16 replies to this topic

[Jergling]

Anyway, is there a plus to longer barrels, or shorter barrels, in real guns, the bullet is straightened in flight by a long barrel, in Nerf guns, the pressure behind the dart is constant, and the dart may even be slowed by friction.

So, what barrel lengths are reasonable? I understand that having a dart protrude from the end is power reducing (thus the use in store-bought guns) but is there any reason to make a barrel excessively longer than the dart itself?

I would tell you about why I'm asking this, but that would be one of those "can it be done?" posts, which is flame inducing. if that makes you mad-





I know this is half the kind of "asking and not doing" post that gets flamed, but I believe it is a valid question.

[Flaming Hilt]

Yes. As you said, in Nerf guns, the pressure behind the dart is constant. Well, to be constant, it has to exist for a certain amount of time. Keeping the dart in the barrel for longer than that pressure exists slows the dart down and decreases range.

But, not keeping the dart in until all the pressure is used up wastes that pressure. Keeping the dart in the barrel for as long as possible allows the pressure to stay behind it until the dart has reached its maximum speed, at which time it's perfect for the dart to exit.

In less words: Yes, and the reason is speed. A 2" barrel only puts pressure on and accelerates a dart for 2". A 4" barrel for 4", etc. until you are out of pressure and out of acceleration, and then (as you said) friction kicks in and the dart begins to slow down again.

[PennyWise]

Well, yes and no. I can't give you any spesifics or anything but it depends on the power of the gun and the fit of the dart. Like you wouldn't put a foot long 1/2" cpvc barrel on a nitefinder, or a 4" 1/2" cpvc barrel on a singled titan. All I could tell you is the longer the barrel the more power and accuracy but there is a point where your barrel length is too long and starts to kill power.

[Jergling]

I think I understand flaming hilt, but:

To be sure I'm getting this right, are you saying the barrel should be only the same volume as the plunger tube? that makes sense.

Revealing a bit more about my 'under wraps' gun (no I haven't started it), I plan to have a plunger tube and spring slightly more powerful than a maverick (for less cocking pressure), and I'm trying to avoid centrifical spray from a moving-and-shooting turret. I'm thinking about 6" barrels with 2-3" shells. I'm inexperienced with this, so my shells may suck, but regardless, would nearly 10" of barrel be reasonable for a maverick strength gun?

Thanks for the quick comments.

[TriggerHappyAzn]

One way to find the ideal barrel length for lower powered guns would be to affix too long a barrel (so that the dart gets stuck before reaching the end of the barrel) onto the gun then firing a dart through it. Halfway between the place where the dart got stuck and its starting point should be where it reaches its max velocity.

[Flaming Hilt]

Quote

To be sure I'm getting this right, are you saying the barrel should be only the same volume as the plunger tube? that makes sense.

In a perfect world, yes. Unfourtunately, there is a frickin' lot of friction so I'd do that and divide by 3 or 4. BS figured out the exact number when he built his FAR; I can't remember what it was.

Here's what I usually use, being the lazy bum that I am.

Edited by Flaming Hilt, 11 April 2007 - 09:49 PM.

[TriggerHappyAzn]

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In a perfect world, yes. Unfourtunately, there is a frickin' lot of friction so I'd do that and divide by 3 or 4. BS figured out the exact number when he built his FAR; I can't remember what it was.

There is no "perfect" / ideal ratio because we all have different parts, spring, seals, etc. This kind of stuff is better determined on a case-to-case basis. When an elastic object (rubber bands, bungee cord,compressed spring, etc.) releases its stored energy, it reaches its maximum velocity about halfway through the action, which in turn translates roughly into the movement of the dart going down the barrel. You're probably best off figuring out the optimal barrel length for your gun on your own.

Edited by TriggerHappyAzn, 11 April 2007 - 10:40 PM.

[Carbon]

You don't want the barrel to equal the volume of the pressure chamber; what you want is the barrel volume to equal the volume of your pressurized pressure chamber. That way, You have the perfect amount of pressure to expel the dart from the barrel. Too little, and the dart slows due to friction. Too much, and it fishtails due to venting of exess air behind the dart.

Of course, there's no really good way to determine what the pressurized volume of the pressure chamber is, as that varies depending on the friction and seal inherent in each gun. Boltsniper figured the ratio on the FAR to be about 4:1, and it seems to be a pretty decent rule of thumb for barrel length. You can vary it from there to match your gun.

