HVBB 6mm prototype

Just build a cannon and want to show it off? Post it in here.

Postby fullmetaljacket » Mon Mar 06, 2006 10:02 am

After managing to exceed <a href="http://localhost/forums/viewtopic.php?t=12632" target="_blank">1,000 fps</a> with my latest 6mm caliber small exhaust valve launcher, I thought it would be interesting to scale up the design with 3 times the barrel length and a slightly larger chamber. The chamber volume is only about 0.7 cubic inches, giving a C:B ratio of 0.44 - not the most efficient in the world, but about as much as I can manage to take to 400 psi with a hand pump without losing patience.

It's not much to look at really:

<img src="http://pictures.greatestjournal.com/userimg/5729342/512443" alt="">

This is what the internals look like:

<img src="http://pictures.greatestjournal.com/userimg/5729333/512443" alt="">

Note the differences from my <a href="http://pictures.greatestjournal.com/userimg/2299449/512443" target="_blank">original design</a> - the piston is now all epoxy, giving very little friction (aided by a good helping of KY ;)) and removing the need for a separate check valve. Also, the rubber tube at the breech provides the extra benefit of holding the projectile a little longer while the valve opens. I also took the liberty of reinforcing the chamber with an inner sleeve of alloy tube of rather convenient dimensions.

I tested it at 50 psi to confirm that everything worked and epoxy didn't leak anywhere it wasn't supposed to, all seems to be in order. I'll allow another 24 hours for the epoxy to reach its maximum strength and then the high pressure testing can begin.

<i>Edit: curse my lack of patience! I've gone and taken it to 400 psi before I should have and made it leak :? make that another 48 hours, bleh
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Postby spudshot » Mon Mar 06, 2006 4:10 pm

what type of epoxy do you use? looks like a great design, but the epoxy here would be shit for a piston
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Postby UnrealityisReal » Mon Mar 06, 2006 7:39 pm

That's a very interesting design and it looks like it would be a pain in the ass to make since it's so small. I'm impressed with just how clean your work is and I think you should make a larger scale version. I've been thinking about making an epoxy piston for a small coaxial gun I've been designing and I was just wondering how you cast the epoxy piston without having the epoxy bond to the side of the syringe tube?
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Postby fullmetaljacket » Mon Mar 06, 2006 11:41 pm

Read through <a href="http://localhost/forums/viewtopic.php?t=11696">this thread</a> for a description of the technique and materials.

It is a bit of a pain in the ass, and the biggest disadvantage is that if something goes wrong inside, everything usualylo becomes a solid mess and you have to throw it away, but if you're careful and jig everything up well while it cures, the results can be very rewarding.

The epoxy will naturally find it difficult to adher to the smooth sides of the syringe tube, I usually smear it with a very light coating of grease or KY to help with release, one small hammer tap and it's free.
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Postby UnrealityisReal » Tue Mar 07, 2006 7:31 am

Ok thanks for that info... I'm actually hoping to scale up your design and make a 1.033" diameter piston to go inside a 1" SCH40 pipe. It should be interesting to see how much the super light piston will effect the performance of the gun.
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Postby fullmetaljacket » Tue Mar 07, 2006 10:03 am

If I could suggest the method:

<img src="http://pictures.greatestjournal.com/userimg/5733710/512443" alt="">

First put a bolt and washer through a rubber disc the same diameter as the chamber tube and glue it in place with PVA glue, makes sure that it's completely sealed so no epoxy will leak out. Then very thinly smear some grease around the inner surfaces of the chamber. Add the foam core (glue it to the bolt), make sure you leave enough wall thickness (at least 1/4 inch) to give the piston strength.

Pour in the epoxy (make sure you have good quality resin, otherwise the result will be useless) and allow to cure, at least 24 hours. Then simply tap it out gently with a hammer <i>et voila</i>, one perfectly fitting piston. You might find that the piston fits a little *too* perfectly and have a sand it lightly to be able to slide smoothly within the chamber.
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Postby nxi » Tue Mar 07, 2006 10:34 am

That´s good work. and your earlier gun 1 000 fps it´s impressive. Damn i want chronograph :)
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Postby fullmetaljacket » Tue Mar 07, 2006 10:54 am

http://www.combro.co.uk/cb625.html

Cheap and effective, plus if you're silly enough to shoot it by mistake, the aftersales service is excellent :D
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Postby UnrealityisReal » Tue Mar 07, 2006 1:29 pm

Thanks FMJ that looks like a great idea I might try that tonight.
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Postby fullmetaljacket » Wed Mar 08, 2006 1:29 pm

The results are in and well, I'm rather disappointed.

The following data applies to 0.12 gram airsoft BBs:

At 100 psi, I got 633,637 and 635 fps for the 12" barrel, as opposed to 655, 657 and 661 fps for the 36" - a slight increase.

Increased to 400 psi, I got 994, 1002 and 998 fps for the 12" barrel and, rather annoyingly, 720, 711 qand 718 fps for the 36" barrel :?

To see the effects with heavier projectiles, I tried with 1 gram lead pellets for the following results:

At 400 psi, I got 515, 513 and 520 fps for the 12" barrel and 566, 567 and 561 fps for the 36" barrel.

Any comments and theories?
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