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Which barrel?

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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Which barrel?

Unread postAuthor: LucyInTheSky » Wed Aug 30, 2006 8:23 am

I have made a new spudgun. But here are some GGTD results for it with different berrels. All 60cm long.

Pipe Size 22mm 15mm 10mm
m/s 131.9 173.7 167.5
f/s 433 570 550
J 69.5 45.3 21
J/cm^2 21.7 31.2 40.8
Distance (m)168 164 212
Ammo(g) 8 3 1.5

Some Questions:

1, Dose this mean the 10mm barrel will penatrate more? My brain is saying that the 15mm one will penatrat better as it is faster and heavier.
2, Why dose the 10mm one fire so far?
3, Which one would you go with?
4, Why dose the 10mm one go slower than the 15mm one?
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Unread postAuthor: boilingleadbath » Wed Aug 30, 2006 3:03 pm

I think the 15mm barrel is shooting faster than the 10mm barrel because the sectional density of the 15mm projectile is about 10% less than that of the 10mm projectile.
This would indicate that valve and/or tank you are using are impracticaly big.

However, I'm surprised that the 10mm projectile is going 20% farther.
However, with projectiles roughly the density of water, this isn't really going to happen; the projectile will tumble in the air, and being about 2cm long, this will substantialy increase it's drag.

#1 would probably depend on WHAT you are shooting at WHAT, as would number 2.
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Unread postAuthor: LucyInTheSky » Wed Aug 30, 2006 4:09 pm

Well I will be shooting spuds, apples etc.
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Unread postAuthor: boilingleadbath » Wed Aug 30, 2006 7:12 pm

"Penetration" in the proper sence is non-existant on any substantial target with edibles.

The way I look at it is this:
You might as well be shooting chunks of water, as it's going to behave essentialy the same. The reason is simple and well demonstrated; at this type of velocity, the impactors fragment into little ity-bitty crumbs. Direct evidence, as far as I am concerned, of the definition of a hypervelocity collision; the strength of the material is ignorable in comparison to the forces of impact.
(This is a convienient aspect of the collisions, and it might be interesting if someone knows of a free hydrodynamics sim.)

Shooting at things like plywood, I suspect you'll want the most impulse/projectile circumfrence. (based on the premis that )
There will be some advantages to large projectiles this due to the small impactors spreading more on impact.

Ya got:
22mm @ 7.6J/cm *adjusted
15mm @ 9.6J/cm
10mm @ 6.7J/cm

(the adjusted means that I multipled the 22mm values by 430/560, to adjust for impact time)
I understand that modeling fluid dynamics with ms calculator (tm) is a really dumb idea, and that these are only approximations.

For things like sheet metal, you can just give up now, although the biggest dent will most likely be left by the 22mm one. (personal experience with ~32mm projectiles at these velocities vs. 4 L paint can gives me the "no go" data)

For fluidic targets, penetration will be nil regardless, and is negligable with blunt water-density penetrators (ice).
The highest energy projectile will most likely yeild the most spectacular splatter.
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