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Sleeve Breech

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Sleeve Breech

Unread postAuthor: PVC Arsenal 17 » Thu Oct 28, 2010 12:22 am

I'm looking to make a simple breech system for a nerf gun barrel with outer diameter .625" and inner diameter .5". There will be a cut made into this barrel for the breech as shown in the picture and a sleeve is to cover this when ready to fire.

My question is: Without pushing the dart further into the barrel after dropping it into the breech, will there be a signifant loss in performance due to air getting around the dart through the breech? (Ignore possible leakage through the sleeve, that is not an issue.)

I've seen this done before with good results, but the difference between ID and OD of the barrel was not nearly as great. My fear is that the darts will "jump" because of this extra space and create a path for air to travel through.

I'm away at school now and normally I'd be able to test something as simple as this quite easily, but now I'm forced to just work on ideas rather than building them, so I've come to you for help.

Thanks!
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 28, 2010 12:47 am

My thoughts are that given the lightweight nature of the dart, the instant it's exposed to pressure it will move forward enough to enter the barrel past the slot and seal, preventing losses though the muzzle before it exits.
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Unread postAuthor: ThornsofTime » Thu Oct 28, 2010 12:51 am

Drop in performance? As to that I would have to say yes. Any time in which you have less than a 100% seal on your ammo you will see a drop in performance.

However, significant?? well thats all relative now isn't it. If you were trying for a defense contract with a pneumatic launcher I would say to you that this is a MAJOR issue that needs redesigning... but this is a nerf gun. Hardly the most advanced ballistic of applications. To that end I dont think you would see too big of a difference either way. I would expect that, in terms of accuracy and power, you wouldn't see more than a ~5% increase even if you were muzzle loading the darts.

As a side note... Usually when making this type of breach you are already saying that this is gonna be a single shot launcher. If you do happen to notice a change in performance it really isn't too big of a deal. You're already dedicated to a reloading procedure between shots... why not just push the dart into the barrel and call it a day.

Just my two cents
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Unread postAuthor: BigGrib » Thu Oct 28, 2010 2:52 am

I agree with Thorny about losing performance with the loss in pressure from the incomplete seal on the breech. So you just need to find a way to seal the breech sleeve. Either find a way to use an o-ring to seal the breech, or a wrap of some sort of tape, you'd gain efficiency there, and I don't really think that you would lose too much out of the end of the barrel due to blow by. Like JSR said as soon as the tip of the dart enters the barrel on the downstream side of the slot, it's sealed and all of the pressure is being applied to the dart.
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Unread postAuthor: Technician1002 » Thu Oct 28, 2010 8:38 am

On a larger scale, it worked fine on a t shirt launcher. PM me if you missed the video.

The amount of performance hit will depend on chamber size. A small chamber can lose a high percentage of air in blow by. A large chamber will lose little pressure to blow by before the projectile seals the barrel.
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Unread postAuthor: Big-E » Thu Oct 28, 2010 10:41 am

I used a coupling to form a breech in my last barrel, I cut a rather long breech opening on the .5" copper (M rated copper I believe) The darts worked great, but there's a trick to it; You need to use the streamlined "clip system" nerf darts, as all others are too tight. If you want a tighter fit in the bore, get a heavier tube, either L or K rated copper, M had a little blow-by, but not much. Higher velocities would demand a tighter fit, but too much pressure or not enough blow-by may rupture the dart in the process of firing it.

Of course, I do chamber the darts, but it wouldn't be required. You see, air under pressure is prone to take the path with the least resistance, so long as your ammo isn't super tight, the air will take the easier path, and carry the dart out of the barrel rather than force itself through the hairline gap between the coupling and the barrel. Yes, it's true it may leak a little, but the majority of your pressure is moving that dart, so the loss isn't that noticeable. Nerf darts are lightweight compared to a AA battery or a lead ball, there's less mass there to resist the air pressure, so I tend to lean towards the idea of using a coupling as a breech slide for nerf darts. just be sure to taper and polish the transition to the barrel so the dart doesn't hang up. Once that dart gets started in the barrel the pressure will do it's job. The gap from your breech opening would only be a problem in that it could potentially hang up the dart, the loss of air wouldn't be as big of a concern to me in that regard.
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Unread postAuthor: PVC Arsenal 17 » Thu Oct 28, 2010 8:20 pm

Thanks for all the replies. Being that this is a spring gun and its "chamber" volume is limited, I've reconfigured it to avoid having to use this type of breech.
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 28, 2010 10:16 pm

PVC Arsenal 17 wrote:Being that this is a spring gun


oh?
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