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Barrel diameter size tolerances

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: BigBang J » Sat Jul 17, 2010 12:30 pm

jackssmirkingrevenge wrote:
BigBang J wrote:Do ya'll think it's a good idea to make a barrel with ptfe (or other flouropolymer material) inner for a good fit and then slide that ptfe tubing into a steel tubing for strength?

If it can take the heat, why not.

That's what I am not so sure about. PTFE can handle 500 deg. F, I know that the combusted propane mix is a couple thousand deg. F, but it is only for a fraction of a second. So what do you think?
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Unread postAuthor: Moonbogg » Sat Jul 17, 2010 5:25 pm

steel spheres? Why not use patches?
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Unread postAuthor: whoa044 » Sun Sep 26, 2010 8:13 pm

I hope this pipe dimensions chart helps with what you are looking for. If you don't already have a pair of calipers, buy them.

I used to have a few glowsticks, 14" long, and 0.585 inches thick. I considered a 1/2 inch (0.622) pipe good enough to shoot them from.

What I consider a perfect fit, is something that you need to push a little for them to fit in the barrel.

I once bought round .50 caliber lead air rifle pellets that were 0.495-0.496 inches. What I used was 3/8 inch pipe, which was 0.493 inches in inside diameter. If you have calipers, take them out, and try to measure out 0.005 inches; that's how much larger the lead balls were than the barrel. This is a perfect fit.

I also had shaped lead pellets, 275 grain, and 0.4995 OD. They made a much tighter fit in the barrel. I preferred these for their heavier weight and tighter fit. Those things weighed 18 grams, but damn did they fly fast!

Link to the 275 grain pellets They also sell the 0.0045 inch smaller balls if you're interested.
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Unread postAuthor: Technician1002 » Sun Sep 26, 2010 11:31 pm

SpudFarm wrote:I would have cut out foam cylinders (like the soft foam used in cheap beds) with a section of the barrel and a hammer. Then glue them to the sphere and they should act as a good wad. Perhaps with a drop of oil in them to reduce friction.

Note that under sudden pressure rise, the foam will collapse. I have managed to get a 2 inch chunk of swimming pool noodle to pass a golf ball in the barrel. I was trying to get higher acceleration force on one by using a larger cross section and launching from a tennis ball barrel. At 100 PSI, the foam compresses enough to pass the golf ball in the barrel. Use a wadding instead. I use shop paper towels or 1/2 a pop bottle.
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