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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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Unread postAuthor: warhead052 » Thu May 19, 2011 7:31 pm

Cool! Looks nice, but I spy some tape. I don't think that it is recoil rated ;)
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Unread postAuthor: Gaderelguitarist » Thu May 19, 2011 9:36 pm

Warhead, I'm going to assume that tape is temporary. I hope you were joking :lol:

This thing is looking better every time I look at it! I love the chamfer on the receiver. I had a similar problem with putting the "bang button" in a grip for my large cannon. I ended up using a forstener bit to countersink the black shoulder of the igniter into the wood. It was done so that the top of the black piece was flush with the face of the wood, and felt very nice.
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Unread postAuthor: warhead052 » Thu May 19, 2011 10:07 pm

Yeah I was. Hence the wink. I agree with you on the looking better every time you see it. I kinda wanna make my own!
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Unread postAuthor: Gaderelguitarist » Fri May 20, 2011 10:43 am

I would make one, but I would probably use oak or cherry for the furniture, then stain it with some concoction. I would also use that ignition circuit I have floating around my house somewhere so that I could have a push button trigger pull rather than a grill igniter.
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Unread postAuthor: rcbif » Fri May 20, 2011 2:35 pm

Sad day :(
Got home from work all excited to play with my launcher tube and try out my new igniter. Got a few one-click shots in, before the launcher flew out of my hands and the new BBQ igniter broke apart. Lost the inner parts in the grass and had no luck finding them. :(
There's 10 bucks down the drain. Guess I'll order another from ebay.


The good/weird news is, that I did confirm once again that adding my "chamber reducing ring/nozzle" does almost double performance. And this was with the same size chamber for each corresponding test. I just cant get similar performance without the nozzle/ring than with it.
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Unread postAuthor: rcbif » Sun Jul 03, 2011 10:16 am

Project is still in the works, but no longer a replica. Due to tooling available, and working with wood (less than precise), and stresses, I decided not to do a break action. I think however, my new solution would be an actual improvement to the real m9 though. Updates coming soon.........
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 03, 2011 10:22 am

rcbif wrote:The good/weird news is, that I did confirm once again that adding my "chamber reducing ring/nozzle" does almost double performance. And this was with the same size chamber for each corresponding test. I just cant get similar performance without the nozzle/ring than with it.


Do you have any chrony data?

Any chance of a diagram of how it looks like inside?
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Unread postAuthor: Gun Freak » Sun Jul 03, 2011 9:31 pm

Unfortunately I'm having the same problem right now haha. In most cases, designing a gun based on aesthetics never works... You must start with functionality then make it pretty.
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Unread postAuthor: PinHead » Fri Sep 16, 2011 2:48 pm

jackssmirkingrevenge wrote: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.



I would tend to agree here Jack, but I think that with the small volume of chamber and thus relatively small volume of gasses being produced, maybe the .75 inch porting is all that he needs to get maximum flow. Once you get past a certain port size, you don't need any more flow; if you increase it any more past that, you're just reducing your velocity.

I've seen guys building race cars who make the same mistake when putting on exhaust of cylinder heads; they think they need the biggest possible. In actuality, you want the SMALLEST diameter possible WITHOUT causing a restriction. Obviously a motor has different needs for different reasons, but you get the idea.

Again, I think the main reason that this works on this cannon is because the air chamber is so small. If the volume of air was greater, I don't know if it would be a good idea or not. Does anybody have a way to calculate an ideal port size, given a set volume of air?
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Unread postAuthor: Technician1002 » Fri Sep 16, 2011 3:25 pm

The projectile should be the "restriction". Any energy loss that is not used to move the projectile is wasted. The goal is not so much fast air behind the projectile, but fast high pressure air behind the projectile to push it and keep up when it moves. For this reason bigger is better as long as the air doesn't have too much "expansion" room. The relation of the barrel volume to the valve and chamber size is important.


Conclusion;
Use the biggest baddest fast valve you can lay your hands on to get the volume and pressure out of the chamber and into the barrel to give that bad boy projectile a royal shove.

Expansion room related to a valve is in both the length of the valve and diameter. Do go large up to your barrel diameter and use as short a flow path from the chamber to the projectile as possible. Let the air expand under pressure on the tail of the projectile, not before.

The valve should hold air when closed and get the **** out of the way when opened.

Now you can talk about performance..

I do have chrony data for my launcher. :D Doubling the speed would make it supersonic.. :D
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 16, 2011 10:50 pm

Technician1002 wrote:The valve should hold air when closed and get the **** out of the way when opened.


Amazing synopsis :D

If we're talking about porting between the valve and the projectile, in this case yes there is a scenario where the porting could be too big, if it allows more flow than the barrel does then this becomes dead space where air can expand to and lose pressure.

In this thread however, the proposal is to restrict flow to less than what can go through the barrel.

If this made any sense, we'd be seeing it on firearm cartridges the world over.

What certainly does work in terms of flow restriction is putting it ahead of the projectile, as many a Russian tanker will attest.

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Unread postAuthor: rcbif » Thu Dec 01, 2011 1:13 pm

Well I lost interest in this project for a while due to some unfortunate design change realizations. But it's back in progress now so I can get this thing off my workbench and working, or atleast hanging from my bedroom wall

Main change is that it is no longer break action. It was just too finicky with the tolerances for closing, and latching and stuff. Now it's fixed barrel, spray down the pipe, and muzzle load. Also probably wont have time to make sights. Just want this thing finished so It wasent a complete waste of time.


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Unread postAuthor: sharpshooter11000 » Thu Dec 01, 2011 1:52 pm

That looks great! I would suggest countersinking that "bang button" but other than that it looks amazing!
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Unread postAuthor: Lockednloaded » Thu Dec 01, 2011 6:28 pm

Aesthetically amazing, but how's the power?
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Unread postAuthor: Gun Freak » Thu Dec 01, 2011 6:45 pm

Looks nice :) same question as LnL, also, the trigger looks a bit tight.
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