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Efficient Barrels

Could combining tight and loose fit increase efficiency?

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#1 ShaNayNay

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Posted 10 September 2013 - 03:43 PM

On a couple of occasions, I've observed discussions debating how having loose fitting/faux barrels after an actual barrel in some cases increases range and/or accuracy.

Basically what has been done is people take a section of tight barrel material, such as CPVC, and hammer it into loose fitting material such as 1/2" PVC.

How this works:
-There is a segment of tight fitting material the size of the dart being used, which allows the plunger assembly to build up pressure as usual.
-Once the dart begins its travel, it enters the loose fitting material, which loosely grips the dart instead of having a continuous tight fit.
-The loose barrel in theory should reduce unnecessary friction on the dart, allowing it to accelerate to higher speeds, resulting in higher range and possibly higher accuracy.

In theory this sounds like it should produce better performance. I will try it out myself with brass and PETG this weekend, but is there anyone out there that has tried this type of barrel, and could shed some light onto if and how much it increases performance?
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#2 rego

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Posted 10 September 2013 - 04:19 PM

It's referred to as a telescopic barrel. The beginning is smaller, and the end of it is larger in od. It has been used before, mainly with brass tubing. It does work if done properly, but not enough for most people to try to make one. If the tighter section is to long, you lose acceleration due to excessive friction, and if it is too short, you loose performance due to the dart moving before the pressure can build up behind the dart.

From what I've seen, they don't increase performance greatly, but will work fine if your willing to go through some trial and error. Dart barrels have not been researched extensively with theoretical data due to the high complexity of the equations and many factors that could affect performance.
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#3 The lord of fish

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Posted 10 September 2013 - 04:29 PM

While the dart is traveling through the 1/2" PVC (assuming this is a stefan)it will not gain any advantage compared to just the the section of CPVC.

Transitioning from 17/32" brass to 9/16" all sheathed in 1/2" PVC has been done before, where the dart is held tightly in the 17/32" section and over comes the initial friction as pressure builds up, then transitions to the 9/16" in the hopes that the dart will retain more velocity since there is now less friction. Many people add "tightening rings", these are crimped areas that reduce the inside diameter, this is sort of the same.

I'm an industrial design major, with only basic knowledge on physics(I'm sure it shows), Doom made a quite lengthy PDF regarding nerf projectile physics, search around and look through it. Faux barrels have the name for a reason, the dart makes no contact with the barrel, nor is the faux sealed to the real barrel. Shoot a stock nerf gun, then put a paper towel roll over the barrel. Shouldn't affect flight, unless the dart makes contact with the inside walls of the roll, which would produce negative results.
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#4 Naturalman7

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Posted 10 September 2013 - 04:32 PM

From what I've seen, they don't increase performance greatly

I concur with this. In addition, with the possibility of a pump-action homemade shooting 130ft+ with a basic hopper/barrel setup, there's not really a need to squeeze a little bit more range out of it anyways, in my honest opinion.
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#5 azrael

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Posted 10 September 2013 - 04:41 PM

Roboman makes some with 5/8" OD aluminum tubes and PETG.


It's a sound practice in theory, the way to think about it is just by applying basic physics. With a tight barrel fit like CPVC in a spring, there is a certain amount of pressure required to build up to allow the dart to break the coefficient of static friction applied by the tightness of the barrel fit. After this, the amount of the friction the dart encounters while moving is much smaller - this is the coefficient of kinetic friction. Ideally, we want this to be as small as possible. This is the idea behind telecoping/efficient/transition barrels.
Now, does it make a significant difference? Not sure. I am sure there is a trade off between a lower coefficient of kinetic friction and the amount of air escaping along the sides instead of pushing the dart. There's also something to be said about the decreasing force of air as the volume behind the dart increases, decreasing pressure behind it.
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#6 Langley

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Posted 10 September 2013 - 05:45 PM

Now, does it make a significant difference? Not sure. I am sure there is a trade off between a lower coefficient of kinetic friction and the amount of air escaping along the sides instead of pushing the dart. There's also something to be said about the decreasing force of air as the volume behind the dart increases, decreasing pressure behind it.


As long as the force applied by the compressed air in the barrel is greater than the force applied by air resistance and friction in the barrel, the dart is accelerating. The acceleration of the dart may be decreasing as the air pressure decreases, but the velocity of the dart will always be increasing while in the barrel, as long as you don't reach that tipping point where the frictional force exceeds the force of the compressed air. Whether or not the force on the dart is decreasing depends entirely on how fast the plunger is moving and how fast the dart is moving in the barrel. It is possible the barrel pressure is actually still increasing when the dart leaves the barrel.

In a theoretical ideal telescoping barrel, the more constricted portion of the barrel acts like the burst valve used in some potato cannons, storing up the compressed air until it reaches a peak pressure. Then the less constricted portion of the barrel applies negligible friction while still sealing with the dart, and allowing it to accelerate.
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#7 Daniel Beaver

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Posted 10 September 2013 - 06:13 PM

As others have mentioned, telescoping barrels and tightening rings have a fairly sound theoretical basis. But they're a lot of hassle for very little observable improvement. I really ought to test some with my chrono.

The actual idea is old as dirt.

Edited by Daniel Beaver, 10 September 2013 - 06:16 PM.

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#8 KaneTheMediocre

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Posted 10 September 2013 - 06:48 PM

These compound barrels CAN be more energy efficient than all tight or all loose barrels, but you're playing a timing game with the plunger that you won't necessarily win. I mainly use them in cases where I don't trust my seal/spring to reliably a dart through a lot of CPVC, and I can't get away with a short barrel for one reason or another.

Some people have told me that they are more accurate, but my experience has been that the coupler introduces another flex point to the already wobbly hopper and doesn't make the darts fly any straighter.
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#9 ShaNayNay

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Posted 10 September 2013 - 09:43 PM

Some people have told me that they are more accurate, but my experience has been that the coupler introduces another flex point to the already wobbly hopper and doesn't make the darts fly any straighter.


I believe I was mixing two ideas together; the faux barrel idea and the telescoping barrel idea. The faux barrel I guess is putting actual faux barrel over a actualy full CPVC barrel of a blaster. this idea sounds stupid, and not worth the effort, as it will probably just hurt range.

The other, the telescoping barrel idea, is the idea I was originally trying to get across.

I am sure there is a trade off between a lower coefficient of kinetic friction and the amount of air escaping along the sides instead of pushing the dart. There's also something to be said about the decreasing force of air as the volume behind the dart increases, decreasing pressure behind it.


Both PETG and 9/16" brass have a perfect airgun fit on my foam, where it isn't too loose that it can slide around. Therefore, I doubt much if at all air flies between the dart and the barrel. So, as long as the barrel is not made too long, there should not be any significant inhibiting factors that could cause the dart to accelerate in the negative direction.

Edited by ShaNayNay, 10 September 2013 - 09:52 PM.

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#10 He Who Mods

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Posted 11 September 2013 - 08:31 PM

In theory this sounds like it should produce better performance. I will try it out myself with brass and PETG this weekend, but is there anyone out there that has tried this type of barrel, and could shed some light onto if and how much it increases performance?


I have tried this with 3" of CPVC and 13" of Sch.80 and it worked well. The barrel significantly decreased the problem of darts fishtailing out of a 14" CPVC barrel on my pumpsnap and new latex-double SNAP design, making the dart fly much more accurate than normal CPVC. I am not sure however, if it increases range too much, if so, only 10ish more feet. Also, I see no flex in my hopper other than the usual, but it does add weight, occasionally falling out.
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