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“Liquid” gas powered longrifle

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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“Liquid” gas powered longrifle

Unread postAuthor: judgment_arms » Mon Apr 21, 2008 5:23 pm

Hey all,
A thought occurred to me the other day, and then I forgot about it until just a few minutes ago while running a few simulations on the HGDT:
I’m trying to make a longrifle (yes, I’m still talking airguns; yes, it’ll have a rifled barrel.) and I want it to have a long barrel, between 4-5 feet, but, a patched paintball must leave the barrel at 300fps.

I was wondering if I could just dump liquid propane into the barrel, and if so how would I figure how much I’d need.

Another, somewhat stupid, question: can I still use a PVC barrel? PVC is all I can rifle, I can get SCH80 if that would be better.

Thanks!
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Unread postAuthor: benstern » Mon Apr 21, 2008 5:52 pm

I think it would be better to have a SMALL chamber. (maybe 2" steel)
and have that hooked up to a triggered burst disk. If you want extra power just heat the chamber to increase the vapor pressure.

As far as PVC barrels go, no you can not. With a mass phase change of propane the barrel will experience extreme coldness.
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Unread postAuthor: Fnord » Mon Apr 21, 2008 6:03 pm

Er... benstern... I doubt cold is going to be a concern unless he putting a ton of shots through it in a short period. No offense, but think you may be talking out of your ass on this one.
(I'd go with a metal chamber though.)

Judge: I don't think getting a paintball up to 300 fps will be too hard with a long barrel. I'm not sure whether the rifling will help or tear it up though.
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Unread postAuthor: judgment_arms » Mon Apr 21, 2008 6:03 pm

Burst disk won’t work; it accelerates the paintball far to fast…

What if I sleeved the PVC barrel in steel or copper pipe?

Edit:
_Fnord, I think he might be right about the cold, I’m talking about dumping liquid propane into the barrel.

The paintball’s will be wadded, I’ve got the barrel all figured out; I just need a way to slowly accelerate the paintball over 4 to 5 feet.
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Unread postAuthor: Fnord » Mon Apr 21, 2008 6:16 pm

In that case, I think the best option might just to be a blowgun hooked up to a propane cylinder. Dumping liquid directly into the barrel probably won't give any advantages, but can be easily avoided by keeping the cylinder slightly upright.
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Unread postAuthor: judgment_arms » Mon Apr 21, 2008 6:24 pm

My thoughts were that the expansion of the liquid propane into a gas would be comparable to the burning of black powder, allowing for a relatively slow acceleration.

My original plan was a 3/8 or 1/4 inch spring loaded ball valve, I might just go back to that.
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Unread postAuthor: bluerussetboy » Tue Apr 22, 2008 7:58 am

300 fps shouldn't be a problem with paintballs.

Silverdooty's Bluefoam Piston:

Barrel:
3/4" X 60" Sch40 PVC

Chamber:
2" X 60" Sch40 PVC
(plus a little extra in various fittings)

Piston:
Bluefoam piston lubed with bacon grease
housed in 2" tee
2 cu. in pilot volume
piloted by a 1/2" ball valve

Ammo:
cheap paintballs from Walmart patched with a worn out t-shirt
average weight: 2.8 grams

At 40 psi paintballs were leaving the barrel intact at just under 500 fps. 80 psi brought them out at around 625 fps. These numbers were from Silverdooty's chronograph, so I trust them.

It wasn't until we tried 150 psi that they were rupturing in the barrel.

http://img113.imageshack.us/img113/1880/im000336jx2.jpg

Paintballs at 300 fps really isn't that hard.
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Unread postAuthor: judgment_arms » Tue Apr 22, 2008 8:11 am

Believe it or not, paintballs at only 300fps is.
I don’t want to get to 300fps by having too long of a barrel; I want it done by slowly accelerating the paintball over a long distance, the reason is two fold:
One, it’ll prevent the paintball from jumping the rifling.
Two, it’ll prevent back pressure which can through off accuracy

I find that three to four layers of paper towel work better the cloth as paper towel compresses more, giving a tighter fit.
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Unread postAuthor: bluerussetboy » Tue Apr 22, 2008 9:51 am

I might be off on this. Due to lack of coffee and a subject I haven't really dealt with for quite a long time.

Rifling jumping,bullet jumping, projectile jumping... These occur when the projectile's nose actually enters the rifling, not if it is already in the rifling. A paintball already seated in the rifling is not going to 'jump'. Using your 3-4 layers of paper towel for a tighter fit is going to prevent this even more. So unless your rifling is very inconsistent, 'jump' is not going to happen.

Now with your tighter 'paper towel' tolerances you are going to require increased pressure to actually start your paintball moving. For me, anything over 10 psi is tight. 3-4 layers of paper towel would be very tight in my opinion. Would your liquid be able to expand enough to actually propel the paintball out of a 4-5' barrel? At what point would you run out of gas expansion for X amount of liquid. Would the projectile even leave the barrel while it is being slowly accelerated to 300 fps?

One more thought. Have you noticed that most paintball markers have barrels in the 12"-14" range. I'm not sure, but I would guess that it is easier to hit the 300 fps safety requirements with the shorter barrel. Not something that is 4'-5' long.
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Unread postAuthor: psycix » Tue Apr 22, 2008 10:02 am

Why the liquid gas part?
Its unneccesairy and overcomplicated.
The liquid will freeze the heck out of everything when it vapors and thus you will need a metal chamber. Also, the coldness will reduce your effective pressure.

