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Longer inner barrel = better accuracy, why?


farewell_red

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Wait... so if you are saying there is no back spin, how does the 'hop-up' produce the effect that it does?

No. He quite clearly said that the rifling doesn't give the BB a spin in the direction of the axis of the barrel. The backspin is still produced by the hop up even in rifled barrels.

 

-Sale

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The barrel length effects accuracy (but to a certain point).

 

A tighter barrel effects accuracy and power output.

 

I'll explain in a bit in laymans terms:

 

When a BB if fired it leaves the hopup bucking but it's trajectory isn't straight and it bounces down the barrel. As it goes bouncing down the barrel the trajectory gets smoothed out. Therefore a longer barrel = tighter groups.

 

A tighter barrel reduces the amount of air escaping around the BB at it travels down the barrel that's why you get a minor FPS increase. The tighter the bore the less bouncing a BB can do so it also increases it's accuracy.

 

In real weapons the longer the barrel the better: more gas behind the projectile, it's constantly accelerated and the bullet is spun faster and for longer in the rifled barrel.

 

However in airsoft if the barrel is too long for the cylinder you start to get negative results: the BB reaches maximum velocity, all the air from the cylinder is now down the barrel but the BB hasn't cleared the barrel. Therefore any distance the BB travels in the barrel at this point causes deceleration due to friction. And here comes a theory: As the BB decelerates in the barrel it is de-stabilised enough to start bouncing again and so accuracy is lost.

 

This is why the TM PSG1 sniper rifle has a longer cylinder for it's barrel length.

 

Sorry I don't have numbers to put up - this is more theory than hard data. However reason stands that there would be a "sweet spot" barrel length for a given bore vs cylinder size. For example (and this is fictitious): 6.04mm X 550mm would net the same accuracy as 6.03mm x 450mm

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Sorry I don't have numbers to put up - this is more theory than hard data. However reason stands that there would be a "sweet spot" barrel length for a given bore vs cylinder size. For example (and this is fictitious): 6.04mm X 550mm would net the same accuracy as 6.03mm x 450mm

The sweet spot is 'pie r squared x height' of the barrel and cylinder's useful working cavity and comparing the two and hopefully having a fraction more air in the cylinder than the barrel for perfect efficiency. However, this would not equate to the accuracy output of a tightbore.

 

In real weapons the longer the barrel the better: more gas behind the projectile, it's constantly accelerated and the bullet is spun faster and for longer in the rifled barrel.

This is not really relevant since real steel is all about range and max out put i.e. longer barrel, greater powder weight, pistol vs .50cal sniper etc whereas in airsoft all guns as a rule are limited to 1Joule at the muzzle.

 

My take on it is that, and this was mentioned by Catman, is that a tightbore reduces or limits the angle of possible bounce in the bore and therefore reduces the amount the bb can deviate off the bore line, due to bounce, once it leaves the muzzle.

Also, I wouldn't be surprised if the longer barrel allowed the rapid exhaust of air from the nozzle to calm down and become more stable the longer it is contained by the barrel and therefore providing a less turbulent, more uniform cushion of air behind the bb to propel it forward.

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There is a big huge thread on this on ASP by tests conducted by GugesMk3.  A tightbore doesn't increase accuracy, neither does a longer inner barrel.

 

And he proved that there is no such thing as "suckback".

If he "proved" there's no such thing as suck-back then what he actually proved was that his hop-up has air leaks. ;)

 

The sweet spot is 'pie r squared x height' of the barrel and cylinder's useful working cavity and comparing the two and hopefully having a fraction more air in the cylinder than the barrel for perfect efficiency. However, this would not equate to the accuracy output of a tightbore.

Don't forget that you also need to account for the quantity of air which passes the BB in the barrel.

 

While it's not easy to directly quantify the exact amount of air that passes the BB it IS possible to account for it simply be calculating the volume of a stock barrel and the volume of a tightbore.

You'll find that the volume of the tightbore is about 2% smaller than the volume of a stock barrel and that 2% is almost exactly what power increase you get with a tightbore barrel.

Probably not the most accurate method but it sort of works.

