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Barrel advice for a compressed air gun

I have some issues with mine.

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#1 dskippy

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Posted 11 October 2013 - 03:00 PM

I have been reading the barrels and darts form for a while and I have been having a hard time figuring out what I should do to make an optimal barrel for my compressed air gun. The gun is a typical 3/4" sprinkler valve gun. I have made two barrels and two chambers for it. The first a 1' long barrel with a small chamber 4x the volume and the second is a 3' long barrel again with a chamber 4x the size. I am compressing my air to about 30 psi, 60psi, and 120psi. Both barrels are 9/16" brass nested inside 1/2" schedule 40 PVC.

With the 1' barrel I get decent shots, 150' at max psi. With the 3' barrel I get varying behavior. Sometimes 200', sometimes the dart stays in the barrel bunched up, sometimes the tip blows off in the barrel and the foam flops out, sometimes the dart dove tails, sometimes I get a weakish shot off target (probably a minor dove tail).

I am wondering if people have an recommendations. I am using standard Nerf Stefans bought off the shelves and I'd like to stick to that ammo.

I have considered making a telescoping barrel based on what I have read. The disadvantage of a telescoping barrel is that pressure cannot build up behind the dart long enough if you make the tight-fit portion too small. However, if your reading comprehension is good you'll remember that I'm using a compressed air gun. :) So perhaps telescoping is a good way to go? Perhaps even just a loose-fit barrel. I could do schedule 80, schedule 40 even, I have not tried CPVC, that seems way too tight. Has anyone experimented with compressed air cannon barrels? Basically, I want to make the dart go as far as possible. This is not for a war so I don't care if it would hurt some one if I hit them. I have other guns for actually shooting humans. This is just for show.

Thanks a bunch,
-mike
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#2 Buffdaddy

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Posted 11 October 2013 - 04:06 PM

Telescoping barrels aren't an option here. Since you already have compressed air ready to go down the barrel, there isn't any need for a tight fit - that is used in springers because the friction allows some pressure to build before the dart is actually dislodged and fired. The looser the barrel fit, the better. There should be just enough friction to actually hold the dart in, while being loose enough that lightly blowing down the barrel shoots your dart. Depending on your dart fit, you may need to take some blunt tubing cutters and make some indentations in your 9/16" tubing so that you can hold your dart in place.

You're using the Nerf Elite darts straight off the store shelves? That is a problem in and of itself. While they perform better than streamlines, at high speeds Elite darts still encounter massive instability due to a combination of weight distribution, length, the hole in the side of the dart tip, etc. While some darts will go straight, others will fishtail or otherwise fly erratically. With the setup you're describing, it's not surprising that you're simply blowing apart some of your darts inside the barrel. Since you insist on this aspect, the flight of the dart is not something you can correct with any changes in setup. Dart integrity, on the other hand, can be maintained just by using less power.

In any case, you will not be able to use the second chamber at maximum pressure with this setup. It's simply too much for your darts to handle. Usually, the highest I've seen modified or homemade air blasters used legally in wars is 30psi. Perhaps higher if you're firing oversized ammo, but that's another issue and we can ignore that.

TL;DR too much power.
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#3 dskippy

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Posted 11 October 2013 - 04:49 PM

Hey, thanks for much for the reply. The reason I'm using the elites is that I wanted a long barrel to give me a long acceleration period and they are standard darts that are all around my office. I guess if these guys can't fly well that well, I might try the velcro or the suction cup ones. Would either of these be any better? I'll try a lose barrel but 9/16 is honestly pretty lose. I can use it very effectively as a blow gun. The barrel alone that is. Perhaps I should get some schedule 80 or just 40 and try that. It seems like the darts with large heads like the velcro or the suction would have a hard time being back-loaded into a very long barrel.

So if I'm open to changing up my darts and my barrel, what setup would you suggest for launching a dart as far as possible? I know I could launched them ungodly far if they are, for example, made of wood, or have metal inserts. I've done this. But I'm interested in maxing out the range of off-the-shelf darts here.

Thanks for the feed back,
-mike
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#4 Draconis

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Posted 11 October 2013 - 05:13 PM

What, pray tell, are you planning to shoot with this? Because if it breathes prior, it is unlikely to continue after.
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#5 dskippy

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Posted 11 October 2013 - 05:33 PM

What, pray tell, are you planning to shoot with this? Because if it breathes prior, it is unlikely to continue after.


