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Topic: Homemade Semi-Auto Idea. Plausible Or No?

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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Unread postAuthor: Vudu » Thu Mar 26, 2009 6:42 pm

Well, there's the two pressure chambers, the left on is the one which has the air to shoot the projectile.

The one on the right is sealed in to prevent air waste and will only consist of around 0.1 cubic inches, at most. It's basically pipe being flush with a size up piece of pipe, with rubber washers to keep it completely sealed, avoiding leaks. The other will be about 3X3X2or3 inches of air space.

And considering I'm switching over to full metal components with a paintball 50/4800, and a reg, I will be using fairly high pressures.
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Unread postAuthor: SPG » Fri Mar 27, 2009 9:09 am

So for GGDT:

Use generic as your valve type, I almost always end up using it for odd valves and it'll give you an idea.

Use the volume between the two parts of your valve as your chamber size, 0.1 ci in your case, the other part of your gun is more like a reserve tank really.

The good thing with GGDT is that for your particular idea you can work out not only the ideal pressure but also use it to tweak the size of your valve chamber.

As for a regulator, I think that depends on: what pressure you're hoping to use, how large your magazine capacity is, the ratio of your valve chamber size to your reservoir size, how often you want to pump the thing up again.

Ideally you'd want one, but you could always start without to test your ideas and then add one later if they work.

I think the whole thing though is very promising, it's a nice simple way to get a repeating airgun.
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Unread postAuthor: Vudu » Fri Mar 27, 2009 6:20 pm

Wow, thanks alot! :) Time to work out the ideal numbers and get this thing into production.
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Unread postAuthor: Vudu » Wed Apr 01, 2009 5:47 pm

So far it's going slow due to me being out so much during spring break. But I am stumped on one last number, I am getting absurd ammounts of FPS and range, and I think it is due to either Flow Coefficiency, Seat Diameter, Open Time, or Dwell Time.

So can anyone help me? I mean, I'm getting 412 FPS off 100 P.S.I, whereas Tippmanns use 340 P.S.I, as a low point, to get around 300.

And yes, all my other measurements are correct, it's just these ones which confuse me.
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 01, 2009 10:22 pm

Vudu wrote:So can anyone help me? I mean, I'm getting 412 FPS off 100 P.S.I, whereas Tippmanns use 340 P.S.I, as a low point, to get around 300.


Projectile weight and diameter?
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Unread postAuthor: Vudu » Thu Apr 02, 2009 1:13 am

.68 diameter, 3.201g mass. Using a .685 diameter barrel.
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 02, 2009 1:55 am

That does sound a bit optimistic, though not impossible. Your unusual valve parameters are hard to model though, particularly valve opening time.
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Unread postAuthor: Vudu » Thu Apr 02, 2009 2:04 am

Hmm, well. I can give you the numbers I have in for the unknown variables listed in above post if that would help.


Flow Coef : 35%

Seat Diam : 2 in (I took this to be what the ID of the pipe will be, as in, the diameter of my rubber washers.

Open Time : 2.5 milliseconds

Dwell Time : 0 milliseconds
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 02, 2009 2:07 am

Vudu wrote:Seat Diam : 2 in (I took this to be what the ID of the pipe will be, as in, the diameter of my rubber washers.


This would be the diameter of the inlet to the barrel

Open Time : 2.5 milliseconds


Sounds very optimistic, as far as I can tell it depends on how fast you pull the trigger.

Dwell Time : 0 milliseconds


Once it opens, the valve remains open until the chamber is charged again, so dwell time should be 999ms
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Unread postAuthor: Vudu » Thu Apr 02, 2009 2:37 am

Ah. Thank you. Back to the program.

I will keep you updated.
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 02, 2009 2:51 am

For completeness' sake, you should also factor in the volume from the valve to where the projectile actually sits as "dead space"
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Unread postAuthor: Vudu » Thu Apr 02, 2009 2:54 am

Well, I have it set as 3in cubed. Because it's just a one inch ninety degree elbow. That should be about right, correct?
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 02, 2009 3:55 am

That's about right, however dead space = bad (since it gives the air space to expand and therefore reduce in pressure before hitting the projectile) , I don't see the need for a transfer tube to be of a larger diameter than your barrel
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Unread postAuthor: Vudu » Thu Apr 02, 2009 11:37 am

So I should use a .5inch elbow? Would that not restrict air flow?

On the program, even with all the dead space I'm still hitting very high speeds and respectable distances. So I don't know if this is very necessary.

EDIT> Tested with the .5 inch elbow, and it cranked up the Velocity but Range decreased so much. Like 83 feet max, is that cause there's not enough air to propel it any further? Or should I bring up the P.S.I, because it's only at 100 and I can go quite a bit higher.

If I crank up the P.S.I, I would think I need an air resrictor or something to lower the velocity of the paintball. Correct?
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 02, 2009 2:02 pm

Vudu wrote:So I should use a .5inch elbow? Would that not restrict air flow?

On the program, even with all the dead space I'm still hitting very high speeds and respectable distances. So I don't know if this is very necessary.


Anything less than the barrel diameter would restrict flow, anything more would increase dead space and be wasteful.

Tested with the .5 inch elbow, and it cranked up the Velocity but Range decreased so much. Like 83 feet max, is that cause there's not enough air to propel it any further? Or should I bring up the P.S.I, because it's only at 100 and I can go quite a bit higher.


That doesn't make sense - greater velocity for a given projectile weight should always equal more range, assuming all other parameters are kept equal.

If I crank up the P.S.I, I would think I need an air resrictor or something to lower the velocity of the paintball. Correct?


That would work but be wasteful though - better to have low psi and the right performance than more psi and restricting it, you'll get less shots per chamber that way.
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