 

Also, if you calculate the volume of a cylinder the diameter of a BB and then the volume of a cylinder the diamater of the inner barrel you could probably use that information to work out how much air gets past the BB as it moves along the barrel.

Somebody smarter than me needs to do that though. ;)

 

As it is, received wisdom tells us that you need a cylinder roughly 1.5x the volume of your barrel.

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Catman: So are you saying that a longer or even longer barrel will always benefit accuracy, as long as the cylinder provides the air? Which would mean that with double the air volume, a 1000 mm barrel would be more accurate than a 500 mm?

 

Surely not.

 

Do you have an estimate of the optimal barrel length of an airsoft gun, and some kind of explanation of how you got to that?

 

One thing to mind is that the projectile velocity we're talking about is relatively low. This means that lock time is increased with extra length on the barrel. In a practical situation this might make the accuracy even worse than it would be from a shorter barrel.

 

-Sale

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Catman: So are you saying that a longer or even longer barrel will always benefit accuracy, as long as the cylinder provides the air? Which would mean that with double the air volume, a 1000 mm barrel would be more accurate than a 500 mm?

 

Surely not.

 

Do you have an estimate of the optimal barrel length of an airsoft gun, and some kind of explanation of how you got to that?

[...]

 

In effect... yes. The longer the barrel: the more the BB's trajectory is "straightened out" - as long as the BB doesn't start to decelerate. However you'd reach the point where the barrel can no longer provide any further benefit to accuracy. So a 1000mm barrel might be overkill.

 

Meh it's just a theory. When you drop a tennis ball it bounces and the height it bounces with each bounce is reduced. If a BB is "bouncing" down a barrel it stands to reason that the more it bounces the more it's path will straighten out, to the point where it's no longer bouncing.

 

Of course, with the low speeds of airsoft: accuracy is very quickly and easily destroyed. I'd love to work out what the optimal barrel length would be but... I r not cleva enough init

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The BB doesn't bounce along the barrel.

It skids along the top of the barrel. You can prove this for yourself. Colour in a bunch of BBs with a highlighter pen and shoot them. You'll find most of the marks along the top of the barrel. :)

 

I do agree with the theory though, sort of. I reckon the BB yaws from side to side as it travels along the barrel. A wide barrel gives the BB opportunity to yaw more whereas a tightbore controls it more rigidly.

This idea works out cos it explains why a tightbore gives accuracy but it also explains why a long normal barrel will also help accuracy because, as you suggest, the yaw will subside as the BB moves along the barrel.

 

Thing is, any movement a BB makes, yawing or bouncing, will decrease as the BB travels along it. In real terms, I KNOW a P90 can place BBs as accurately as an M16 so, somewhere along the way, the excess length of the M16 barrel becomes unneccesary.

 

As well as this, I doubt many people will buy a standard length tightbore barrel AND a long tightbore barrel (hidden by a silencer etc) in the same gun.

They immediately buy a long tightbore, fit it and then assume the long barrel is helping. If they'd fitted a standard tightbore they might have found the same improvement.

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Thats a pretty good method of checking...I'll have to try it.

 

For what it's worth, I just calculated, through the Bournouli equation combined with Newton's equations for circular motion, that a .2g bb has to be rotating at 1635rps MINUMUM, to maintain a level trajectory.

 

This means that the surface of the bb moving along the top of the barrel of a gun shooting at about 1j will be moving at around 95m/s, so that BB will either slide across or bounce past about 5% of the barrel.

 

If 1635 rps sounds fast, as it did to me, consider this: a .2g bb rotating at 1635 rps still has only .0019J of rotational energy.

 

I did this calculation making the false assumptions of no slip between the air and the BB, air is incompressible, and the flow is invisid, all of which are logically impossible, so in reality, the velocity required is a fair bit higher, but I still thought it was cool to get an estimation.

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So we are in agreement that rifled barrels (For bbs) Dont work properly, and do jack squat, correct?

 

Yeah I don't think that a rifled barrel would help if what we have read here is true.

 

1- two people have confirmed that the BB rolls allong the top of the barrel. They proved this by marking the BB with pen. This make sense with the back spin. Therefore a rifled barrel would negate the HOP UP effect by forcing the BB to spin on access with the barrel. Someone talked about the rifles creating an air cushion all around the BB but this seems unlikely. Riffling should engage the projectile like with a minie ball.

