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Small diameter barrels in combustion launches

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Small diameter barrels in combustion launches

Unread postAuthor: queensland spudder » Thu Nov 19, 2009 2:28 am

Geeze being a while since i have been on here :roll:. Anyway here is what im trying to make. I trying to make a a spud gun that is use ammo that is 0.75 inch in diameter and i was asking wether i could make this thing powerful but with a barrel that is not too long.

i tried to make a combustion camber that is long and wide enough that i can install a ignition system yet keep a good C:B ratio while not making the barrel to long. Has anyone else have this delime before.

Anyway here is some specs to feast your eyes on.
Chamber length 7.9 inch/201mm

Barrel length 39.4 inch/1001 mm

Barrel diameter 0.75 inch/19mm

Chamber diameter 1.590 inch/40mm

C:B ratio calculated to 0.775 to 1.000

however are there any other ratios that will work well so that i can make my chamber short and fat cause i have heard that is the most efficient style chamber
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 19, 2009 3:40 am

If power is what you're after, think of going pneumatic. You might also want to consider a combustion with a large barrel and use a sabot for a small diameter projectile.
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Unread postAuthor: queensland spudder » Thu Nov 19, 2009 3:50 am

thankyou for your advice , hehe i hav done done that before with a 12 inch metal spike :D wnt through 1 inch of fiber board it had a hell of a kick to it (i was firing it like a shoulder mountd firearm) the spike least weighed 1kg.

If i use all pesure rated pipe thn could i use propane rather than spray 'n' pray cause have a 3 kg bottle of propane begging me to make a small cannon for it and i always wanted to make a rilfe siz spud gun.
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Unread postAuthor: Moonbogg » Thu Nov 19, 2009 4:55 am

If you use metal pipe you can make a hybrid with UBER power :D
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Unread postAuthor: queensland spudder » Thu Nov 19, 2009 2:46 pm

He he the tech shed at my school has a number steel tubing in different sizes and lengths but i don't think my teacher want me to use one, he will probably be to suspicious of me. :lol:.

hey if i maybe i can do a under over type setup and have a larger chamber yet a keep a good ratio. I hope he force of detonation doesn't put to much stress on the frame.

Moonbogg what is you experience with under over type combustion launchers and how does a under over type launcher affect preformace and structual integrity of the cannon.
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Unread postAuthor: spencetron » Wed Nov 25, 2009 2:54 am

You could try to install a device that would hold on to the ammo, but release it at full pressure, acting as a burst disc of sorts. My old cannon had a rubber washer epoxied onto the barrel, which would hold the ammo, power was very impressive too, >600fps with 6mm .2g bb's. http://www.spudfiles.com/forums/moonlig ... 17777.html
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Unread postAuthor: Boom_erang » Mon Dec 07, 2009 2:03 pm

How powerful? I just completed a cannon kind of similar to what I think you have in mind, although mine did not turn out "small". It's a 1" bore (48" long) with a C:B ratio of about 1:1 (3" chamber, 5.5 inches long).

One thought is that for more power, go with a larger chamber although some of that power is wasted in the "bang".

I would also try to keep your chamber as close to spherical as possible. Long thin chambers don't work as well IMO. Maybe for a 3/4" bore, I would suggest a 2" chamber.

I agree with Spencetron's comment about retaining the ammo until greater pressure is achieved - it works.
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Unread postAuthor: McCoytheGreater » Thu Dec 17, 2009 1:27 am

You might also want to consider a combustion with a large barrel and use a sabot for a small diameter projectile.


I've had great success with dart style sabots (20d nails with a bunk epoxy tail cone). I take a scrap piece of barrel, spray some great stuff in the bottom, set the dart on top, and fill around the dart with the foam. Slide it out from the bottom, slice the foam in two and BAM...Sabot.
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