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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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Barrel diameter size tolerances

Unread postAuthor: BigBang J » Thu Jul 15, 2010 4:18 pm

How close do you want the ID of your barrel to the OD of your ammo?
if using 1/4" OD ammo can you use a barrel with an ID of 0.245" or 0.249"? Or should the barrel be slightly larger? Or should it match perfectly. ID of ammo 1/4", OD of barrel 1/4"?

I am having a little dificulty finding a barrel - ammo match.
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Unread postAuthor: Ragnarok » Thu Jul 15, 2010 4:25 pm

Larger, but by as small an amount as you can manage.

While firearms have the pressure to swage a projectile to the barrel, spudguns do not, and there needs to be enough tolerances that the projectile doesn't jam in the barrel.
However, you don't want to waste power by losing pressure past an overly loose fitting projectile.
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Unread postAuthor: BigBang J » Thu Jul 15, 2010 4:35 pm

Thanks, so I definatley want larger. Does it matter by how much, I mean being practical, I know 1/4" ammo with a 1/2" barrel is retarded.
But would 1/4" ammo go well with a .263 barrel?
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 15, 2010 4:44 pm

BigBang J wrote:But would 1/4" ammo go well with a .263 barrel?


That's 0.0543 square inches for the barrel, and 0.0491 square inches for the projectile base area, meaning it's like having a 0.08" diameter hole drilled in the barrel behind the projectile...
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Unread postAuthor: Ragnarok » Thu Jul 15, 2010 4:46 pm

BigBang J wrote:Does it matter by how much.

As little as possible is the rule of thumb.

But would 1/4" ammo go well with a .263 barrel?

Well, I'm not fluent in fractions of an inch, so... 6.35mm and 6.68mm.

Well, that is a 10% gap (working by area, not diameter before anyone gets confused), which workable, but less than ideal, but it does depend on things like the launcher and, indeed, what exactly these 1/4" projectiles are.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 15, 2010 4:53 pm

To put it in real terms, I had done a comparison test comparing a 4.5mm i/d barrel with a 5mm i/d barrel, for 4.5mm lead balls.

Using the same barrel length, pressure, valve and projectiles that 0.5mm (0.02") difference gave about 25% less power for the wider barrel, which is very significant for what does not on the face of it look like a huge difference in diameters.
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Unread postAuthor: Stolly32123 » Thu Jul 15, 2010 8:26 pm

i would use the same. Just push it down with a ram rod.
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Unread postAuthor: Hotwired » Thu Jul 15, 2010 9:23 pm

For preference a barrel larger than the ammo and use wadding/some kind of gas seal on the projectile to close any gap.
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Unread postAuthor: jhalek90 » Thu Jul 15, 2010 9:25 pm

depending on the ammo, the projectile may expand while under the pushing force of the air, and actually jam in the barrel if it is to tight.

A 1'' bouncy ball has a tendency to do this in 1'' pvc at higher pressure...

Once, the ball is stuck, pressure builds in the barrel, and the ball usually, but not always, explodes into 3 or 4 chunks, and comes out like that.
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Unread postAuthor: BigBang J » Thu Jul 15, 2010 10:29 pm

Wow, well when you start breaking it down to milimeters it starts looking pretty bad. I guess I'll just have to keep looking for a better fit.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 15, 2010 10:51 pm

With a cylindrical projectile, if you have say a 0.05" gap, you can easily make up the difference by wrapping with duct tape ;)
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Unread postAuthor: Moonbogg » Fri Jul 16, 2010 2:30 am

It depends on the kind of projectile. In my cannon, I use limes that I have to really shove in there hard, so they fit ultra tight and can only come out with the cannons pressure. Limes are shockingly strong and able to withstand 2-3x hybrid shots, fitting tight at 1.87" diameter. If its a hard projectile then you need a close, but loose fit so it can move freely or with a gently push.
I shot some sureshot UHMW rounds that were jammed in super tight. So tight that I could in no fashion ever get them loose, but they shot out anyway =)
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Unread postAuthor: BigBang J » Fri Jul 16, 2010 7:33 pm

Hmmm.

I am planning on shooting steel spheres.
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?
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Unread postAuthor: SpudFarm » Fri Jul 16, 2010 7:42 pm

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.

Or you caould just cast some wax or something around the projectile, then cut it up in three pieces lengthwise to make it act as a sabot for the sphere.

It all depends on your cannon.
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 17, 2010 7:03 am

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.
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