 

2- a sphere like a BB wouldn't benefit from the aerodynamic benefit of spinning on axis with the barrel the way a bullet shaped object does. Maybe if someone made American Football shaped BBs then a rifled barrel would make then fly farther but that's not the case.

 

 

I have a theory if I might throw it out there. Maybe airsoft doesn't want a perfectly round barrel? A trianglish shaped barrel might be more ideal? And here is why...

 

If the BB rolls allong the top of the barrel and it is the lack of precision in the barrel that causes it to move slightly side to side, like a ball rolling down a gutter, as someone put it. Would maybe a tirangle shaped barrel with the peak up keep the BB pushed up against the top with no side to side play.

 

Of course it couldn't be a true triangle because of all the air leakage around the barrel but maybe a slight taper up top to wedge the BB in a way. Wish I new how to make something like that :) Just an idea.

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Im gonna try again: does barrel length (i.e. M4 inner barrel vs. M16 inner barrel and a silencer) affect BBs travelling distance? Especially horizontal travel distance?

If there's a slight increase in muzzle velocity, then you'll theoretically get a bit more range too. The thing is, we have limits at skirmish sites (328 for the UK, 400 for typical US fields), and you can easily upgrade the M4 to those limits. Even if you have a longer barrel, the limit is the same. In this case there is absolutely no (zero, nada, zip) difference in the travelling distance (range) of the BB.

 

Catman, so you're saying that eventually the BB will stabilize in the muzzle, and it has more time to do so in a longer barrel. How do you know it isn't as stabile as it's going to get in a 300 mm barrel?

 

I have no personal interest in this except to share knowledge. You can keep building guns with 590 mm barrels, but don't be amazed if you're outshot by people who put more effort in the pneumatics of the gun and other upgrades that have an effect on accuracy, but still run barrels half the length you do.

 

-Sale

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A long barrel WILL give you more power.

The ultimate example of this is a GBB. A Glock 17 firing at 1J will shoot at nearly 2J if you manage to fit a 300mm barrel to it.

This happens for the same reason that an real-steel M16 can fire a bullet further than a real-steel M4.

It's cos the charge behind the projectile can continue to accelerate the projectile for longer.

The problem, however, is that (corny as it sounds) power is nothing without accuracy and repeatibility.

Personally, I fit barrels for accuracy nd springs for power. I don't see the point in, for example, fitting a 50cm tightbore barrel in a P90 which means you need to keep the silencer on at all times.

Far better, IMO, to fit a standard tightbore barrel to achieve the accuracy and then fit either a stronger spring or other parts to make the gun shoot harder.

 

As for the TK twist barrels, I have several issues with them...

Firstly, the price.

If a standard tightbore costs $40 due to the accuracy needed in manufacture, I can't see how a more complex item like a twist barrel can be manufactured to the same quality for less cost.

 

Secondly, the principle.

You must have seen those window displays in shops where a beach-ball is balanced on a column of air, right?

The ball spins. The flow of air against the ball makes it spin.

The same principle applies to a twist barrel.

The idea of a twist barrel is that the air moves around the grooves and creates a laminar layer of air between the wall of the barrel and the BB.

The BB hits the hop-up and begins to back-spin as it travels along the barrel as normal.

The problem with this is that, as with the beach ball, the spinning air moving along the barrel MUST exert some force on the BB, just as the air blowing on the beach ball does.

Also, with regard to the cost, I can't see how it's possible that all the grooves are identical, thus the layer of air won't be smooth so it'll be a bit like shooting a BB down a slightly bendy barrel.

 

Thirdly, they don't work on rifles.

I have used a TK twist barrel and it wasn't as good as a decent 6.03 barrel.

What it WOULD do is shoot 2 or 3 very accurate shots and then shoot half a dozen flyers.

If I then swapped the TK barrel for a 6.03 barrel I would find I could keep putting shots within a 3" group all day. No flyers.

 

I'm told that TK pistol barrels work best, giving superior range. I can understand this. If the BB normally spends it's time skidding along the top of the barrel then it'll lose power due to friction.