I'm firing them off into the park at work. I'm going for distance. So... I'm shooting a very unlucky piece of grass. Hopefully about 200' away.

-mike

p.s. You're not really going to kill anything with a any stock Nerf dart, even if you max out at 120 psi. Sure it can hurt a lot but it's similar to a paint ball gun.
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#6 Draconis

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Posted 11 October 2013 - 07:31 PM

I'm firing them off into the park at work. I'm going for distance. So... I'm shooting a very unlucky piece of grass. Hopefully about 200' away.

-mike

p.s. You're not really going to kill anything with a any stock Nerf dart, even if you max out at 120 psi. Sure it can hurt a lot but it's similar to a paint ball gun.



You'll probably be disappointed. Stock darts tend not to have the combination of mass and low wind resistance needed to achieve that kind of range. And if they do? Almost never where you were pointing. Honestly, at pressure like that, I would not be surprised of every dart you put in that thing shreds on the way out. Especially stock darts. If the pressure remaining in the barrel when the dart exits exceeds the maximum pressure that the foam tube can handle, it will explode. For the new MEGA darts, this pressure is about 20PSI.
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#7 dskippy

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Posted 11 October 2013 - 07:39 PM

You'll probably be disappointed. Stock darts tend not to have the combination of mass and low wind resistance needed to achieve that kind of range. And if they do? Almost never where you were pointing. Honestly, at pressure like that, I would not be surprised of every dart you put in that thing shreds on the way out. Especially stock darts. If the pressure remaining in the barrel when the dart exits exceeds the maximum pressure that the foam tube can handle, it will explode. For the new MEGA darts, this pressure is about 20PSI.


Well perhaps I shouldn't have quoted a random number. Maybe 200' is not possible. But what is possible? That's what I want to find out. Also accuracy is not what I'm going for here. Just distance. Other projects I'll go for reasonable. This gun is impractical for reasons beyond the scope of this article.

But other folks on this forum have achieved distances nearing 200' by their reports. Also I've gotten a not of luck getting velcro headed darts to go upward of 175' with just a 2' schedule 40 barrel. This speaks to BuffDaddy's point that looser fits are better for compressed air guns. Having had better luck with that and getting BuffDaddy's feed back I will try much looser fits.

-mike
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#8 Doom

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Posted 11 October 2013 - 09:14 PM

With the 1' barrel I get decent shots, 150' at max psi. With the 3' barrel I get varying behavior. Sometimes 200', sometimes the dart stays in the barrel bunched up, sometimes the tip blows off in the barrel and the foam flops out, sometimes the dart dove tails, sometimes I get a weakish shot off target (probably a minor dove tail).

I am wondering if people have an recommendations. I am using standard Nerf Stefans bought off the shelves and I'd like to stick to that ammo.


I'll reiterate the point that you are limited by your darts. Stock darts were not designed to be shot from a gun that powerful. Homemade darts would serve you much better.

But other folks on this forum have achieved distances nearing 200' by their reports.


Tests have shown that few people actually shoot perfectly level, even when they claim they are shooting level. Slightly angled shots get much higher range.

- - -


In terms of getting more range, consider the following:

Heavier darts usually improve the energy efficiency of the air gun and get better range to boot. Stock darts usually are not heavy enough.

Are you are aware, the best barrel fit is tight for the initial stages of the barrel and just loose enough to seal for the remainder. The initial part is not so important for air guns with fast opening valves. I'm not sure how you are actuating your valve, but I would suggest air piloting over using a sprinkler valve's electronics. I'd also suggest QEVs over sprinkler valves. QEVs have higher flow rates, are more durable, and don't require modification to use with a pilot valve.

Holding everything else constant (gas chamber volume, valve, etc.), a barrel length that will maximize range exists. Assuming that your darts are good (e.g., ignoring jams, darts breaking, and dart stability issues), you can find this by testing a few barrel lengths as you have. The results should be fairly consistent. As you found, if your darts aren't good then you won't get good results. My suggestion is to try several barrel lengths with your smaller chamber and see what length gets the best range on average.
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#9 Mully

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Posted 12 October 2013 - 10:38 PM

As basically everyone else in this thread has said so far, stock elites aren't really that great for what you're trying to do. At 120 psi, it's impressive that you actually managed to get them to fire!
It sounds like you wanna keep the stock-dart firing capability; what you could try instead of using elites and brass, is just a piece of standard 1/2" with taggers and whistlers. Basically a blow-gun, I used to use one, they're quite accurate. If you decided to give it a shot, I'd use about 3-5', or whatever feels like the right size.

