SpudFiles

Shame about overly restrictive gun legislation, if I had my own machining facilities I'm sure this project would have gotten off the ground a long time ago.

It would be great to see multiple cartridges in a gun, good luck with your progress.

Thanks, as it is there is a lot of promise in that even without the benefit of a breech to contain the expanding gasses, the thing still happily throws itself across the room indicating it should be capable of operating a direct blowback mechanism, countering the force of the bolt spring as well as the friction caused by the following cartridge pressing against it.

It's a shame that I can only use it at 6x when I know that piezo ignition allows me more than twice that, but freedom from the weight and complexity of a burst disk outweighs this disadvantage. Also, the great thing about cartridges is that it makes the design modular, so in future I might yet find a better sealing material or construction technique that will let me up the pop-off pressure.

Heating was a big problem for the barrels, mine warped from using thin walled tube.

Heat is a particular concern as the barrel I'll be using is cast from epoxy and the breech is a section of 3/4" PVC pipe, so there's no metal in this area at all. The consolation is that most of the heat is absorbed by the cartridge body which is ejected so this shouldn't lead to problems, especially as this is going to be a semi-auto launcher.

Currently curing is a 1.25" long breech attached to a 6" barrel, HGDT predicts a fairly low 250 feet per second for a 3/8" bearing which isn't exactly spectacular but I think it will be enough to inspire me to actually make a working cartridge fed prototype.

More testing planned for when it cures, as always watch this space

if I had my own machining facilities

Are you sure there are no ready made fittings that suit your needs?
I've been using tap cartridges (I hope that's what they are called) for some time. They can be converted easily into compression fittings and detents
pic

It might not be exactly the fitting you need but at least you get the idea. For mass production something like that would be ideal

I've seen some that look pretty much like the one in my drawing

I have suitable threaded fittings like the ones I used in my mini, but they add too much weight to the cartridge.

If I could find suitable plastic ones I might just change my mind...

i just through together a rough gun with a 3.5ml camber 5mm burst disk and a 45cm x 8mm barrel and at 8x it has a bit of power and is reasonably quiet so your cartridge has some hope for it yet

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Looking good, the combustion throws everything - projectile, cartridge, barrel/breech - all over the place, love it and it penetrates 1/4" ply rather nicely even with the cartridge unsupported.

Didn't penetrate the bottom of a soup tin, but that's the second ignitor today that's been ruined by having its cables ripped out

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Didn't penetrate the bottom of a soup tin,

That's because the energy normally intended for the soup tin was making a bid for freedom in the opposite direction and ripping cables out of things.

What are you letting that cartridge smack into when it recoils? I'd worry about stressing the epoxy. Also, how much does the cartridge weigh in relation to the projectile?

Yeah. You need to better secure that cartridge (and the rest of the launcher, to be frank) on firing.

I know you want blow-back, but if there's enough spring pressure that it's only cycling by a fairly small margin (enough to keep it reliable), that's less energy lost from the projectile.

Naturally a lot of energy is lost from having it unsecured, this was just some preliminary testing. I'll put together a bolt and ejector system tomorrow and see how that goes before moving on to adding a magazine.

What are you letting that cartridge smack into when it recoils?

Rolled up jacket, I'm being nice

how much does the cartridge weigh in relation to the projectile?

Haven't weighed the cartridge but it's fairly light.

I know you want blow-back, but if there's enough spring pressure that it's only cycling by a fairly small margin (enough to keep it reliable), that's less energy lost from the projectile.

That's what I'm aiming for, the critical thing will be pushing the bolt far back enough for a new round to be fed in the breech by the magazine spring before it starts to return. Adding weight to the bolt or perhaps a friction seal in the bolt mechanism could be a solution in this case.

wow that looks great, cant wait to see the actual gun when you make it, and how is your seal holding up dose it still have the same pressure capability?

ilovefire wrote:how is your seal holding up dose it still have the same pressure capability?

I've fired it about 20 times so far and it will still retain the 75 psi I'm using it at.

As to cartridge weight, it's around 16 grams, so only 4-5 times the weight of the projectile, a strong bolt spring is definitely in order

Currently curing is a breech similar to the one I had made for my last combustion cartridge experiments, hopefully this one won't blow apart

jackssmirkingrevenge wrote:Adding weight to the bolt or perhaps a friction seal in the bolt mechanism could be a solution in this case.

Weight is probably the better option. It'll slow the blowback down and give the bolt more inertia so that it continues to cycle even after the cartridge has ejected.
Between those things, it should also give the magazine more time to feed the next round.

Here's the breech and bolt setup curing with cartridge in place, to be tested later today

I can't wait to see it in action. It looks great. Have you given much thought to how the cartridges will eject? (Side ejection/magazine feeding) Is the sheer force enough without anything guiding the motion?

lozz08 wrote:Have you given much thought to how the cartridges will eject? (Side ejection/magazine feeding)

The plan is to have it feeding from a fairly conventional vertical bottom mounted magazine, and ejecting sideways to the left. As pitured it's lying on its right side.

Is the sheer force enough without anything guiding the motion?

There's an edge visible half way between the end of the cartridge and the bolt block, the base of the cartridge strikes this on firing and it causes the cartridge to tip out sideways, something like this:

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The problem with the above is that the bolt starts to return the instant the cartridge stops, in order to avoid this I plan to add more weight to the bolt in order to give it more momentum and continue moving back even after the cartridge has been tipped out.

wow it works... congrats

The problem with the above is that the bolt starts to return the instant the cartridge stops

use a check valve - air would be allowed to exhaust from the cavity behind the bolt but on the return stroke it would be slowed down by vacuum

the added benefit is that you've the bolt housed inside a length of pipe so it reduces the chance of injury

if the bolt is housed inside the pipe you might even use a floating oring as teh check valve