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Decimator video UPDATE 07-20-09

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Technician1002 » Sat Jul 25, 2009 11:04 am

jeepkahn wrote:On that note, Because the brass brazed end was so ugly,I did have this hydrotested before being put into use with way more than a 4:1 safety margin...

The joint on the right is actually using a rod called Blockade...

Here's a link to the website. http://www.harrisproductsgroup.com/cons ... .asp?id=31

I only had a sample of it to use that's why there are 2 different rods used, had I known how well the blockade was going to work I would've gone ahead and bought some more....


Thanks for the update on the product used. My job was done with the old industry standard Stay-Silv 15 listed in your link.

Reading the product literature, the improvement appears to be the ability to braze to brass without flux which the stuff I use doesn't do very well. It wants to ball up and roll off brass instead of wetting it.

Again thanks for the update on the other connection.
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Unread postAuthor: jeepkahn » Tue Jul 28, 2009 5:34 pm

Here's some pics of a home safe that I've been using for target practice...
These are with from 1" steel bearings. And the safe is constructed of .25" AR plate(abrasion resistant, very hard stuff).

Yes, it stopped the bearings but the funny thing is that the bearings literally hit and drop straight to the ground, no bounce at all, the safe itself weighs about 75lbs and moves about a foot with each shot... I was amazed at the depth of penetration against this hard @$$ plate steel...
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Unread postAuthor: Technician1002 » Tue Jul 28, 2009 7:08 pm

WOW.. Do you have chronograph results? :shock:

As far as dropping rounds.. it shows the energy transfer is close to 100% like a Newton's cradle. Nice selection on projectile mass to achieve this.. Well done. :wav: :headbang: :salute:

http://www.physicsforums.com/showthread.php?t=59032
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Unread postAuthor: Mitchza89 » Tue Jul 28, 2009 7:46 pm

:| Dear god...

That must have sounded like a train smash when those hit! Very well done mate.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 29, 2009 12:17 am

Great stuff... how about a dart type projectile in a sabot though, you will probably see daylight through that safe :)
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Unread postAuthor: jeepkahn » Wed Jul 29, 2009 9:05 am

Technician1002 wrote:WOW.. Do you have chronograph results? :shock:



When I'm target shooting I normally don't chrony, but I do know that these steel balls travel at just over 950fps from when I was chronying...

as a point of referance, both with same pressures...
decimater shooting 62gram steel balls=950fps(ggdt predicted 813fps)
decimater shooting 10gram superballs=1200fps(ggdt predicted 1987fps)

GGDT chokes on that one, that little differance in speed for that much weight differance...


@JSR, .25" darts coming this weekend :twisted:
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Unread postAuthor: Technician1002 » Wed Jul 29, 2009 11:52 am

jeepkahn wrote:
Technician1002 wrote:WOW.. Do you have chronograph results? :shock:



When I'm target shooting I normally don't chrony, but I do know that these steel balls travel at just over 950fps from when I was chronying...

as a point of referance, both with same pressures...
decimater shooting 62gram steel balls=950fps(ggdt predicted 813fps)
decimater shooting 10gram superballs=1200fps(ggdt predicted 1987fps)

GGDT chokes on that one, that little differance in speed for that much weight differance...


@JSR, .25" darts coming this weekend :twisted:


On your disparity in speed, I took a stab at it in this thread. I didn't want to cross post, but it belongs in both places. Follow the link to a possible reason for the supersonic disappointment. Part of my 3 inch build is to address some of these real world limitations. The 3 inch valve is for well subsonic through the valve due to a large area. A venturi will be used to transition smaller without bends or sharp edges. This fall I should have some real world data with lightweight projectiles.
http://www.spudfiles.com/forums/viewtopic,p,254045.html#254045
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Unread postAuthor: jeepkahn » Wed Jul 29, 2009 12:43 pm

I was refering to the fact that ggdt undershot the heavy balls, and overshot the light(due to gas terminal velocity in the barrel)....


Maybe one of you gas dynamics experts can answer a question of expanding gas in a cylinder....

If you open the end of the cylinder does the gas expand like a sweater unraveling, or does it expand like a spring???
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Unread postAuthor: Ragnarok » Wed Jul 29, 2009 1:02 pm

Like a spring, in that it all starts expanding at once (subject to some minor notes) - although at different speeds depending on where in the chamber.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Technician1002 » Wed Jul 29, 2009 1:22 pm

jeepkahn wrote:I was refering to the fact that ggdt undershot the heavy balls, and overshot the light(due to gas terminal velocity in the barrel)....


