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M79 Grenade Launcher SpudGun Project

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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M79 Grenade Launcher SpudGun Project

Unread postAuthor: rcbif » Fri May 13, 2011 7:52 pm

Well it all started here.............
http://www.sksboards.com/smf/index.php?topic=87163.0

Well that was just a little one weekend project. It was a fun little toy for the first few shots, but now I want something bigger and with more firepower

Image
(google reference picture)

So now comes this project. It will be a spud gun dressed as an M79. My project will be AS ACCURATE AS I CAN MAKE IT for ease of building. It will not be a high performance gun. The tests showed 70 yards with alcohol fuel. (I figure not bad for the overall size and chucking a 3oz projectile)

My spudgun will have a constant barrel to chamber diameter for realistic looks. Instead of the large chamber and skinny barrel, my gun's barrel will use a High-low pressure system inspired by the real M-79's 40mm rounds design. Instead of the typical spudgun chamber screw cap, my gun will be break action like the real M79 for refueling. The rounds are foam tipped plastic bottles, and weight about 2-3 oz. The gun will be muzzle loaded due to design features and to maintain spudgun status.


So let the fun begin.................


Rough stock shape layed out and ready to cut on bandsaw.
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the two stock pieces and the final template with the barrel. Barrel is 1.5" SCH 80 and is 14" long.
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The two stock pieces were epoxied together, cut to precise template shape, and then had the edges sanded on router table.
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work on the front hand guard next. Made of 3 pieces of wood with some 45* cuts lengthwise to allow for barrel.
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pieces epoxied together looking kinda ugly
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trimmed and front curve added.
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lower edges hit with router for radius. Upper edges chamfered and sanded round. Rear relief cut. Inner channel sanded round for barrel
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showing fit
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mock-up at the end of the day
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Thoughts.......

-For some reason my stock is 1inch too long. I may fix it, or may not.
-The receiver section is gonna be a nice challenge to make :)
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Unread postAuthor: JDP12 » Fri May 13, 2011 8:40 pm

WOW! Incredible, welcome aboard!!!! Can't wait to see the rest of this..
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Unread postAuthor: Gaderelguitarist » Fri May 13, 2011 9:03 pm

That receiver should be no problem to mock up. And if you have a belt sander, it wouldn't be too much extra effort to deepen the contours of the stock. The reference picture shows a bit more of a gradual round-over on the upper portion, especially around where you hand must grab.

I love seeing projects like this though! Great work so far.
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Unread postAuthor: rcbif » Fri May 13, 2011 9:28 pm

Gaderelguitarist wrote:And if you have a belt sander, it wouldn't be too much extra effort to deepen the contours of the stock. The reference picture shows a bit more of a gradual round-over on the upper portion, especially around where you hand must grab.


Ya, I was just looking at that. I'll probably try to make a template to wrap some sandpaper around, to get that contour. I hate free handing stuff like that with powertools.
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Unread postAuthor: Gaderelguitarist » Fri May 13, 2011 11:02 pm

It can be scary sometimes to try to free hand things when the slightest slip could make all of your work worthless.

Half of the fun is the risk though. Can't learn without making a few mistakes.

Are you thinking in terms of a sanding block? I would get fed up with that and probably resort to hammer-and-chisel carving the thing to shape.
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Unread postAuthor: jackssmirkingrevenge » Sat May 14, 2011 1:35 am

Amazing little project and beautifully finished, your next one is looking to be even better, kudos :) I really recommend metering gas with a syringe as opposed to spraying in order to get the best performance out of your size constraints.

Also have a look at the launcher used for the above tutorial, it similarly uses the same tube for barrel and chamber and illustrates the difference in performance for different projectile starting positions in the barrel.

Combustion will not generate more than 100 psi, so no need for a high/low pressure system - because it's already starting out at low pressure ;)
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Unread postAuthor: Doctore » Sat May 14, 2011 5:32 am

haha i love it
keep us updated :D
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Unread postAuthor: rcbif » Sat May 14, 2011 8:13 am

jackssmirkingrevenge wrote:
Combustion will not generate more than 100 psi, so no need for a high/low pressure system - because it's already starting out at low pressure ;)


This is interesting, because my tests showed about a 40% increase in distance with the high-low set-up. There was also a significant recoil and noise difference. Maybe I'll repeat the tests.

For my high-low set-up, I'm using a ring that fits in the barrel snugly, with an 3/4 opening. (so 1.5" I.D. reduced to .75" I.D.). I have found putting the ring 5" from the rear (giving a 5" combustion chamber) gives me the best performance. I have also tried these tests by simply measuring the distance I slide the round in (without the ring). So isnt there a slight pressure increase due to the smaller ring opening the gasses have to travel through?
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Unread postAuthor: jackssmirkingrevenge » Sat May 14, 2011 2:26 pm

All you've done if I understood it correctly is added a flow restriction which isn't really going to help performance. Granted, it will increase gas velocity, but reduce the rate of pressure increase in the barrel.

The 40mm round has a "burst disk" which generates high pressure in the cartridge but leaks relatively low pressure into the barrel. I would just leave the tube plain, and experiment with projectile position for optimal performance.
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Unread postAuthor: LeMaudit » Sat May 14, 2011 3:14 pm

Beautifully done so far!
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Unread postAuthor: rcbif » Sun May 15, 2011 2:52 pm

Got some work on the receiver today. Making a wooden receiver :lol:
Had a bad feeling starting off on my wooden receiver, but I'm feeling a bit better about it at the end of the day.

Also did some work on the front hand guard as suggested by Gaderelguitarist.

3 main sections cut out.
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pivot hinges re-enforced with one of the greatest building toys ever (erector set pieces)
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all glued together into a block. Bottom edges routed
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Test fit. Top edges rounded around barrel.
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showing my next step. Making a chamfer/radius combo to get the stock flush with the receiver. This is one of the less scale parts of my build. Because the barrel is a bit bigger than the real ones, I had to use 3 layers laminated for the receiver/forend and 2 for the stock. In other words, the rear section of the receiver will have a bit more dramatic width change than the real thing.
Image

My next step will be bedding the barrel to the receiver with bondo and doing some other bondo work for a smooth metal appearance. After than will be the receiver chamfering/radius thing and then probably more bondo :P
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Unread postAuthor: Gaderelguitarist » Sun May 15, 2011 9:35 pm

Thats looking really great. I just don't remember suggesting working on the foregrip. I mentioned the contours of the stock. :lol:
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Unread postAuthor: rcbif » Mon May 16, 2011 2:31 pm

Gaderelguitarist wrote:Thats looking really great. I just don't remember suggesting working on the foregrip. I mentioned the contours of the stock. :lol:


ah, ya misunderstood you. In either case, both needed done.
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Unread postAuthor: jakethebeast » Mon May 16, 2011 3:33 pm

this is turning out pretty nice :) . i just hope youll get it finished soon!
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Unread postAuthor: rcbif » Thu May 19, 2011 6:56 pm

A small update......

Some more work on the receiver, rounding/chamfering it down to meet the stock size. And also installed the "Bang Button". Drilling that hole was pretty scary, but it turned out alright. I'll find some way to dress up/ hide the threaded area of the bang button. It was a bit bigger than I though, so it cant go further into the stock, but it actually works out.
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