accuracy between barrel bores is minimal. I've had very accurate builds with 6.02 and 6.03 barrels, and poor performance with 6.23 widebores, so I think the barrel bore for stability isn't a huge deal.

[ak2m4]

25 minutes ago, Muzzer said:

Say what! Did you mis type you mean longer more accurate, surely 

you read right 🙂 260 to 300 seemed the sweet spot.  Of course this is my own testing, the longer ones still performed well but just not as good as the shorter ones.  

[Pseudotectonic]

@Adolf Hamster Re air cushion the most convincing article I've seen is here but it only shows that air cushions exist in theory, it doesn't discuss barrel lengths and volumes. But I find it interesting it says "BB's in tight bores follow a more stable path than wide bores" but @Sewdhull @Impulse suggested otherwise. My current believe is wider bore is more accurate, not because of better or worse air cushioning but because it is less susceptible to defects or imperfections or dust ingress or vibrations. Of course again this is pure theory.

 

@ak2m4 @Skara Shorter more accurate!? Last 3 or 4 inches greatly affect accuracy? That is very counter-intuitive.

 

Is shorter more accurate a linear relationship or how is the curve? Was the air volume appropriate for each barrel, same joule, same hop, same BB?

 

What might be the theory behind this? Could it be that the air cushion in the last few inches (where it matters most) has a higher pressure in a shorter barrel than the longer barrel (because to achieve the same joule the shorter barrel needs a stronger spring / air pressure to accelerate the BB within a shorter timeframe) therefore has a stronger stabilising (i.e. centering) effect?

 

And what do you mean 260 to 300 being the sweet spot? You mean it doesn't improve much if it is shorter than this?

[Sewdhull]

Air pressure behind the BB is probably the most important criteria for stability in a barrel. Enough to push past the BB and stabilise it. Longer barrels will need the same pressure for longer and shorter barrels may other issues of their own like insufficient time to stabilise or excess pressure on exit causing issues, I don't know.

 

I do know that I can balance a Malteser in the stream of air from my lips using he same principle that exists in when air pushes past a BB in a barrel. The problem is that even those ppl who try to control the environment leave numerous variables unaddressed and then claim a conclusion.

 

HPA guns stand the best chance of showing the value or not of a longer barrel ( On accuracy) because of the constant pressure, but the complex interactions before the BB exits the barrels make drawing conclusions from experimentation tricky.

 

 

 

 

[Colin Allen]

9 hours ago, Fatboy40 said:

 

My son's backup AEG, a modified G&G Firehawk with a 160mm inner barrel (120mm is stock length for it), says otherwise. Shooting 0.32g, with a Max Model M4A-PRO + o-rings rather than a spring in front of the hop unit, it has a huge amount of range and is very accurate.

 

Looking at inner barrel length in isolation is not the whole picture.

 

(I've also a CYMA MP5K, running a Snipetor r-hop patch + Modify flat bucking + 0.32g BB's, and it's range and accuracy is also way better than you'd expect)

My 150mm barrel length stubby M4 has astonishing range and accuracy that also says otherwise.

[ak2m4]

4 hours ago, Pseudotectonic said:

Is shorter more accurate a linear relationship or how is the curve? Was the air volume appropriate for each barrel, same joule, same hop, same BB?

2 identical aeg's for testing, one with a 3/4 and other with a closed, same hop, same rubber.  I imagine many people will disagree but it's just my observation.  Maybe it's down to the super tightness of the barrels I was testing, most around 6.01 +/- 0.01.  

I didn't get too technical for linear relationships and curves, if it flies straight and long I'm happy.

I should have 6.03 / 6.04 stock in soon so will be interesting to see what they're like.  Ultimately the shorter barrels just seemed to give me a lot less grief. When testing when I come across ones which I'm not happy with I just put them to one side, I bore-scope them to see what might cause the curve but I can never find what it is.

I've been refining the process over the past 6 months, I'm a little sick of it tbh as it's a lot of extra work in deburring and polishing.  I won't be doing this when the new barrels come in thankfully.

[Impulse]

8 hours ago, ak2m4 said:

I should have 6.03 / 6.04 stock in soon

 

 

Hopefully you'll have them in long boi lengths 😂

[ak2m4]

Yep all lengths in AEG.  I'm just waiting on the 2nd sample to arrive, hopefully end of next week.  They sent the 1st sample to me couple of weeks ago but it didn't have the internal polishing.  I'll post up some video of it.  Again some interesting stuff with in that you all might find interesting.

