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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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Does that thing kinda look like a big cat to you?
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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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