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A few pneubie questions...

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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A few pneubie questions...

Unread postAuthor: LHawk » Wed Jul 02, 2008 12:09 am

I'd like to start off with saying hello to everyone! I've finally decided to pursue my dream of building a pneumatic rifle, and throughout my many hours of research, I've probably spent a good 10-12 hours lurking around spudfiles in the last day or so. I am still shocked at the gold mine of information here on such an underground topic.

Let me apologize up front for any typos/illogical phrases (such as the word illogical), I am running on little to no energy typing this.

I've read the first 15-20 pages of topics in the pneumatic cannon board as well as many other boards, and although I've learned quite a bit, I still have a few questions (the search box returns hundreds of results for any search, which isn't always practical).

A quick bit of background on my project before I get to the questions:

I am looking to build a sniper rifle, probably with a 1/2" or 3/4" barrel, breech/bolt loading, possibly interchangeable (more on that in the questions), and I plan to use a modded sprinkler valve. I Will have a bit of scrap 2" pvc left over from a home plumbing project, I was thinking I could use a portion of this for the chamber/tank (assuming it is pressure rated).

Below I'll list any questions that I can think of, but I promise I'll forget a few, and I promise that some previously discussed questions will make an appearance on my list, but here it goes:

1.) Is 2" pvc acceptable for a chamber/tank when the barrel is going to be 1/2" or 3/4"? Should I use something more/less narrow? (remember the 2" will be free if I decide to use it)

2.) What would be the most effective barrel length for 1/2" and for 3/4" for a sniper rifle?

3.) With the above barrel length, what would be the most effective chamber/tank dimensions? (I've been playing around with GGDT but I think I'm doing something wrong).

4.) Is CPVC worth it for any portion of the gun? (remember I'll be using a modded sprinkler valve, so max psi will be <150psi).

5.) Would it be possible/efficient to make interchangeable barrels with a 1/2" and a 3/4"? Remember, both would be breech loading, so they'd be inside of a thicker tube. What kind of parts/adapters would I need to make this possible?

6.) In addition to the previous question, would it be possible to have a third barrel, probably 1" or so (whichever size is used for golfballs)? this wouldn't be breech loading.

7.) Back to the chamber/tank, is it worth it to put a larger piece of pvc snug around my 2" tank for extra support, or is this not needed with my psi range (<150psi)?

8.) For the barrels, which type of pvc would be best, ex: sch40, sch80, cpvc, etc? If it helps, the ammo being used would probably be:
1/2" - probably marshamallows in residential areas, maybe some small marbles, AA batteries, and possibly small paintballs (which caliber for this type of pvc?)
3/4" - Maybe marbles if they don't work well with the 1/2", paintballs (which caliber?)
?" - Golfballs?

9.) I know I forget many of the questions that I had, so I'll edit them in between these lines when I think of them:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1.) Woo already remembered one! Would it be worth it to put both a schraeder (spelled that wrong) AND a quick connect on my chamber/tank, or will more holes make it weaker?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Thanks for taking the time to read this unorganized wall of text! (I promise it won't happen again)

Hopefully I can hit up Home Depot tomorrow morning if I can plan out a shopping list for my build before then.

I've reread this once or twice, but it is tiring, so if something doesn't make sence, please let me know.

[Good god this is like 650 words, sorry again for that]
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Unread postAuthor: starman » Wed Jul 02, 2008 12:24 am

1 - 2" is fine for 1/2" or 3/4". It's more about volume than absolute size.

2 - pick a size and go for it

3- go for a c:b volume ratio (chamber/barrel) of 1:1 to 1.5:1

4 - No need to fool with CPVC

5 - yes

6 - yes

7 - keep everything pressure rated. Don't worry about the extra layers

8 - sched 40 is plenty, make sure its pressure rated.

sub 1 - it's best to pick one or the other but if you must do both, drill/tap one hole and setup a tee with one of each. I prefer the quick connect stuff.

Welcome to Spudfiles!
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Unread postAuthor: rednecktatertosser » Wed Jul 02, 2008 12:30 am

Welcome to the forums! Good job doing your research!

1) Yes, 2" pipe will be fine for a 3/4" barrel I have done it before.

2) Most effective length depends on a few things... but 3-4 feet is a good average length.

3) Chamber to barrel ratios is more for combustions than pneumatics. I wouldn't worry about that.

4) 3/4" cpvc barrel will allow you to fire paintballs very well.

5) as far as breech loading goes... http://members.aol.com/sph911/spud/breech.html

6) If you use a threaded valve, then you can just use threaded adapters to put whatever barrels you want on them, and golfball barrel is 1.5" SDR-21. you can usually get it from tractor supply co.

7) no, that is not needed, and it will just waste time and energy.

8) generally I just shoot whatever fits in the barrel, frozen cranberries, and gumballs work good in half inch. atomic fireballs work great in 3/4" .68 calibre paintballs fit perfectly in 3/4" CPVC, you should be able to get the PVC to CPVC adapters near the cpvc fittings.

