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air sprung nerf dart gun

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air sprung nerf dart gun

Unread postAuthor: cfb_rolley » Wed Nov 03, 2010 2:45 am

so i thought of this today, and would like this theory to develop into a working gun, if it's feasible.

basically, in a nerf n-strike blaster, such as a longshot cs-6, there is a mechanism that is usually modified to increase power. it consists of a bolt that when pulled back both makes way for a new dart, and pushes a piston back to a position where it locks in. the bolt is then pushed forward, sliding over the dart ready for firing, and leaving the piston in place. when the trigger is pulled, the piston is released and slammed forward by a spring behind it, forcing the air between it and the dart to force the dart out the barrel, and the cycle can repeat after this.

modifying them usually involves a spring replacement or the addition of an extra spring that fits inside the main spring, so that the piston moves faster.

my idea is very similar, but involves o-ring seals, stronger parts/ materials, and the use of compressed air in place of a spring, and the preloaded air pressure could be increased or decreased via a schrader valve in the rear, depending on the power needed and the strength of the user (to be able to pull the bolt back).

the reason? nerf wars are a ton of fun, so is sneaking up on someone unsuspecting a shooting a barrage of foam darts at them, even if you're probably too old to be mucking around with toy guns :lol: not fun however, is how flimsy they can be and how often they can break, so a stronger, better nerf gun would be awesome right?

extra details:
-the piston and bolt need to travel at least 85mm to be able to load the dart from a magazine
-as far as i know, the larger the bore, the more power it has
-a modified longshot gets ranges of 60-100 feet, i want to try to double that.
-all of the parts can be cnc machined from either pvc, polycarbonate or 6061 alloy.

so, what does everyone think? could it work? is there anything i need to think more about?
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 03, 2010 3:38 am

Exactly the same principle behind gas ram air rifles, the theory is sound ;)

Why doesn't the head of your piston match the conical breech though? Not only does it eliminate dead space but spreads out the impact force reducing wear on the piston head.

Charging it might be a chore however if you increase the "spring" strength too much, you might want to consider a charging lever as on underlever spring piston airguns.
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Unread postAuthor: cfb_rolley » Wed Nov 03, 2010 3:43 am

sweet. good idea on the dead space thing. how would i calculate how much force it would take to pull the bolt back at different preload pressures? it would be proportional to the amount of compression wouldn't it?
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 03, 2010 3:51 am

You can use some basic calculations to estimate strength.

For example imagine a 1" diameter piston with 4 inches of travel preloaded to 50 psi.

At rest, the 50 psi is acting on a piston area of 0.79 in<sup>2</sup> so it's exerting a force of approximately 40 lbs. If you pull back the piston 2 inches, gas spring volume is halved so pressure is doubled, and you'll now need 80 lbs of force to hold the piston back.
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Unread postAuthor: lozz08 » Wed Nov 03, 2010 3:52 am

There will be a sweet spot for the ratio of piston diameter to projectile diameter.

The wider the piston, the harder it can push the projectile at a given piston speed, because it is moving more air at once. The sweet spot is balancing the flow choking due to air being squeezed into a smaller orifice, the friction due to all the seals and air volume being pushed.

All these factors make optimisation of this project extremely complex both mathematically and mechanically.

First up, the air behind the piston pushing it won't be optimal. It might push hard on the piston at the beginning at the stroke, but the pressure will decrease a lot by the end of the stroke.

You want as close to constant force as possible.

Image

This way, when cocked, the volume of the reservoir does not change by much, giving you a more constant force.
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Last edited by lozz08 on Wed Nov 03, 2010 3:53 am, edited 1 time in total.
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Unread postAuthor: Crna Legija » Wed Nov 03, 2010 3:52 am

i tryed to use pre-pressurized air to load ammo for a semi but found it really hard to push the piston back, that was only 15psi in 1/2inch cpvc.
i think if you don't have a leaver it would be nearly impossible to recock

what i just said it even in the link in jsrs posted.
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Unread postAuthor: lozz08 » Wed Nov 03, 2010 3:55 am

Like I said, that is because the volume behind the piston is changing too much at fully cocked state. The pic above allows you to have a lower peak force and more energy overall.
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Unread postAuthor: cfb_rolley » Wed Nov 03, 2010 4:13 am

how about if the reservoir was larger? so that the piston still only travels back 85mm, but the pressures would not increase as much, however there would still be preload pressure to push the piston. remembering that i'm not looking for maximum possible performance, just looking to fire a nerf dart 120 feet or more (40 meters)
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 03, 2010 4:23 am

cfb_rolley wrote:how about if the reservoir was larger? so that the piston still only travels back 85mm, but the pressures would not increase as much, however there would still be preload pressure to push the piston.


That would work, a bit like the air spring I had put into the action of my first combustion cartridge prototype.

Image

Any reason why you're going for a spring type launcher as opposed to a pneumatic one? I see the benefits of having it self contained without having to worry about pressure reservoirs, but if you're looking to increase performance then a pneumatic launcher would make it easier to achieve and I'm sure that using a blowgun as a valve would suffice, something like I proposed here:

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Unread postAuthor: lozz08 » Wed Nov 03, 2010 4:55 am

I totally agree, I don't think there is anything to gain by making a spring gun other than no need to pump it up. I tried to make a spring gun and it nearly killed me it was that annoying and difficult.

The setup above is a very good one. I used pretty much that same setup and it was awesome. Power is easily adjustable too.
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Unread postAuthor: cfb_rolley » Wed Nov 03, 2010 5:24 am

lozz08 wrote:I totally agree, I don't think there is anything to gain by making a spring gun other than no need to pump it up. I tried to make a spring gun and it nearly killed me it was that annoying and difficult.

The setup above is a very good one. I used pretty much that same setup and it was awesome. Power is easily adjustable too.


that's the idea, some nerf wars can go on for a very long time, so needing to stop and pump up can be quite hindering unless you have a big tank, which is needless to say, bulky when running.

i have done the blowgun idea, results are actually pretty good especially using a kind of detent, but again, looking to eliminate the need for needing to pump up all the time. this is one idea i want to get working before revisiting the blowgun and air tank reservoir idea
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 03, 2010 5:47 am

How about combining the ideas and making a single stroke pneumatic like the Parker Hale Dragon?
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Unread postAuthor: Hubb » Wed Nov 03, 2010 7:43 am

A single stroke pneumatic is an idea I've been tossing around for a while now. Out of all the ways of constructing a Nerf blaster, it seems (to me) that this is the most promising. It will allow for a portable use but still be powerful enough to achieve primary status.

The current design I'm working on now is estimated to put around 10 psi into a chamber, which, giving the chamber volume, should be plenty of force to achieve at least 100 to 120 feet.

And from my experimenting and investigating, it seems that a single stroke pneumatic may even be easier to construct than a springer.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 03, 2010 8:02 am

Hubb wrote:The current design I'm working on now is estimated to put around 10 psi into a chamber, which, giving the chamber volume, should be plenty of force to achieve at least 100 to 120 feet.


I would be more inclined to use a long stroke small diameter pump to give a much higher pressure but smaller volume, then use a small QEV to get the most of that small amount of air.

Also, nothing wrong with having a 2 or 2 stroke pneumatic, unlike commercial hammer valve rifles with an integral pump like the Crosman 760, a QEV style valve doesn't have a "sweet spot", more pressure will always equal more power :)
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Unread postAuthor: lozz08 » Wed Nov 03, 2010 8:05 am

Why not undermount a hand bike pump, one pump should be enough to get 40 psi into a smallish chamber. SSP does seem like the best option here.
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