 

[ruskitseller]

This is an interesting topic but I remain confused... 🤣 I saw this vid on the subject...but then I have been pinged myself by a Mk23...

 

[jack25]

Barrel length should have no effect on accuracy, but I'm not sure.

[Pseudotectonic]

I think ultimately it depends on the context and detail of the question.

 

Are you asking about a physics problem, where all other variables including cylinder size and spring and hop are unchanged, would simply a longer or shorter barrel affect accuracy? Yes, because changing the length would unmatch the air volume, which affects fps, in turn affect reach, and then accuracy. Is it day and night difference? It will not be day and night difference, if your barrel length is not day and night difference.

 

Are you asking about swapping a different length barrel on the same gun, while re-tune the cylinder and hop so that the fps is unchanged? Probably depends more on whether your tech skills can tune it to the same fps and hop. Anecdotally a shorter barrel feels more accurate (see above comments) but it is unknown how air volume factor into this, and whether it is measurably more accurate in a target grouping test. Therefore, in the same gun, the effect of the absolute length of barrel is negligible and unpredictable, because of all these other variables you are inevitably changing at the same time. Some might say it makes "no difference" in a practical sense. Unless you are doing something extreme, of course. Incidentally polishing the same barrel will make a bigger improvement than swapping to a longer or shorter barrel, so is changing hop rubber, or use a heavier BB; barrel length is the last thing you need to worry about, when upgrading the same replica.

 

Are you asking about choosing between two entirely different guns with two different barrel lengths, like an MP5 vs an AK? Or a pistol vs sniper? Now the pool of variables are even bigger, because ergonomics, skills, play style, and even situations where you use them would be so vastly different it would be comparing apple and orange. Even if they all shoot the same fps, the difference in weight of the replica alone would probably make a greater difference in your aiming posture and control than say the barrel length difference. So the barrel length difference, in this question, is quite irrelevant.

[Pseudotectonic]

Bonus question:

 

In a physics problem, comparing two barrels, one longer and one shorter, assuming in both situations the BB will exit with the same velocity of 300 fps while spinning at the same RPM for hop, would the longer or shorter barrel provide more accuracy? This is an unsolved problem, both theoretically and experimentally. In pure geometry a longer barrel should suggest a more precise exit vector with less angular variation, however the question of BB ballistics inside the barrel is a complex situation combining the Magnus effect, Bernoulli's principle, Coanda effect, harmonics, interactions of plastic vs brass (or steel), and inevitably the probabilities of defects and surface finish variations, etc. We are still very far from a comprehensive theoretical model that describes everything that happens in this simple system, and as far as I know no one has done such an experiment. So really it is still up to debate.

[ParHunter]

Is there somewhere a formula in matching your inner barrel to your piston and spring?
I understand that the volume of the cylinder (the active part - how do you actually figure that out? I guess you could measure how far you pull back the bold handle) should be higher than the volume of the inner barrel but by what factor?

 

And how does the spring come into the equation?

I got a TAC41 LS and I was told the barrel is too short for the gun and I should get a longer barrel for it. And that was mentioned as the first improvement I should do. Apparently I could go down to the weaker 100NM spring and pretty much achieve the same distance but with better accuracy. They also mentioned that the bb would be pushed offline when the air pressure is too high when the bb exists the inner barrel.

 

On another note, would it be possible to 3d print a ‘compensator’ with build in barrel extension? I guess you would not be able to get a good air seal. 

[Pseudotectonic]

@ParHunter Very roughly this is a guide for voluming, generally you want to follow a rough ratio of say > 2. The cylinder volume is just literally the cylindrical shaped block of air, inside the cylinder, in front of any port opening on the cylinder. That is the block of air that is being compressed when the piston slams forward.

 

Stronger spring can compress a certain air volume faster, which gives the BB more acceleration, thus fps. (In car: how hard you press the gas)

 

A bigger air volume, will accelerate the BB for longer (given a long enough barrel to make use of it). (In car: how long you hold the gas)

 

You can accelerate to a certain speed by pressing the gas harder, or by holding the gas longer. If you press hard enough, you can get to a high speed using less runway distance. And if you have a long enough runway, you don't have to press the gas as hard, you can just hold the gas for longer, so when you reach the end of the runway you would have gained enough speed.