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auto qev (using piston to load chamber).

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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auto qev (using piston to load chamber).

Unread postAuthor: i-will » Thu Nov 06, 2008 2:50 pm

please let me know if it's been done already.

i wanted to make this before i showed everyone but i don't have the time and my budget is $0. it didn't feel right to hold back an idea that long anyways so here it is. i came up with this when wanted to make a small coax handgun and semi-auto snipe for airsoft. i reminded myself of a personally important method. find energy and use it. the piston has a lot of it and i intended on using it.

there is a blow-back and blow-forward system and they both pretty much work exactly the same. this is just a rough mock-up to share the basic theory so try not to bash the pics though u will be tempted to do so.

my attempt at an explanation is that when the piston pulls back to fire it carries a tube with it. this tube is the main mech for the breech and has openings close to the piston for the flow of gas. when the piston opens it also closes the breech. the brown thing is hop-up (for those airsofters). because the breech is so simple u can use any ammo that fits in the barrel but if u want a mag system u should stick to one type of ammo. the blow-back system carries the whole barrel back to close the breech. the blow-forward works more like a paintball gun and closes the breech from behind and leaves the barrel in place. it's my opinion that the blow-back seals better.

<a href="http://s90.photobucket.com/albums/k273/PROGRESSKATE/?action=view&current=autocoax.jpg" target="_blank"><img src="http://i90.photobucket.com/albums/k273/PROGRESSKATE/autocoax.jpg" border="0" alt="Photobucket"></a>

what do u think? would u build it?
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 2:55 pm

have a look through this thread ;)

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Unread postAuthor: Ragnarok » Thu Nov 06, 2008 3:02 pm

I've seen the idea before, but I wouldn't build it, because consider the huge weight of the barrel will massively slow valve opening and performance.

I also don't know exactly what you're planning, but if you do try this - build it with a full shroud - you won't want to get your fingers caught up with the kind of forces piston valves can create.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 3:16 pm

Ragnarok wrote:I've seen the idea before, but I wouldn't build it, because consider the huge weight of the barrel will massively slow valve opening and performance.


That and it would make o-rings necessary, hence my suggestion in the diagram that the barrel be fixed and an inner bolt is connected to the piston instead to block off the mag.
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Unread postAuthor: i-will » Thu Nov 06, 2008 3:30 pm

the barrel that i want to use is a 6 inch aluminum airsoft barrel so it's pretty light.

where would i need o-rings?
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 06, 2008 3:56 pm

i-will wrote:where would i need o-rings?


Scratch that, assuming the piston seals against the tube the barrel runs it o-rings won't be necessary.

If you make it an auto you'll get the cool M2HB/Bofors/Mk19 recoiling barrel effect :D
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Unread postAuthor: i-will » Thu Nov 06, 2008 4:32 pm

thats what i like about this. recoil feels nice and realistic. i was thinking about connecting the barrel to the slide when i make the hand held version. give it that blow-back action and recoil.

thanks for the warning ragnarok. for the rifle version i plan on using a floating barrel method like the magpull masada. the barrel will float inside the hand guard.

how do i figure out what size the coax should be compared to the length of a 6mm barrel?
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Unread postAuthor: psycix » Fri Nov 07, 2008 3:43 pm

Point is that the whole bolt assembly first accelerates and then stops dead, which neutralizes eachother.
Accelerate: An X amount of impulse is transferred onto the gun
Stop dead: The X amount of impulse is transferred onto the gun in reverse direction.
Because all of this happens within a few ms and thus you will not feel any recoil. By the time force is applied onto your body from the bolt accelerating, it already stopped dead.

The complex bolt assembly will indeed decrease opening times (and thus performance) due to its weight.
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Unread postAuthor: keep_it_real » Fri Nov 07, 2008 6:29 pm

I've tried this: link
I've been working on it a bit since that post and made some improvements. I can now get it to fire 250-300 fps at about 1 bps. The biggest problem I've found is the slow refill time but I think I have an idea to fix that. My version does move the whole barrel with the piston which makes for a tiny delay but once it fires, it fires.

Some recommended changes:
Bigger piston relative to barrel.
Near perfect fit between barrel and whatever supports it. I used 3/4" cpvc inside 1" cpvc. It is a pretty close fit but not good enough.
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Unread postAuthor: i-will » Sat Nov 08, 2008 4:51 pm

keep_it_real, i'm glad to hear that someone else has tried this. this gives me a chance to learn a thing or two.
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