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Double Shot (High Pressure Mini)

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Novacastrian » Tue Nov 06, 2007 1:45 am

Thanks for the ideas guys, i think i may go with JSR's idea with the red dot scope. That way i can use it on all my "appliances". :P
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Unread postAuthor: Novacastrian » Fri Nov 09, 2007 5:13 am

New addition, breech loading. ( don't even mention dead space :D )
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 5:27 am

*struggles to keep mouth shut*

nyarghhh!!!

*regains composure*

There are some theories that the air in the dead space gets compressed on firing which somehow boosts velocity. I don't really buy them though...

edit: I might be able to test this however with a 6mm piston valve and a chrony, maybe this afternoon.
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Unread postAuthor: Novacastrian » Fri Nov 09, 2007 5:35 am

jackssmirkingrevenge wrote:*struggles to keep mouth shut*

nyarghhh!!!

*regains composure*

There are some theories that the air in the dead space gets compressed on firing which somehow boosts velocity. I don't really buy them though...

edit: I might be able to test this however with a 6mm piston valve and a chrony, maybe this afternoon.


When i first built the launcher it did not have enough flow for proper activation. HOWEVER, i found that if i muzzled loaded a round only say half way up the barrel- the said projectile traveled further than a round jammed all the way down. So there 8) *sticks tounge out in childish fashion*
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 5:47 am

I'll do some chrono tests later, I might well be wrong perish the thought :D
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Unread postAuthor: Novacastrian » Fri Nov 09, 2007 5:56 am

jackssmirkingrevenge wrote:I'll do some chrono tests later, I might well be wrong perish the thought :D


I'd like to see that :wink:
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 6:32 am

The results will be on your desk this time tomorrow :P :D
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Unread postAuthor: spanerman » Fri Nov 09, 2007 7:20 am

waiting with baited breath jack :lol: im also curious as my current pb gun has a rather large dead volume....i would also test with a larger bore but i dont have a chrony....maybe on the Xmas list though :D
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Unread postAuthor: Hawkeye » Fri Nov 09, 2007 9:27 am

I can see it being true if the projectile has a near airtight fit. Then there could be a sort of gas ram effect. Can't see it being true with a less than optimal fit of ammo.
Hotwired did some tests with a gun that basically fired a piston down a barrel which compressed the air ahead of it and then fired the projectile. Not sure how conclusive the results were.
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Unread postAuthor: spanerman » Fri Nov 09, 2007 10:27 am

IRRC his piston got jamed too often and the power wasnt to great...
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 10:42 am

I'm going to do some test with a 6mm mini-piston and wadded BBs, the problem is though that while I can vary the dead space, it will not be the only variable - increasing the dead space means technically shortening the effective barrel length, meaning the results might not be valid.
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Unread postAuthor: ALIHISGREAT » Fri Nov 09, 2007 12:24 pm

jackssmirkingrevenge wrote:I'm going to do some test with a 6mm mini-piston and wadded BBs, the problem is though that while I can vary the dead space, it will not be the only variable - increasing the dead space means technically shortening the effective barrel length, meaning the results might not be valid.


well if the results still are better with more dead space then it doesn't matter but if the results are worse or the same then they will be inconclusive!

i hope dead space does increase performance, and wouldn't the air get to a greater velocity before hitting the projectile so would this give an increase in performance?

ooo and another idea, could you not just put it through ggdt? i know ggdt sometimes gives unaccurate results but it could give you an idea and it would calculate with the same barrel length every time.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 1:56 pm

Well ladies and gentlemen, the results are in.

The test setup involved a mini-piston launcher in 6mm caliber and a 16 inch barrel, charged to 110 psi and firing 16 grain 0.22 pellets. I first started out with the pellet loaded at position "zero" - ie touching the piston, chronographed three shots, then moved the pellet position to one inch away from the piston, chronographed another three shots and so on.

Velocities in feet per second:

At position "zero" : 261, 269, 258

At 1 inch away from the piston: 262, 251, 240

At 2 inches away from the piston: 232, 240, 244

At 3 inches away from the piston: 235, 231, 232

As you can see, more dead space (as well as, it must be noted, a corresponding decrease in effective barrel length) results in lower velocity and therefore lower performance. I'm sorry to report that this is now the case with the launcher that is the subject of this thread, alas, alas.

*dramatic shakespearean gesture*

As a sidenote, pretty friggin' consistent results for a homemade launcher :D
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Unread postAuthor: ALIHISGREAT » Fri Nov 09, 2007 2:12 pm

hmm interesting, if only you could try with equal effective barrel lengths :( because 3" is over 1/5 of the barrel length so i think it is the effective barrel length that effects the velocity more.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 3:20 pm

The 16 inch barre is made up with a ten inch barrel and a 6 inch extension, so I took the opportunity to measure shots for 10 inch effective barrel length, starting from zero and 6 inches away from the piston respectively. I got 225, 227 and 221 fps for a normal 10 inch barrel, and 198, 190 and 196 fps for the 16 inch barrel with the projectile 10 inches away from the muzzle.

An almost 13% decrease in velocity for the extra dead space, meaning 26% less muzzle energy. A very limited test, granted, but food for thought nonetheless ;)
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