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HPA pistol musings........

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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Unread postAuthor: High-PSI » Sun Apr 29, 2012 2:43 pm

jackssmirkingrevenge wrote:
High-PSI wrote:So, 200 ft lbs of muzzle energy would be awesome in a handgun


You would have to to have to go hybrid for that, and in a compact pistol package that would mean pretty high mixes.

Not according to GGDT. At 1,000 psi and a 12 inch long 3/4 inch barrel (.8" bore) the muzzle energy is shown as 197 ft lbs.

Of course, this is all theoretical. In any event, it would have some kick for sure.

Too many projects, too little time.

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Unread postAuthor: jackssmirkingrevenge » Mon Apr 30, 2012 12:03 am

Right, I had forgotten you were going for "Howdah pistol" bore hehe!

If you load up a 0.75" lead ball that's around 41 grams, you'll get your 200 ft/lbs even if it's loafing along at a lazy 375 feet per second...
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Unread postAuthor: High-PSI » Mon Apr 30, 2012 7:06 am

Actually, it is showing a touch over 200 ft lbs at over 700 fps with a 14 gram ball.........

There are so many variables to this stuff. Back when I started with this stuff, I began modding these brass valves and found some mods drastically increased muzzle energy (punching holes through sheets of plywood) and some mods reduced energy.

The short barrel requires huge pressure and large bore size to do any real damage.

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Unread postAuthor: jackssmirkingrevenge » Mon Apr 30, 2012 7:11 am

High-PSI wrote:The short barrel requires huge pressure and large bore size to do any real damage.


Not to mention a fast valve, the same way that pistols need faster burning propellant than rifles.

With pneumatics, heavier projectiles almost always yield lower velocities but higher energies.
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Unread postAuthor: High-PSI » Mon Apr 30, 2012 9:57 am

Right, with a heavy slug, the air has much more time to pile up behind the slug and translate into more power. This would be especially true with a pistol.

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Unread postAuthor: jackssmirkingrevenge » Mon Apr 30, 2012 10:03 am

High-PSI wrote:Right, with a heavy slug, the air has much more time to pile up behind the slug and translate into more power. This would be especially true with a pistol.


Yep, it's also why breech detents work - you're increasing the time it takes for the projectile to start accelerating, giving the valve more time to open and maximise flow.
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Unread postAuthor: High-PSI » Mon Apr 30, 2012 4:55 pm

I considered that back when I started this. It seems to me that a detent of some sort that holds the slug tightly will allow the valve to open fully (or nearly fully) before the slug moves. This should net a noticeable gain.

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Unread postAuthor: High-PSI » Tue May 15, 2012 5:57 am

OK, I am moving ahead with this gun build.

I have a question on this that I wanted your opinion on;

GGDT shows no noticeable change in muzzle energy or velocity with various changes in vent diameter size. I programmed it for 800 psi with a pilot volume of 1.5 cubic inches (a .8 inch barrel coax design). I gave it a .3 inch pilot vent diameter and a .1 pilot [I even went down to a .05 inch pilot opening) with less than 1% difference in shown performance. Does this sound right? It may be due to the extremely high pressure being used as well as the relatively tiny pilot volume, but that sure seems odd to me.

Do you guys have any thoughts on this?

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Unread postAuthor: jackssmirkingrevenge » Tue May 15, 2012 6:25 am

High-PSI wrote:relatively tiny pilot volume


The piston has the inertia of its own mass to be overcome, this provides a time interval during which the pilot can vent. Did you try changing the mass of the piston?
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Unread postAuthor: High-PSI » Tue May 15, 2012 7:04 am

Yes. There is very little difference in the performance with a varied piston mass.

One interesting thing, however;

The accelleration curve changes with a nearly identical exit velocity. What GGDT shows is a very different behaviour in the barrel when the vent diameter and piston weight changes, while the exit velocity is nearly unchanged.

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Unread postAuthor: Brian the brain » Tue May 15, 2012 7:14 am

What you could do is make the piston diameter only slightly larger than the seat diameter.

This will give you better porting but it will also make the piston pop open faster.
The air has less surfaceon the front to push on when it is closed position, so the pilotpressure needs to drop further before the piston can start moving.

When it does, pressure will hit the full diameter of the piston while the pilot is at a lower pressure already.
This will cause it to snap back even harder than in a "normal" pistonvalve.

Most homemade pistonvalves are scaled after QEV's.
I'll use them as an example.
Those QEV's have a large diameter piston yet a fairly small port.
This makes them very sensitive.
A property that makes them perfect for draining aircilinders.

When it comes to pushing an object out of a short barrel with the highest possible velocity at the other end however, priorities aren't the same.
You want full opening as fast as possible, utilising every mm of barrellength for acceleration.

The "out"port shouldn't pertrue into the T-valve too much either.
Right over the reservoir port or even a hair to the front will provide unrestricted flow.
Another downside to QEV's.
It requires a 180 turn for the air to rush out.
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jackssmirkingrevenge » Tue May 15, 2012 7:26 am

Since you seem to have reasonable machining facilities to hand, have you thought of going valveless for ultimate performance?

http://www.spudfiles.com/forums/pac-wit ... 23760.html
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Unread postAuthor: High-PSI » Tue May 15, 2012 7:40 am

Brian the brain wrote:What you could do is make the piston diameter only slightly larger than the seat diameter.

This will give you better porting but it will also make the piston pop open faster.
The air has less surfaceon the front to push on when it is closed position, so the pilotpressure needs to drop further before the piston can start moving.

When it does, pressure will hit the full diameter of the piston while the pilot is at a lower pressure already.
This will cause it to snap back even harder than in a "normal" pistonvalve.

Most homemade pistonvalves are scaled after QEV's.
I'll use them as an example.
Those QEV's have a large diameter piston yet a fairly small port.
This makes them very sensitive.
A property that makes them perfect for draining aircilinders.

When it comes to pushing an object out of a short barrel with the highest possible velocity at the other end however, priorities aren't the same.
You want full opening as fast as possible, utilising every mm of barrellength for acceleration.

The "out"port shouldn't pertrue into the T-valve too much either.
Right over the reservoir port or even a hair to the front will provide unrestricted flow.
Another downside to QEV's.
It requires a 180 turn for the air to rush out.


I wondered about that. I know the beauty of this type of valve is the radical pressure differential once the piston opens the tiniest bit.

I appreciate the input and the explanation. I am still relatively green in this hobby, so any input is appreciated.

As for valveless, don't even get me started!

I think the only downside to that is the limited ammo that could be used. But, the performance would be insane!

Oh, also, sealing would be difficult at the pressure I will be running.

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