9) I have put both plus a pressure gauge, and a safety pop-off in a chamber. all threaded fittings in their own holes. as long as the holes are reasonably spaced, they should do fine.

again, welcome to the forums, and I cant wait to see the cannon!
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Unread postAuthor: LHawk » Wed Jul 02, 2008 12:42 am

Thanks to both responses! The fact that you both mostly agrees helps a lot with my decisions.

As for the tank, looks like I'll be going with 2", now I just need to determine a length, somewhere probably between 6 and 12". I want enough volume for maximum performance, but I don't want it to take forever to fill up, so I guess I have to find some middle ground.
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Unread postAuthor: rednecktatertosser » Wed Jul 02, 2008 12:49 am

what would you be using to fill it?
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Unread postAuthor: LHawk » Wed Jul 02, 2008 12:53 am

Probably a compressor if I am near electricity, if not I might buy one of those 5 gallon compressor tanks for like $30. It would be cool if I could use some sort of pump to fill the compressor tank if it runs out, but I don't know if/how that'd work.
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Unread postAuthor: starman » Wed Jul 02, 2008 1:10 am

LHawk wrote:Probably a compressor if I am near electricity, if not I might buy one of those 5 gallon compressor tanks for like $30. It would be cool if I could use some sort of pump to fill the compressor tank if it runs out, but I don't know if/how that'd work.


The smaller your air chamber the easier it will be to pump up by hand...another factor you might take into account in your design. Having to be near a compressor will get old. Also, you can get 10 gallon tanks for not much more than the 5 gallon ones. The larger tank has obvious advantages.
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Unread postAuthor: rednecktatertosser » Wed Jul 02, 2008 1:15 am

if you have access to a compressor, go with the foot long chamber.
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Unread postAuthor: LHawk » Wed Jul 02, 2008 1:16 am

Alright, thanks, I'll look into a 10 gallon.

Any suggestions from anyone on how long to make my 2" pvc chamber? The barrel will be either 1/2" or 3/4" (maybe both), about 3-4ft long.

~~~~

rednecktatertosser wrote:if you have access to a compressor, go with the foot long chamber.


Well I have a large, stationary compressor, and well as a smaller one, maybe a 2-4 gallon pancake tank on it, but I would like to not have to be tethered to a house or extention cord.
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Unread postAuthor: starman » Wed Jul 02, 2008 1:35 am

If you keep your barrels 4' or less you'll only need 6-7" or so of 2" chamber. Any more will be going to waste.

You can do your own volume calculations with the formula : pi * radius * radius * length

You won't need a c:b ratio any greater than say 2:1...even that is overkill with a nice fast piston. Again a c:b of 1:1 - 1.5:1 is considered an optimal window.
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Unread postAuthor: LHawk » Wed Jul 02, 2008 2:56 am

Alright, I went ahead and did a bit of number crunching using the dimensions found here:

http://www.harvel.com/pipepvc-sch40-80-dim.asp

Here are the results that I got, let me know if anything looks off.

~~~~~~~~~~~~~~~~~~~~~~~

Barrel

3' of sch40 1/2" = 38.814553164293045
3.5' = 45.283645358341886
4' = 51.75273755239073

3' of sch40 3/4" = 51.58188517751789
3.5' = 60.17886604043753
4' = 68.77584690335717


Chamber

6" of sch40 2" = 27.339697501496356
7" = 31.89631375174575
8" = 36.45293000199514
9" = 41.00954625224453
10" = 45.566162502493924
11" = 50.12277875274332
12" = 54.67939500299271

~~~~~~~~~~~~~~~~~~~~~


Now all I need to do is figure out what to do...

From the results (correct me if I'm wrong) it looks like a 12" chamber would be just over 1:1 for a 4' barrel of sch40 1/2", or 3' of sch40 3/4".


~~~~~~~~~~~~~~~~~~~~~
Edit:
Ugh it seems like spudfiles doesn't like my [TAB] spacing for my results, oh well...

Also, for the calculations I used: http://grapevine.abe.msstate.edu/~fto/t ... inder.html since is after 3am here, and I'm just a bit tired....

My numbers (for ratios) seem to contradict what is here in this thread, as well as other threads on this forum, by saying a 12" x 2" chamber is needed for a 1:1 with a 4' barrel of 1/2". Why does this not seem right?
~~~~~~~~~~~~~~~~~~~~~~
Edit:
Oh well, I'm going to sleep now, I'll check back here in about 5 or 6 hours before I head out to home depot at 9ish, hopefully I'll be able to make up my mind by then. Regardless, thanks to everyone so far for all the help, as well as the warm welcome(s)!
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Unread postAuthor: vovka351 » Wed Jul 02, 2008 9:44 am

GGDT says "Barrel potentially choking flow"
I say its fine.
I'd say a longer barrel is better, at least looking in GGDT, the velocity increase is practically linear past about 40 inches.

As for pumping: On my first gun- it had a 42"x2" chamber, and we used a bike pump for it. Its not actually that bad...
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