CB ratio for coaxial gun

Jolly Roger · Sat Jun 25, 2011 3:39 am

Does anyone know how to optimise the diameter and length of the inside barrel on a coaxial design, and how that would affect the actual CB ratio? Any help of suggestions would be much appreciated.

Cheers

jackssmirkingrevenge · Sat Jun 25, 2011 4:48 am

- what are yout shooting
- how much power do you want
- how loud can it be
- can the chamber be as long as the barrel
- what pressure are you firing at
- what are you using to pressurise

Jolly Roger · Sat Jun 25, 2011 5:00 am

- what are yout shooting
Most probably lead cast rounds from molds, then machined down on lathe. 15-20mm diameter

- how much power do you want
As much as possible from 150x propane air mix. Same duplex stainless as Lardas

- how loud can it be
Doesn't matter

- can the chamber be as long as the barrel
If that is the most efficient, otherwise probably no more than 4 times the chamber length, as measured from the outside

- what pressure are you firing at
Roughly 2200psi pre-ignition. Peak will vary

- what are you using to pressurise
Scuba

jackssmirkingrevenge · Sat Jun 25, 2011 5:10 am

With those specs, you will have awesome power with the tiniest of chambers, even a 0.01:1 ratio is probably overkill.

I would suggest a short fat chamber to optimise combustion as opposed to a thin one along the whole length of the barrel.

Jolly Roger · Sat Jun 25, 2011 5:46 am

Yeah I think that's very true. Although I was hoping of some way to figure out what barrel and chamber diameter ratio would be optimal, to save on using more air than needed.

jackssmirkingrevenge · Sat Jun 25, 2011 5:50 am

have you played with HGDT?

Crna Legija · Sat Jun 25, 2011 5:54 am

download HGDT read the usage tab, then plug in the numbers and find out what works best.

Jolly Roger · Sat Jun 25, 2011 7:14 am

That's what I'm looking for

Thanks heaps guys

saefroch · Sat Jun 25, 2011 9:36 am

HOLY SHITE!!!! This will be interesting... I'm curious to know why you want to do a coaxial though...

warhead052 · Sat Jun 25, 2011 11:12 am

Wow, I saw all the specs that you posted and I had the same reaction as everyone else. Let me see if I can find the link of one of the coaxials I have seen.

Edit, here ya go. This may turn some gears. http://www.spudfiles.com/forums/high-pr ... 12803.html

Crna Legija · Sat Jun 25, 2011 12:39 pm

warhead052 wrote:Wow, I saw all the specs that you posted and I had the same reaction as everyone else. Let me see if I can find the link of one of the coaxials I have seen.

Edit, here ya go. This may turn some gears. http://www.spudfiles.com/forums/high-pr ... 12803.html

the OP is taking about a hybrid not pneumatic,

Jolly Roger wrote:- what are yout shooting
Most probably lead cast rounds from molds, then machined down on lathe. 15-20mm diameter

- how much power do you want
As much as possible from 150x propane air mix. Same duplex stainless as Lardas

- how loud can it be
Doesn't matter

- can the chamber be as long as the barrel
If that is the most efficient, otherwise probably no more than 4 times the chamber length, as measured from the outside

- what pressure are you firing at
Roughly 2200psi pre-ignition. Peak will vary

- what are you using to pressurise
Scuba

warhead052 · Sat Jun 25, 2011 12:42 pm

I know, I was just saying...

DYI · Sat Jun 25, 2011 1:01 pm

With those specs, you will have awesome power with the tiniest of chambers, even a 0.01:1 ratio is probably overkill.

Now let's not get too carried away here Jack - performance would be terrible with a ratio that low, it'd totally defeat the purpose of using high pressure.

In the case of a 20mm bore barrel (and a coaxial configuration), I suggest a chamber internal diameter of 60mm or so, probably 15-20cm long, for reasonably good performance (about 600-650 m/s for the slugs you propose). You'll not get firearm like performance at 150X, and 600m/s with full bore lead slugs is about the best you can aim for without the dimensions getting overly large.

Also, don't put too much stock in HGDT's results - just remember that it IS a 0-D simulator and therefore tends toward inaccurate output at high propellant gas densities and/or speeds.

Jolly Roger · Sat Jun 25, 2011 8:06 pm

DYI wrote:
With those specs, you will have awesome power with the tiniest of chambers, even a 0.01:1 ratio is probably overkill.
Now let's not get too carried away here Jack - performance would be terrible with a ratio that low, it'd totally defeat the purpose of using high pressure.

In the case of a 20mm bore barrel (and a coaxial configuration), I suggest a chamber internal diameter of 60mm or so, probably 15-20cm long, for reasonably good performance (about 600-650 m/s for the slugs you propose). You'll not get firearm like performance at 150X, and 600m/s with full bore lead slugs is about the best you can aim for without the dimensions getting overly large.

Also, don't put too much stock in HGDT's results - just remember that it IS a 0-D simulator and therefore tends toward inaccurate output at high propellant gas densities and/or speeds.

Yeah I think when approaching high mixes with a piston valve the amount of chamber ignition points become critical. With a low number of ignition points, the projectile will likely have left the barrel before maximum chamber pressure is reached, simply because the pressure release is gradual, compared to a burst disk which will release it all at once at a specified failure pressure (higher the better).

And DYI, you are fairly spot on there with the chamber ratios. I'm getting 50mm x 170 as the sweet spot. Anything below or above 50mm seems to reduce performance. But again I think that has to do with more space without ignition points.

saefroch · Sat Jun 25, 2011 8:15 pm

You can make do with fewer ignition points simply by designing the valve only to open very near maximum pressure. MrCrowley and SB15 did this by pressurizing the pilot.

CB ratio for coaxial gun

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