Maybe one of you gas dynamics experts can answer a question of expanding gas in a cylinder....

If you open the end of the cylinder does the gas expand like a sweater unraveling, or does it expand like a spring???


It depends on the pressure. At low pressure like a balloon pop, it is like a spring. The air mass has little affect. At high pressure with supersonic flow, it is a mix of both, like a line of cars taking off at a traffic light, but the rear impatient to get rolling. Air mass and the wave of pressure release has a high enough amplitude transversing the cylinder to set up pressure gradient waves. The pop echo has an effect on the pressure over time as it travels the cylinder. This is why a short fat cylinder is considered better than a long one for supersonic attempts. More air is already in the vicinity of the valve and needs less acceleration to reach the entrance of the valve when the fast valve opens.

It is why the chicken cannon has a shorter fat chamber.

A competing school of thought is a long chamber will accelerate the flow before the valve. The thought is good but ignores the flow resistance in a smaller pipe and the effects of transonic flow or supersonic flow in the valve. Air arriving at the valve arrives at higher speed, but lower pressure.. Pressure and compression will make it obvious that the volume of air reaching the valve is BIGGER in volume as it arrives at a LOWER pressure. This contributes to a slower terminal velocity as the lower pressure chunk of air is bulkier and has to take more time to get it all through the valve into the barrel.

It is better to get the bulk of the HIGH pressure air through the valve quickly and let the expansion take place in the barrel where it will do the most good.

I hope this somewhat non technical and totally absent of math description helps.
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Unread postAuthor: jeepkahn » Wed Jul 29, 2009 2:02 pm

Now that you've answered that, look at my chamber like an I instead of an H.... and you'll undrstand why I designed it like I did.... As pressure drops at the barrel breech, pressure "refills" that area from both sides which are still at a high pressure, as the pressure drops in each of the sides of the main chamber, pressure refills from both ends of the secondary chambers....

It's not designed that way for gas speeds in the chamber, it's designed that way to maintain higher breech pressures than a coax or toolie or over under... In fact the chamber gas speed can be much slower without effecting the pressure available at the breech...

Because the gases DO expand in all directions relative to the center of mass for the gas volume, it maintains more pressure at the barrel breech because the expanding gases don't have to move as fast to "refill" the low pressure area created by gases exiting through the barrel...
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Unread postAuthor: jeepkahn » Tue Aug 04, 2009 5:36 pm

Been a few days so...Doublepost...lol

Made some Glowstick/cyalume missiles and was able to shoot them 2785' as measured by gps.... took 5" glowsticks, cut the string hanger off, used heatshrink and duct tape to put 1/2oz fishing weight on the top, and used a superball as a wad.... 4 shots all landed with 15yds of each other... not sure of Cd or total weight, but they were gone post haste... shooting angle was between 35-40 degrees... wife and kids in the landing zone said they looked awesome coming in, said that once they heard the shot, the glowstick was visible about 2 seconds after that...

Shooting from my back yard over a road across 1/4mile of woods, into one of our hay fields....
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Unread postAuthor: shadowzzz » Tue Aug 04, 2009 8:09 pm

That's an amazing gun you have there! But I am interested in one of your other contraptions, also. A while back you posted in a thread of yours that you are working on a cane gun.

I am very curious if you ever finished the actual 'cane' / wooden part of it? If you have I for one would love to see how it came out !

See ya !
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Unread postAuthor: jeepkahn » Wed Aug 05, 2009 10:46 am

Details on the 'cane, I can only do descriptives of.. the purchaser doesn't want any finished pictures posted/circulated due to the nature of his line of work(personel protection)...

the handle features a twist to pierce 12g cartridge to fill the chamber with, it is a push button actuated pilot, and the shaft was made from 2 pieces of cherry that were hollowed to encase the copper coax, and then the two haves were pressed/glued together around the coax, and then it was turned and finished on a wood lathe...
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Unread postAuthor: shadowzzz » Wed Aug 05, 2009 1:06 pm

No pictures? Too bad, but I understand. I am planning on making one with a blowgun as pilot valve. The blow gun will be encased between 2 hollowed out pieces of wood and will double as cane handle, no CO2 for me :). Does it sound feasible ?

The range on the Decimator sounds amazing! Congratulations on that :p
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