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Old 02-09-2012, 09:18 PM   #51 (permalink)
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Ooops.
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Old 02-09-2012, 09:20 PM   #52 (permalink)
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The long and short of it is... Converting mechanical energy into air pressure to propel a projectile is a waste of energy.
I'm not sure about that. Ignore the airow gun because it's badly thought out.

I think the real waste of energy would be making the draw weight heavier and heavier to get 300 fps with a paintball, when we have more than enough energy except it's represented by torque instead of speed. We already have enough energy here, we just need to change it into a form that we can use. An air piston is the best way to do it, IMO.

If you try and shoot the paintball using the band or string, you'll have to make a crossbow that can snap back at 300 fps. If you convert the existing energy, we can probably get away with a crossbow that normally snaps back at 150 fps.
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Old 02-09-2012, 09:26 PM   #53 (permalink)
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Russ it is easy to make a direct contact powered gun to get a projectile up to speed. I do it all the time when I build band spearguns for fishing. The shaft speed underwater is 190 fps. The spear shaft is quite a bit heaver than a paintball, and air is less dense.

I agree the airgun was not a great application.
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Old 02-09-2012, 09:27 PM   #54 (permalink)
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I limitedly agree with both of you guys (Ty, Russ)... The draw of a bow almost certainly has way more than enough energy to make the conversion inefficiency a moot point.

Meanwhile, I'm not sure that we would have to increase draw weight to increase FPS... The trick is getting string speed up without increasing stored energy.

I would prefer to build a design that runs on string alone... But I can already see an elegant way to build the gun with an air piston, using modified automag components.
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Old 02-09-2012, 09:29 PM   #55 (permalink)
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Let's make both.
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Old 02-09-2012, 09:30 PM   #56 (permalink)
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Let's make both.
Or break a few things trying. Back in the 80's Sportsmansguide used to sell a dart gun that used surgical tubing to operate an air piston to shoot a dart. A paintball gun could be done that way too.

edit: Found a picture of one:

http://airgunenthusiast.weebly.com/t...-5-pistol.html
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Old 02-09-2012, 09:40 PM   #57 (permalink)
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My current "light bow" at 30 pounds @ 29" draw shoots a heavy arrow around 225 FPS.

I haven't chrono'd my 150 pound bow, but I could this weekend

Ty
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Old 02-09-2012, 09:54 PM   #58 (permalink)
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Ty, I think Russ' concerns about draw weight are extremely valid... What can you do to increase string speed, while decreasing draw weight?
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Old 02-09-2012, 10:01 PM   #59 (permalink)
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Converting the energy stored in the bow to pneumatic does create a loss of energy. The question is whether or not the pneumatically stored energy it wastefully creates is more effective/efficient at transferring its energy to the paintball than directly firing it.

To directly fire a paintball and get it up to speed, the string/band/arrow/plunger/whatever needs to get up that speed and come to a sudden stop while momentum carries the ball.

You need something that can transfer ~12 joles of energy to the paintball as efficiently as possible, without destroying it in the process. You can compress enough air to do that without the string having to reach 300FPS.
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Old 02-09-2012, 10:02 PM   #60 (permalink)
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Or break a few things trying. Back in the 80's Sportsmansguide used to sell a dart gun that used surgical tubing to operate an air piston to shoot a dart. A paintball gun could be done that way too.

edit: Found a picture of one:
http://airgunenthusiast.weebly.com/u...92/9666971.jpg
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