SpudFiles

thats what I thought. I'm gonna have to do some experimenting with silicone tubing to find the best diameter- I believe I had PMed you about this awhile ago, but I had to clean out my inbox so i dont have it anymore.

What size silicone tubing do you typically use?

JDP12 wrote:What size silicone tubing do you typically use?

In this case I'm using polyurethane tubing, it's tougher and retains the projectile to a higher pressure, 12mm O/D and 9mm I/D.

Here's the latest test:

[youtube][/youtube]

Pretty nice clip even if I say so myself, because you can clearly see the ignition spark jumping from the bolt, as well as the flash shining through the cast epoxy breech.

You can also see the chip of cartridge fly off where I underestimated the strength of the blowback and didn't put a rubber buffer on the ejector, and also the bolt face itself has been deformed by the impact.

Power wasn't too impressive either, just a dent in steel sheet, however a stronger bolt spring would likely significantly increase that.

When are you going to be able to try out feeding ?
I reckon the stroke might need to be longer with the ejector slightly further back, to give more time for the next cartridge to move in.
Although there does seem to be a slight delay on the bolt return, maybe from the off centre force twisting the bolt as the cartridge was ejected.

al-xg wrote:When are you going to be able to try out feeding?

I'm going to have to make a new cartridge and bolt, and reconstruct the bolt housing.

I reckon the stroke might need to be longer with the ejector slightly further back, to give more time for the next cartridge to move in.
Although there does seem to be a slight delay on the bolt return, maybe from the off centre force twisting the bolt as the cartridge was ejected.

I've lengthened the stroke, but there really is a lot of energy in the shot to eject the cartridge, meaning that the return spring can be very strong, meaning I can afford a strong magazine spring that will quickly load the next cartridge without too much friction preventing the cartridge from moving.

looks good....

JSR why haven't you considered using aluminum pipe instead of PVC?

I did some calculations, a .745" ID and 7/8" OD aluminum pipe from mcmaster with a safety factor of 4 can be used up to 1900 PSI...

aluminum is fairly lightweight.. so any considerations??

JDP12 wrote:JSR why haven't you considered using aluminum pipe instead of PVC?

You mean like this one

To be honest all that aluminium (I've used it for several other cartridge bodies if you look through this thread) came from a single shower curtain rail which is now reduced to a short stump, I've never actually purchased aluminium tube maybe it's about time I looked for a local supplier.

a safety factor

yea... ordering some would definitely be an option.. although I know how much you like to scrounge....

and yea thats true... with your multiple epoxy fails and such... your safety factor is much lower than the normal persons... haha

JSR-- have you considered trying to delay the bolt somehow? i'm onto an idea for a combustion cartridge... sort of. will post it later today...

But obviously the main problem is the cartridge ejects and much of the gas is wasted. Have you considered trying to add o-rings to the breech so that the cartridge takes longer to eject? it seems like this would be a better option than a stronger bolt spring, as a bolt spring must be overcome for the whole travel of the bolt, whereas an o-ring or greater friction at the breech only has to be initially overcome.

Just a thought.

one other question... how "flexible" do you find the tubing you use has to be to retain the projectile? By that I mean wall flexibility- I.E. surgical tubing is very flexible, and an aluminum pipe obviously would be extremely rigid. Does this make sense?

yeah, safety first.... don't worry JSR - I heard that some women find amputees very sexy

JSR-- have you considered trying to delay the bolt somehow?

yeah, consider using air to delay the bolt

? POLAND_SPUD, I don't understand... are you mocking me? Or suggesting an air cylinder or something.

actually that would be a good idea.

you could just have the actual shaft of the cylinder act as the bolt rod, and attach the bolt face to it. That way you don't have to worry about any linkages

POLAND_SPUD wrote:yeah, safety first.... don't worry JSR - I heard that some women find amputees very sexy

That implies only a major limb is amputated!

Now, if a man's third leg missing, that is a whole different situation!

JSR had better invest in a Kevlar nut cup!

POLAND_SPUD I don't understand... are you mocking me?

Nahh I am just pointing out that JSR might acctually get hurt one day

Or suggesting an air cylinder or something.

Now you're mocking me

you could just have the actual shaft of the cylinder act as the bolt rod, and attach the bolt face to it. That way you don't have to worry about any linkages

That's what I'd do. Just because it is an off the shelf part that can be easily added to any design (Most air cylinders can be threaded in standard fittings & they've got threaded shafts)

dewey-1 wrote:

POLAND_SPUD wrote:yeah, safety first.... don't worry JSR - I heard that some women find amputees very sexy

That implies only a major limb is amputated!

Now, if a man's third leg missing, that is a whole different situation!

JSR had better invest in a Kevlar nut cup!

im sure it was his first epoxy creation [/b]

JDP12 wrote:have you considered trying to delay the bolt somehow?

Altering bolt weight and stroke is delay enough, after all it's the principle behind straight blowback submachineguns like the Sten:



Remember it worked perfectly here

But obviously the main problem is the cartridge ejects and much of the gas is wasted. Have you considered trying to add o-rings to the breech so that the cartridge takes longer to eject? it seems like this would be a better option than a stronger bolt spring, as a bolt spring must be overcome for the whole travel of the bolt, whereas an o-ring or greater friction at the breech only has to be initially overcome.

Friction in the breech will let you have a weak bolt spring and still have power *but* on the return stroke when it comes to chambering a new cartridge, the bolt won't be up to the challenge. I would advocate a low friction breech and strong bolt spring/heavy bolt.

one other question... how "flexible" do you find the tubing you use has to be to retain the projectile? By that I mean wall flexibility- I.E. surgical tubing is very flexible, and an aluminum pipe obviously would be extremely rigid. Does this make sense?

I'm not sure I understand the question, but the thing about friction seals is that if it's easy to push the BB in, it's going to be easy for even low pressure to pop it out so high mixes are not an option.

One thing to note is that when you put a BB in flexible tubing with a smaller I/D that the BB O/D, the tube O/D also expands. If the tube restricted however (eg cast in epoxy) this expansion is not allowed and the friction is greater.

CZTUCZNY_CHUJ wrote:Most air cylinders can be threaded in standard fittings & they've got threaded shafts

Ironically, if one's "third leg" were to be violently removed by high velocity fragments, a threaded shaft is exactly what you would need

I am just pointing out that JSR might actually get hurt one day

More a question of "when" than "if" at this rate

Anyway, back to the subject of this thread, I made a new cartridge and bolt which are currently curing, I shortened the cartridge length to 3 inches (increasing stroke), now that there's a sturdy copper structure I can experiment with different ejector locations. To avoid damage to the cartridge as in my previous test I'll bolt on a rubber block as the ejector.

ah, I see what you're saying. fair enough, makes sense.

Thanks for the tips on the tubing.