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Barrel Transition

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Barrel Transition

Unread postAuthor: PVC Arsenal 17 » Sat Feb 13, 2010 11:47 pm

Suppose I fired a Nerf dart through a .5" bore barrel and then wanted it to travel through a .527" bore barrel extension for about 4 inches. Assuming the joint between the two barrels is completely sealed and centered, would this transition cause any adverse effects to the projectile?

Being that the transition is from small to large, there is little chance of the dart impacting the opening to the extension, but I'm worried that backpressure would be able to make its way around the dart when it's inside the larger bore and cause some sort of negative effect.

Any input is greatly appreciated.
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Unread postAuthor: Marffy » Sat Feb 13, 2010 11:55 pm

I'm no expert at this but i would think that since nerf darts aren't perfectly balaneced and are not perfectly aerodynamic i would think that there would be a small chance maybe 50/50 that the dart could tumble and fly at an angle. then wind might pick up one side of the dart a send it off trajectory. but since it is only 4 inches it probably woudn't happen. and, usually when i make silencers the silencer bore is usually bigger and i experience no inaccuracy. but then again the there is holes in the silencer bore which allow the silencer to swallow the air and sound that may lead to inaccuracy.what really interests me in this is why would u do this? why not use the same barrel material? best of luck with what ever you plan to do.
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Unread postAuthor: inonickname » Sun Feb 14, 2010 5:55 am

I don't know why he wouldn't just use the one, correct diameter of barrel. If it's a spring powered gun there may not be enough gas to propel it through a tightbore barrel efficiently (you'll notice a lot of nerf guns have a fairly loose bore).

Worst comes to worst, is the dart jams. Unlikely though, seen as it probably wont be at very high velocity, and the dart is fairly long and smooth.
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Unread postAuthor: psycix » Sun Feb 14, 2010 1:11 pm

An artichoke??
The last piece of barrel is the piece that determines the largest part in accuracy, thats why modern barrels have a choke where the last part is extra tight.

This WILL make your barrel less accurate. How? Search for "golfball in a drainpipe"
It will not cause jams.
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Unread postAuthor: jackssmirkingrevenge » Sun Feb 14, 2010 2:45 pm

psycix wrote:Search for "golfball in a drainpipe"


True, true.

Not my quote though.
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Unread postAuthor: btrettel » Sun Feb 14, 2010 3:53 pm

I really don't get how this would make the dart less accurate. There is a big difference in the diameter ratio between here and a golf ball in a drain pipe. Not to mention the length of the dart. The dart won't rattle around. I've used aluminum 0.527 inch diameter barrels with Nerf darts before and they're a pretty good fit for the foam I can get.

As far as I know, choked barrels are used only in shotguns to control the spread of the pellets. (I'm no expect on guns though.) How would choking the end of a barrel with a normal projectile improve accuracy? I think I might be missing something.
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Unread postAuthor: PVC Arsenal 17 » Sun Feb 14, 2010 4:51 pm

I can't discuss the details of the project too much as it's going to be my entry in the Nerf Gun Challenge, but I will say that the first length of "barrel" ...if you can call it that... will be made of plastic to save on cost, whereas the second length of "barrel" will be made out of metal for certain reasons. The metal and plastic have different bore sizes, and I cannot use the plastic for the second length of barrel.

I can avoid having different bores by sleeving the inside of the larger metal tube with a brass tube, but I'm asking this question because it could potentially save me lots of time sanding the brass tube to fit inside, and also because there's a chance the increase in bore size could actually benefit the design, provided it doesn't cause any negative affects.
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Unread postAuthor: jackssmirkingrevenge » Sun Feb 14, 2010 11:51 pm

btrettel wrote:I really don't get how this would make the dart less accurate. There is a big difference in the diameter ratio between here and a golf ball in a drain pipe.


The expression is just an exageration to illustrate the point. If these foam darts actually expand to some extent it might not be a problem in this case.

As far as I know, choked barrels are used only in shotguns to control the spread of the pellets. (I'm no expect on guns though.) How would choking the end of a barrel with a normal projectile improve accuracy? I think I might be missing something


Not sure about firearms but many air rifles have choked barrels.
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Unread postAuthor: btrettel » Mon Feb 15, 2010 5:14 pm

Seems I read things too literally. :lol:

I did some research, and the only reason I can find to justify putting a choke on an airgun is to ensure that the pellets leave the barrel at a consistent size. Consistency is important for accuracy, so this makes sense.

However, foam is fairly elastic---small deflections will not permanently deform it. So the reasoning used for airguns can not be used for Nerf, spuds, or any other projectile that will not be permanently deformed by a choke.
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Last edited by btrettel on Mon Feb 15, 2010 5:51 pm, edited 4 times in total.
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Unread postAuthor: Ragnarok » Mon Feb 15, 2010 5:34 pm

psycix wrote:Search for "golfball in a drainpipe"

I went to Google for that one.

First link was a hilarious story about how a man had blocked his shower drain by dropping a golf ball down it.

@btrettel: That's my understanding of air rifle chokes.

However, as an aside I'd note that usually they're double choked.
One choke from the breech down to the main barrel, then another down to the size of the muzzle.

That way you've got the low friction for loading at the breech, then a tighter bore to stop excessive blow-by in the barrel, and then a final (higher friction) section that forces the pellet down to a consistent size.

It's actually a very impressive thing to observe. I've managed to recover a few pellets undamaged (mostly), and it's interesting to see the consistency in both size and roundness before and after firing.
Also, the rifling marks are quite neat.

Well, it's brilliant right up until you load a pellet that's slightly too large a diameter and it gets stuck in the muzzle choke, then have to go off muttering to find the bore rod.

As you say - workable on a projectile that will deform plastically. Not on one that deforms elastically.
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