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'New' blow back / forward design

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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'New' blow back / forward design

Unread postAuthor: The Engineer » Thu Oct 16, 2008 10:44 pm

Essentially this is the same thing as a couple of other designs on the forum, but it is made from different parts (as much as I can tell). I'm just wondering how well it will work.

The pressure builds up and pushes the piston back, and it passes the hole. The pressure is then released through the hole and into the barrel. The spring pushes the piston back forward past the hole and the process repeats. This would make a fully automatic gun that doesn't eat up BBs like crazy. It would also have an adjustable rate of fire which depends of the strength of the spring. In other words, stronger spring = lower rof and weaker spring = higher rof.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 17, 2008 1:41 am

It looks good on paper, but let me tell you what will happen for a constant flow. You'll get one good shot, then the spring will return the piston far enough to fart air through the barrel with a pathetic *pfffff* noise.

I know this only too well, check out the auto piston ideas that were floated around a while ago, and then some.

It would work well for semi-auto though, but you should use the piston motion to work some sort of breech to reload BBs from a magazine.
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Unread postAuthor: jimmy101 » Fri Oct 17, 2008 10:12 am

Like Jack said. The design is basically a pressure regulator. The first shot'll be OK but after that the piston will just sit so that the port to the barrel is open just enough to maintain enough pressure drop to hold itself open.

To get this to work I think you would need to severely restrict the air flow through the blowgun. If you can dump air out the barrel much faster than the blowgun can resupply it then the piston will have a chance to move back to the fully closed position. It'll then take a "long" time for the chamber to repressuirze enough to open the piston valve again. "Long" time might be something like a couple tenths of a second. If the barrel can dump the pressure significantly faster than that it may work.
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Unread postAuthor: ALIHISGREAT » Fri Oct 17, 2008 10:49 am

maybe something more like BTB's snapvalve? sorry i don't have time to find a link.
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Unread postAuthor: The Engineer » Fri Oct 17, 2008 6:40 pm

that's what i was worried about. thanks for telling me before i tried it. could have been a real waste of time. but you said it would work for semi automatic, right? thats fine with me, so i may still build it and then try to modify it some and, like jimmy101 said, restrict the airflow or something like that.
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Unread postAuthor: psycix » Sat Oct 18, 2008 5:22 am

When looking at that design I am worried about the flow.
You press the blowgun, air enters chamber, pressure builds, piston gets pushed further and further as pressure builds, and then when the piston opens up the gap, the pressure drops, shutting it again.
It may oscillate (aka fart) that way or keep constant.
Anyhow, you will theoretically not be able to get more flow then the blowgun can put out, because that is the amount of flow that is vented through before the pressure starts dropping, and thus the system will be in equillibrium when the flow in is the same as the flow out.

One aspect has not yet been spoken of now: Once the piston has a certain velocity, it does not stop on the point where the resulting force becomes 0.
Its moves on a little and then reverses.
Using a heavy piston would make sure it completely opens up, and when it closes again, the projectile would already have left the barrel.
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Unread postAuthor: The Engineer » Sat Oct 18, 2008 3:12 pm

That's a good point, I thought about that some. I used a ton of epoxy on the piston to make it heavier (epoxy for a reason, you would have to see the actual gun). Thanks, I wasn't sure if I was correct on that aspect.
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Unread postAuthor: psycix » Sun Oct 19, 2008 10:09 am

If you need more weight, add lead inside the epoxy casting.
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Unread postAuthor: The Engineer » Sun Oct 19, 2008 4:58 pm

Good idea, I can melt some into shape over a fire with a mold.
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 19, 2008 5:37 pm

The Engineer wrote:but you said it would work for semi automatic, right? thats fine with me, so i may still build it and then try to modify it some and, like jimmy101 said, restrict the airflow or something like that.


The thing is though that your design makes no provision to reload the BBs taking advantage of the piston movement.

Have a look here and here for some inspiration ;)
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Unread postAuthor: SEAKING9006 » Sun Oct 19, 2008 5:45 pm

Some have said that my design in particular would be fairly simple to implement, relying on basic electronics with physical triggers for timing, rather than pure mechanical precision during construction, as well as requiring only basic materials and components, the most complicated single part having under 5 machining processes necessary, and all parts can be made with only hand tools, though a drill press would help ALOT. I'm not going to link to it right now, because there's a crapload of stuff I'm trying to multitask right now, so just search X-ACS.
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Unread postAuthor: The Engineer » Sun Oct 19, 2008 7:31 pm

I already have a way to load BBs into the barrel, I simply left it out of the drawings for simplification.
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Unread postAuthor: kablooie » Sun Oct 19, 2008 7:48 pm

Here is my attempt at what you have described in your design. It worked, but it sucked air like nothing else (like a normal bbmg) and the power was relatively weak. Here's the link:
http://www.spudfiles.com/forums/auto-ra ... 15721.html

On the second page there is a vid of it working. If you will just be shooting bb's I highly recommend using an o-ring (or rubber tube) breech in a cloud bbmg type setup, it works very well for me and the rate of fire is adjustable by the incoming flow, unlike with the ram design.
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Unread postAuthor: The Engineer » Sat Nov 01, 2008 1:10 pm

I finally tested my design after adding more epoxy in front and behind the O-ring for stabilty. It also served to add much more weight to the piston. And guess what? It worked! It had a very high rof, so i plan on slowing it down with either a stronger spring or more weight. I also have some plans for a magazine loading system.
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Unread postAuthor: jonnyboy » Sat Nov 01, 2008 1:19 pm

Can you post a video or some pictures of the thing.
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