If you now send a BB down a barrel where it's floating along on a cushion of air then that friction won't be present and the BB should leave the barrel without losing so much power.

 

When I did my tests I was shooting at a range of 25m. It's entirely possible that the TK barrel might have had greater ultimate range than a 6.03 tightbore but, to me, that's pointless if 75% of your shots are going to be wild anyway.

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[...]

 

Catman, so you're saying that eventually the BB will stabilize in the muzzle, and it has more time to do so in a longer barrel. How do you know it isn't as stabile as it's going to get in a 300 mm barrel?

 

[...]

 

I don't ;) As long as the barrel isn't too long for the cylinder - no harm, no foul imo.

 

Airsoft is pretty inaccuracte anyway, a little breeze and it's off track, why most people use high-caps and walk their rounds to the target.

 

I know just the man to ask about this.... stay tuned :)

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A long barrel WILL give you more power.

The ultimate example of this is a GBB. A Glock 17 firing at 1J will shoot at nearly 2J if you manage to fit a 300mm barrel to it.

This happens for the same reason that an real-steel M16 can fire a bullet further than a real-steel M4.

It's cos the charge behind the projectile can continue to accelerate the projectile for longer.

It depends hugely on the gun. If you want a realsteel example, a .22 LR is more powerful from a 300 mm barrel than a 600 mm barrel. There just isn't enough powder to keep accelerating it. Airsofts are different animals. The guns that benefit most from longer barrels are NBBs, but those physics can't be applied to GBBs or AEGs.

 

Being as the spring of an AEG is limited and gives a steady shove instead of releasing pressurized gas, there's an optimal barrel length after which the velocity just doesn't go up, even if you increase barrel length and add a suitable cylinder. That's why an M4 and M16 in airsoft form have about the same muzzle velocity, accuracy and thus, range.

 

If we talk about the increase of accuracy thanks to a longer barrel, you need to compare weapons with similar power. This is especially important, since most players go to skirmish sites with limits.

 

-Sale

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Pretty much agree with you on all counts there.

 

I guess I should have said that a longer barrel will always give more power up until the point where it's too long for the cylinder to push the BB all the way along it.

 

You can verify this by taking a gun with a type 0 cylinder, test firing it and then putting the barrel off an MP5K in it. You'll see the power drop off by about 20fps.

 

Of course, that proves the theory but it doesn't affect much in practice because, as you say, if you have an MP5K or a G3 you can still only tune it to 1J power.

Once you've got your 1J power it's the other elements, such as the quality of the barrel, hop-up, nozzle etc, that will determine accuracy.

 

I guess GBBs are a bit less technical than AEGs. Either that or maybe it's not possible (or manufacturers simply don't bother) to regulate the amount of gas fired with each shot as accurately as it's possible to tune the air discharged from an AEG?

There must be stacks of gas venting behind the BB in a G17 because, as I say, when a friend fitted a long barrel to one the power went up by a MASSIVE amount.

 

Incidentally, I deliberately used the M4 vs M16 as an example of longer barrels making a difference in real-steel because I know it to be true with those two guns.

Obviously, a gun which fires a smaller charge will only benefit from a suitable length barrel.

Kinda like airsoft then? ;)

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I was under the impression that the rifled barrels were meant to be used without the hop-up engaged.

You were misinformed. :)

 

Seriously, this is one of the bits of dodgy information that is going around about TK barrels.

 

People automatically assume they're designed to make the BB spin axially and then the next assumption is that this wouldn't work with hop-up so they assume the hop-up is not used.

 

Common sense tells you this is nonsense. You still NEED hop-up to generate the back-spin that gives a BB it's range. An Axially spinning BB may (or may not) be accurate but it will generate no lift so it'll only fly about 30m.

 

The idea of the twists (I think) is that air travelling along them has further to go so it moves slower. Air behind the BB is going faster. As a result you end up with a smooth layer of slow-moving air around the edge of the barrel which the BB can move through but which controls the path of the BB.

 

It should work in theory but I, personally, just don't reckon the barrels are well enough made to capitalise on the idea.

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