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Short question topic

A place to ask general spud cannon related questions.
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Re: Short question topic

Unread postAuthor: farcticox1 » Wed Nov 30, 2016 7:12 pm

https://www.amazon.com/Legris-0610-Sint ... ir+muffler

wow $9, that's a lot of cat food, Legris is a good brand though, I get mine here

https://www.automationdirect.com/adc/Sh ... 22&start=0

They will quite things down, but with a slight restriction if that doesn't cause problems. I guess it's like a porous material, maybe a scotch brite pad would work

https://www.acklandsgrainger.com/en/pro ... 01001757:s

or just a scouring pad
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Re: Short question topic

Unread postAuthor: wyz2285 » Sat Jan 14, 2017 12:25 pm

So went to the US and bought myself a QB79, also bought a 13ci 3000 PSI tank for it.
I have previously modded a QB78, choked the barrel/sling shot hammer, improved the accuracy by about half (1 inch grouping to 1/2 inch at 10m, the quality of the barrel was extremely poor, the bore wasn't even centered so...)
Now this one looks well made, the cuts were rounded, no sharp edges, rifling seems clean, bore is centered etc. So I think this one worth more upgrades. First I'm gonna chock the barrel and install fiber optic sights and do the sling shot hammer mod. However I'd like to cut down the barrel and install a suppressor. Now will I lose power with a shorter barrel? I know it sounds silly but I read somewhere that HW77 is weaker than HW97 because the former has a longer barrel. I know they're springers and the air cylinder is limited so a long barrel may cause extra friction instead of acceleration. So for a PCP there would be such a sweet spot too right? I'm thinking of cutting off about 20cm of the original barrel.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Short question topic

Unread postAuthor: Ragnarok » Sat Jan 14, 2017 5:38 pm

wyz2285 wrote:So for a PCP there would be such a sweet spot too right?

There will be, but it's difficult to say where that is.

My gut feeling is that taking that much off the barrel of a PCP will cost you velocity - a back of the envelope guess would be around 10-15%.
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Does that thing kinda look like a big cat to you?
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Re: Short question topic

Unread postAuthor: wyz2285 » Sat Jan 14, 2017 5:46 pm

For these HPA airsoft guns I just turn the valve open time to max then gradually reduce it until the FPS drops and I consider that optimal for a given barrel length at given pressure.
For hammer valve I suppose I would be adjusting spring tension or hammer weight. My QB78 after modding the hammer was able to pull out 80 good shots at 700-800 FPS however I never went looking for the sweet spot.
The QB78/9 have a really long barrel, I rarely see one so long in modern PCPs, maybe because they are designed for CO2?
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Short question topic

Unread postAuthor: Ragnarok » Sat Jan 14, 2017 10:12 pm

wyz2285 wrote:The QB78/9 have a really long barrel, I rarely see one so long in modern PCPs, maybe because they are designed for CO2?

It's possibly a factor. CO2 is after all generally less powerful, given the lower pressures available.

Anyway, I do think that a PCP is going to be able to make use of a longer barrel than a spring-piston (particularly if the valve has been tuned for maximum velocity rather than for air efficency), so I do suspect that shortening the barrel will cost you power. I can't quantify exactly how much though.

Personally, I'd be inclined to leave it at its full length, as that's about the same overall length as something like my Air Arms TX200HC (and the problem with that is not really the length, but the balance), but I guess it is a subjective matter.
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Re: Short question topic

Unread postAuthor: wyz2285 » Fri Jan 20, 2017 11:29 am

The thing is I want to install a suppressor on it so it's more neighbor friendly and that would add an extra 15-20 cm. Guess I will have to have it tuned first and measure the muzzle velocity, don't need much for target shooting anyways so if it's on the hot end I will cut it.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Short question topic

Unread postAuthor: wyz2285 » Tue Mar 21, 2017 5:53 pm

I had this co2 bottle for quite a while and it seemed like a good chamber for 10x hybrid. However the threading isn't very user friendly and small so I drilled and tapped the button with 1/2 BSP and epoxied a modified 1/2 union. Is this a bad idea? The thickness of the button allowed about 10mm of thread.
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Re: Short question topic

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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Tue Mar 21, 2017 6:44 pm

That's about 5 turns in, sounds reasonable for a 10x hybrid.
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Re: Short question topic

Unread postAuthor: WizardNoodle » Wed Apr 05, 2017 2:55 pm

I am preparing to build a piston hybrid cannon. After a lot of searching, I finally found a suitable material that was pressure rated enough for my uses. Then I went to their website and found the pricing list.
OK, I scroll down the pricing list and under the "Standard Seamless" chart I look for the 3" diam category. Then I find the longest length they have, in this case 12". I look at the price and... 357.59???
That price cannot be right for 12 inches! Could the supplier have mixed up how one ' is feet and " is inches? Or am I reading the chart totally wrong?
If it is that much, would buying this pipe and paying a machinist to put on threads (NPT?) be less pricey, or does anyone have any other ideas for materials? This is definitely giving me a headache :lol:
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Last edited by WizardNoodle on Wed Apr 05, 2017 5:13 pm, edited 3 times in total.
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Re: Short question topic