[Flaming Hilt]

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There is no "perfect" / ideal ratio

What I meant was, there is friction in the universe. I could have been more clear. You are entirely right, however.

Quote

what you want is the barrel volume to equal the volume of your pressurized pressure chamber.

Heh, again, could have been clearer but that's what I meant.

So, listen to Carbon and Trigger Happy because they have their heads on straight today... unlike someone here. XD

[Master Yogurt]

A 10" barrel for a Mav-power gun is probably far too much. The Mav has very little power, 10" is more suited to a BBB or something. Six should be a sufficient size.

When European traders traded guns for deer fur to Native Americans, a gun was worth its height in piled deer skins. They realized that they could make the barrels longer to get more furs, and the NA's thought that the longer barrels were better, so they ponied up the furs for guns with barrels that were far too long. Just a random historical sidenote.

[Retiate]

I think 6" might even be too long. A Nitefinder barrel replacement is usually around 5" long, and a Maverick spring is quite a bit weaker than a Nitefinder spring. So maybe a 4"-5" barrel would be good. And that might even want to be shortened a bit if your using shells that are 2"-3" long.

[Recruit]

Ok, Flaming Hilt, here is what you do. Take your plunger assemblynow "cock" it so it is ready to fire now plug it so the preassure built up by the plunger remains in the plunger tube. Now meassure how far the plunger was from the end of its reach does this quickly so no air can leave. Now use your calculator figure the volumn of the area from where the plunger stopped to the end of the plunger tube (I will refer to it as ECA=empty chamber area). Now you take the volumn of the ECA and take the measure of the circumference of a nerf barrel and use that to find the right length of barrell for your gun. This should I say should because I am not 100% sure that this will work with nerf guns. I just know this works reasonably well with spud launchers

[elchupathingy]

Another thing a good ratio of 3-4: 1 is good if its a airpowered gun, but if its a plunger a ratio of about 2-3: 1 is good.

Also the airpressure in a air or plunger gun is not constant it may seem constant but it is not. It has to accelerate to its top speed and the point where the does not accelerat any further is the point that the nerf dart has gotten all of its speed. The distance from the first point to the other point is hte length of the barrle. Usually just finding hte volume and such is good enough then just multiply or divide it by 2 or 4 and you got your self a barrle or a tank.

[Prometheus]

Recruit, on May 2 2007, 11:21 PM, said:

Ok, Flaming Hilt, here is what you do. Take your plunger assemblynow "cock" it so it is ready to fire now plug it so the preassure built up by the plunger remains in the plunger tube. Now meassure how far the plunger was from the end of its reach does this quickly so no air can leave. Now use your calculator figure the volumn of the area from where the plunger stopped to the end of the plunger tube (I will refer to it as ECA=empty chamber area). Now you take the volumn of the ECA and take the measure of the circumference of a nerf barrel and use that to find the right length of barrell for your gun. This should I say should because I am not 100% sure that this will work with nerf guns. I just know this works reasonably well with spud launchers

No. Definitely not. Friction, and lack of a perfect seal result in your ideal barrel being too long. You need to shorten it up a bit.

[Recruit]

Ok, Prometheus you are right.

[Jergling]

Wow, I didn't expect posts back on this any more, but I have some more information I'm going to disclose:

I built the plunger tube for my gun, and I'm getting the fishtail thing, my plunger tube is reverse of normal homemades like the SCAR/FAR, with a large tube moving over a small one (the barrel). I installed a very basic breech, and rubber bands force the plunger forward since I have no springs. This plunger easily has several times the power of the Mav, so scratch that.

Moving on, the breech sucks. Where do I get foam sheets? I've been using duct tape for seal... also, right now the only thing really hindering the performance to an average of 20' is that half of the shots are misfires due to the duct tape's rubberiness. I think with a smooth (and sealed) breech and plunger I could raise my range to at least 60' (good enough for what I want to use this for)

[Prometheus]

Just to elaborate on the effects of barrel length, I put an 8.5" 17/32" brass barrel on one of the barrels on my AT3K turret, and another 12" 17/32". the 85" got ranges around 70'-80', where as trhe 12" got 60' tops. Also, shotgunning the darts (2 of them) with the 8.5" got 40', minimal spread, where as the 12" got one 45', the other 5'. So you see, bigger is not always better.