Maybe it also freezes up your paintball.... Getting hit with a frozen one would hurt.
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Unread postAuthor: Ragnarok » Tue Apr 22, 2008 4:54 pm

That's something of an unnecessary barrel length, given that paintballs can quite easily be accelerated to 300 fps in 6", let alone 4' to 5'.
(Something I have in the pipeline - "Project 3vo" - would do it in one and a half inches)

I know you've explained your reasons, but it still sounds weird - at 300 fps, extra back pressure is the least of your worries - wind will be a bigger problem for something that slow.

But, it's not my place to comment. So, first question: Is this for paintball field use, or is the 300 fps limit in place for a different reason?

Now, I've spent some time thinking about accuracy - as I hope to hit a target at 1/4 mile with a spudgun, it's somewhat necessary.

As the longer barrel will have problems (not least, if it's not supported right, it will bend under it's own weight), I suggest you go with the shorter 4' barrel. Even then, your average barrel pressure has only got to be around 7.5 psi above friction.
If you were prepared to shorten it further, 3' would be a better move, especially if this were intended for field use. (My personal choice would be 2' at most.)
It will make a shorter and more convenient package, the barrel end won't droop under it's own weight (of course, a metal barrel would help with this), as well as a few other advantages.

Talking about the barrel a little longer, applying a dry Teflon or silicon lubricant will help, but venting/porting the end of the barrel works wonders for accuracy.

For this purpose, using liquid propane is a bad move - it will evaporate at different rates depending on temperature, and will create a very large temperature dependency for your long rifle - not good for accuracy.

Again, it's still not great for accuracy, but a better choice would be CO2 sparklet bulbs regulated down to an appropriate pressure.
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Unread postAuthor: judgment_arms » Tue Apr 22, 2008 5:01 pm

Bluerussetboy, by “jumping” I mean being hammered so hard be the air that the wadded paintball skips right over the shallow groves of the rifling, with a 1 in 14 rate of twist it’s a vary real possibility.

Most paintball makers use high pressure Co2 and HPA through a hammer valve, which gives a fast acceleration requiring only a short barrel.
I, on the other hand, require a long barrel with a low flow valve to slowly push the paintball; rather than hammer it like a paintball gun does.

Psycix, you make a good point, regardless of what it does to the barrel, if it freezes the paintball it’ll be to dangerous… I think I’m going to scrap the idea of liquid gas power, to dangerous and to much hassle.

Thanks all!

Edit:
Ragnarok, 300fps is in place for safety (I live in a semi residential area) and for possible use in actual paintball games.

The 4-5 foot barrel is for both accuracy and looks; I plan on making it look like a Kentucky longrifle.

It’ll have a full length stock to support the barrel.
I was going to use lubricated patches on the round ball… er, paintball. :)
I thought about a ported barrel but decided that it would ruin the look.
I have a Co2 setup… but the Co2 powerlet gives me an idea…
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Unread postAuthor: Ragnarok » Tue Apr 22, 2008 5:20 pm

judgment_arms wrote:The 4-5 foot barrel is for both accuracy and looks; I plan on making it look like a Kentucky longrifle.

I imagined there was a lot of looks involved. Still, the 4' option would probably be wiser - no need to make it too long, as actually, you'll have some trouble keeping the PB to just 300 fps anyway.

It’ll have a full length stock to support the barrel.
I thought about a ported barrel but decided that it would ruin the look.

Porting doesn't need to be visible. If you're thinking of a full length stock anyway, you could have the porting on the underside of the barrel, venting into a space inside the stock.

Just a thought.
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Unread postAuthor: bluerussetboy » Tue Apr 22, 2008 6:16 pm

Well if your going with looks and authenticity you should know that the Kentucky(originally from PA) had a barrel length around 30". The Kentucky eventually evolved into the Pennsylvania rifle which had 42" or longer barrels. :wink:
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Unread postAuthor: mobile chernobyl » Tue Apr 22, 2008 9:13 pm

I'm not sure if paintballs have changed, but last time i checked they were round gel capsules with some sort of oil (fish oil sometimes) and die in the center. That presents a liquid filled capsul.

There have been endless discussions on rifling in paintball, and countless tests have proven it to be of no use, sometimes more often than not hampering accuracy, and it also has the added effect of degrading efficiency; if you don't believe me visit pbreview.com, or pbnation.com and search rifling, its kinda been beatin to death like the old plane on a conveyor belt argument. The problem with rifling a paintball barrel is that you induce a spin on said liquid filled capsule, well inertia's a [female doggy] in that sense, becuase of the short impulse placed on a paintball when accelerating during shooting. the liquid in the center presents inconsistent results and the accuracy of the paintball is jeopardized. Think curve ball coming from a pitcher. This is a VERY SHORT and BASIC explanation, and its not worth getting into on a SPUDGUN forum lol, if you disagree, go search on the aforementioned websites, and/or get learned in physics.

In general, if its round, smooth bore, if its oblong and doesnt have stabilizing assistance, rifled bore.

Also you will be able to use very low pressure compared to a paintball gun if your using even a 4ft barrel, since a spud gun operates in more of a HVLP manner, given the barrel is .68 ID, being biased towards .695 or .7 might actually help given the tolerances of pvc or whatever your going to buy in 4-6" is no where near that of a peice of machined aluminum hydraulic tubing, aka paintball barrel.

::edit:: I'm not even sure if its worth asking, but I was assuming you were referring to spiral rifling, and I'm pretty sure you are reading through the topic, but are you aware of linear rifling? It's currently being tested in paintball, with somewhat neutral results compared to porting patterns. It in theory keeps the paintball straight, not inducing any spins by letting the paintball "ride" on a set of rails, rather than the entire circumfrence of the bore. This in turn gives it a little less friction, but also decreases efficiency since some air is allowed to escape past the ball on the lower portions of the rail set.
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Last edited by mobile chernobyl on Tue Apr 22, 2008 9:30 pm, edited 1 time in total.
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