Unread postAuthor: wyz2285 » Wed Apr 05, 2017 3:02 pm

First, what you found are seamless steel pipe nipples, not pipes.
Second, it's plain steel, you need to apply anti-corrosion coating on it. According to their price list, you can have it galvanized, for double the price.
Keep looking.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Short question topic

Unread postAuthor: WizardNoodle » Wed Apr 05, 2017 3:14 pm

wyz2285 wrote:First, what you found are seamless steel pipe nipples, not pipes.
Second, it's plain steel, you need to apply anti-corrosion coating on it. According to their price list, you can have it galvanized, for double the price.
Keep looking.


I know they are nipples, they are actually what I am looking for as I have absolutely no welding experience.

You have a point though, so should I look for pipe like this and ask a machinist to put the threads on for me? I really hoped to not have to pay a machinist or welder...
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Thu Apr 06, 2017 6:08 am

You've gone a little bit in the deep end going for seamless stainless construction.

Galvanized iron is fine...
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Re: Short question topic

Unread postAuthor: WizardNoodle » Thu Apr 06, 2017 9:04 am

jackssmirkingrevenge wrote:You've gone a little bit in the deep end going for seamless stainless construction.

Galvanized iron is fine...


Thank you for pointing that out! And now I am following up with another question... :D

So I found a decent supplier, and they list it with pressure ratings, yay! For Sch 40 24" Length 2" Diam (Had to downsize from 3" which is a problem) pipe, they rate it to 940 psi @ 650 Degrees F. Obviously the cannon won't be anywhere close to that temperature but is it still safe to fire with pressures around 1100 psi? I understand that it is the working pressure and the burst is probably far off but I would rather have a concrete YES or NO before I put myself at risk of obliteration...

Also, as I am not happy with the size of the pipe (I'd like to be able to force my hand down it to adjust ignition wires chamber fan etc,) could I buy Sch 40 3" Stainless Steel pipe from Online Metals for the main chamber pipe, pay to have a machinist to thread male threads on so I can fit this(it says it is threaded here) over, and pay a welder to take a reducer to threaded 2" and weld it on to the other end, and screw Galvanized into everything else? Probably costs more than all galvanized but I feel that I need to have full access to the inside of the gun.

Thank you for your time.

EDIT: I don't know how I missed this, but online metals has pipe nipples too!! Of course more expensive, but good to know.
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Thu Apr 06, 2017 3:28 pm

Obviously the cannon won't be anywhere close to that temperature but is it still safe to fire with pressures around 1100 psi? I understand that it is the working pressure and the burst is probably far off but I would rather have a concrete YES or NO before I put myself at risk of obliteration...


Hybrids generate a brief pressure spike that is limited by the disk bursting, so the idea that you would have a catastrophic failure even if you're beyond the working pressure of the pipe is most unlikely. I doubt anyone will want to go on record saying that you'll be OK doing working close to or beyond the limits, but you decide what level of risk is acceptable.

I made a PVC hybrid cartridge that was fine at 7x:



Took it to 8x, also fine, though it blew up my shitty epoxy breech :)



Catastrophic failure is always a possibility when dealing with this level of energy, the best we can to is make it unlikely.

For Sch 40 24" Length 2" Diam (Had to downsize from 3" which is a problem) pipe, they rate it to 940 psi @ 650 Degrees F


I personally would be fine using this pipe for a 10x hybrid, provided burst disk failure pressure was below the rated pressure and the projectile was not too tight or heavy.

Also, as I am not happy with the size of the pipe (I'd like to be able to force my hand down it to adjust ignition wires chamber fan etc,)


You really don't need a fan for a hybrid, pressurizing the mixture is more than enough to agitate it.

In terms of ignition, if you want accessibility and serviceability, threading your chamber for a spark plug is a better idea.
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Re: Short question topic

Unread postAuthor: WizardNoodle » Thu Apr 06, 2017 5:23 pm

jackssmirkingrevenge wrote:I personally would be fine using this pipe for a 10x hybrid, provided burst disk failure pressure was below the rated pressure and the projectile was not too tight or heavy.


First, I will make the valve a piston like MrCrowley's Mjilnor cannon rather than a burst diaphragm because it is cleaner, faster, I am hoping the pilot exhausting will slightly counter recoil, and I know it won't fail unlike the burst diaphragm which you could accidentally put too many layers on and end up with a pipe bomb in your hands.

jackssmirkingrevenge wrote:You really don't need a fan for a hybrid, pressurizing the mixture is more than enough to agitate it.

In terms of ignition, if you want accessibility and serviceability, threading your chamber for a spark plug is a better idea.


No matter how little the fan will optimize the performance, I definitely want to make it as efficient as possible!

I will use the spark plug method, however I will also modify it so it will pass the spark through a spark strip. Moar sparks! Moar power!! :twisted:
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