Edited by Mully, 12 October 2013 - 10:39 PM.

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#10 dskippy

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Posted 13 October 2013 - 12:01 AM

As basically everyone else in this thread has said so far, stock elites aren't really that great for what you're trying to do. At 120 psi, it's impressive that you actually managed to get them to fire!
It sounds like you wanna keep the stock-dart firing capability; what you could try instead of using elites and brass, is just a piece of standard 1/2" with taggers and whistlers. Basically a blow-gun, I used to use one, they're quite accurate. If you decided to give it a shot, I'd use about 3-5', or whatever feels like the right size.


That fits with what I've found. Basically what I want to know is this:

What stock Nerf dart can travel the furthest? What setup achieves this goal?

Also regarding the firing angle comment from another post, I am firing at 45 degrees when I am getting near 200'. I didn't realize there was a standard for reporting firing distance horizontally but naturally that makes sense. Anyway. I just want to know how far I can get a stock dart to go. 1/2" schedule 40 with taggers has been my best so far but that was just convenience. I figured I must be able to do better but perhaps I just got lucky.
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#11 Doom

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Posted 13 October 2013 - 12:59 PM

If you want better range, 35 degrees is probably better than 45. 45 is only optimal for the case without drag or a starting height. Add a starting height and the optimal angle decreases. Add drag and I'm not sure what happens off the top of my head, but there are a few papers online that you can find that'll offer clues. You could do some tests to figure this out too.

Unfortunately, the answer to your question is probably "no one knows yet". I'll offer some advice. Look for stock darts that have the lowest value of A/m, where A is the frontal area of the dart and m is the dart mass. Some theoretical work I've done suggests that the lower this value, the greater the range (holding everything else constant including muzzle velocity).

You'll want these darts to be stable (i.e., not rotate appreciably) when they leave the barrel. Lighter darts seem to be bad for that, so heavier darts again are what you should be looking for.

In terms of what the gun should look like, what I've said about valves and barrels applies and this is approximately an independent problem from the aerodynamic effects (aside from the dart mass, but heavier is better here too).
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#12 Draconis

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Posted 14 October 2013 - 01:20 AM

In practice, I think that it is relatively accepted that stock tagger darts are the most stable and achieve the longest ranges. They also fit nicely in cheap 1/2" Sch 40 PVC.
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#13 dskippy

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Posted 14 October 2013 - 10:55 AM

Curses. Full circle to what I started with accidentally. They did fly pretty damn far at an angled firing position at 120 psi. I'll go back to them and start playing with barrel length to experiment. Thanks everyone.

-mike
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#14 Datum

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Posted 16 October 2013 - 02:49 AM

The brass I know only comes in 1' sections, but I'd recommend 9/16" brass to fit your darts for compressed air propulsion. Double check when you get your barrel material by bringing a dart with you to make sure it's right. Also, in regards to accuracy, I've been wanting try porting the muzzle similar to how a muzzle brake works, only instead of controlling recoil it would vent high pressure gas just before the dart leaves the barrel, such that when the dart actually leaves the barrel there is no muzzle blast to destabilize it, and also it wouldn't risk bursting the dart as much with higher pressures. I know it's all theoretical and has no evidence of working, but food for thought.
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#15 Doom

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Posted 16 October 2013 - 09:18 PM

Also, in regards to accuracy, I've been wanting try porting the muzzle similar to how a muzzle brake works, only instead of controlling recoil it would vent high pressure gas just before the dart leaves the barrel, such that when the dart actually leaves the barrel there is no muzzle blast to destabilize it, and also it wouldn't risk bursting the dart as much with higher pressures. I know it's all theoretical and has no evidence of working, but food for thought.


There's tons of evidence that muzzle brakes and porting improve projectile stability. But it's not really necessary, as if you use an optimal barrel length the pressure inside the barrel will be pretty close to atmospheric when the dart leaves, so any muzzle blast that exists is probably completely negligible in terms of the destabilizing effect.
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