SpudFiles

joannaardway wrote:Yeah, but why do anything if it's easy?

Everyone loves a challenge eh

Sometimes it's easier to just take the difficult route instead of puzzling over a simpler way of doing things.

Actually, looking at the figures again, if I shorten the cartridge, and narrow the inner tubing to counter this loss of volume a bit, I can get the length down to only just over 3 inches.

It does obviously reduce power, but by curious accident, gives it a muzzle velocity with a PB of fractionally over 300 fps from 45cm of barrel. Adding some porting after that would bring the velocity to just about the same as a regular PB gun (instead of shortening the barrel, which would cause problems with the tube mag, should I try that).
BB buckshot would have a slightly higher velocity, which would give an assortment of .2g BBs an energy between 1J and 1 ft. lb, which is in the acceptable range for airsoft.

Obviously I wouldn't be foolish enough to actually take the thing to a field (because of the legality issues), but it could be used for airsoft wars I very occasionally have with my friends.
It's obviously not going to beat anything else I have for power, so I might as well tailor the power slightly to make it usable for that instead.

Which kills two birds with one stone - it now has something that can be done with it, and increases magazine capacity. Oooh, I'm going to like this.

Since power is not an issue for reloading, you might as well vary the power simply by changing pressure.

Off on a tangent, I was looking at the XM26 lightweight shotgun system - short barrel, lightweight combustion cartridges, hmmm...

Or I could scale the idea up.

Gah, so many concepts, so little time

jackssmirkingrevenge wrote:Since power is not an issue for reloading, you might as well vary the power simply by changing pressure.

I'd rather keep the pressure up, and instead curb power with valve diameter.

Reducing the pressure means that when the projectile reaches the barrel end, a vacuum will form behind it. Reducing valve diameter will reduce the acceleration the whole way along the barrel, but without the negatives suckback could cause.

It's not a major power loss. I was never going to get more than about 25 ft lbs from the larger cartridge, so bringing it back to 11 or 12 is fine.
If I want to throw energy at a paintball, I could put 250 ft lbs into one without breaking a sweat with one of my larger launchers.

The first thing I thought of when I saw that XM26 shotgun was "Splinter Cell" - because it features the modular F2000 rifle (under the name SC20k), which can be outfitted with interchangable underbarrel shotguns or launchers (as well as having a very intriguing front ejection system)
I'm not sure the F2000 has yet progressed to the point where it can be out fitted with an underbarrel anti-materiel rifle, but it's cool enough in game. (few other stealth games give you the opportunity to use semi-automatic shotguns and anti-materiel rifles - not subtle, but fun.)

Actually, there's an idea...

joannaardway wrote:I'm not sure the F2000 has yet progressed to the point where it can be out fitted with an underbarrel anti-materiel rifle, but it's cool enough in game. (few other stealth games give you the opportunity to use semi-automatic shotguns and anti-materiel rifles - not subtle, but fun.)

Given the fact that anti-material rifles are usually at least 0.50 cal and fitted with a long barrel for the long range accuracy and power they require, I think it's more likely that you'll have an AMR with an underbarrel assualt rifle.

A bit like the Objective Individual Combat Weapon idea when you think about it

Man thats just off the hook. I dont know how easy this would be but id say Jackhammer all the way.

The main reason I abandoned this design was the lack of power blowing back the cartridge, mostly because the massive (in comparison to the 6mm valve port) 0.75" barrel that didn't allow sufficient pressure buildup. I was aware of this point but SPG brought it up recently and got me thinking again.

I did a small experiment and blocked off the breech with a 6mm aperture to see if it would make a difference. To say that it worked is a bit of an understatement, even without a projectile the thing blew back with such violence into my hand that it turned the tip of my finger purple

As such, I decided the project merits revisiting, but rebarreled in 6mm calibre. If the burst disk cartridge fails to materialise, I will certainly give it a go. I would prefer the former - simpler (though probably more expensive due to the threaded section) to manufacture, no dead space and a faster valve would mean more power - but the mini-piston isn't too hard to make either, I would happily churn out ten or so.

One concern with the piston design is that the firing pin would prematurely fire the cartridge if it encounters too much friction while pushing the cartridge, so perhaps I should have it free floating and open the valve by inertia once the face of the cartridge has stopped against the breech.

maybe you could put a weak spring in front of the bolt to push the cart forward but when fireing the spring easily compresses and the pin cac hit the shrader. But where will the cart eject?

spudchucker wrote:But where will the cart eject?

That's easy, all you need is an open port to one side and a ridge of some sort on the other, when the base of the cartridge hits the ridge it will tip out of the port. I used this idea successfull in my cartridge-fed pneumatic.

if you really needed more recoil/blowback force your could add water behind the projectile and sacrifice some of the power. and yeah that

I think it would be pretty awesome if you used some of the extra length in the new design to shoot mini spears like projectiles

I think you should try to modify the schraders like for a rubber schrader, remove the valve and use a nail or something that seals against the rubber stub and then just make some spring return system, it should have higher flow or try something because schraders really suck

iknowmy3tables wrote:I think it would be pretty awesome if you used some of the extra length in the new design to shoot mini spears like projectiles

I think you should try to modify the schraders like for a rubber schrader, remove the valve and use a nail or something that seals against the rubber stub and then just make some spring return system, it should have higher flow or try something because schraders really suck

Schraders suck for larger launchers, but when you're down to this scale they're perfectly adequate as pilots. Also, increasing its flow would increase the force needed to strike it open, which is not a desirable trait in this case.
I prefer to use "conventional" short projectiles like pellets and cut down on the pilot volume, here's a design I've been pondering with a solid rubber piston. The pilot volume is increased (no, I'm not high or anything ) but this should be offset by the fact that dead space is almost zero compared to the original design.

Wait is dead space bad or am I high?!

Dead space is that volume which the air can expand into after exiting the valve before it hits the projectile - for example if you only load your projectile half-way down the barrel, then the first half of the barrel counts as dead space. As it expands, it loses pressure, so ideally it should be as close to zero as possible.

Ok I'm not high

Is it posible for you to use a diaphram as a valve in the cart?
Now after looking at the diagram you made I would like to know where the air gets into the cart chamber is there a equilization hole?
I think you should get a quote from your friend on those threaded fittings