Air cartridge prototype - ".338 Lupra Magnum"



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Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #20 on: February 09, 2019, 09:15:35 AM »
Concept for a mechanically locked pressure actuated air cartridge in the same vein as some airsoft 40mm grenades:



By having the two o-ring diameters on the piston close to each other, differential pressure and therefore the amount of friction that resists the motion of the "firing pin" can be kept to a minimum, thus allowing for a possible blowback repeater.
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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #21 on: February 09, 2019, 02:05:14 PM »
The only thing I question is the retention of the front O-ring on the valve piston.... You might be better to use a conventional poppet and seat arrangement at that point.... Just a thought....

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #22 on: February 10, 2019, 07:55:46 AM »
The only thing I question is the retention of the front O-ring on the valve piston.... You might be better to use a conventional poppet and seat arrangement at that point....

A valid concern, especially with higher pressures.  With this configuration however there is no pressure to be put against the seat because the net pressure by definition needs to be pushing the piston away from the seat.

Here's how I imagine a balanced poppet could work, similar to how the cartridge designed by Michael Saxby but with a much smaller pressure bias keeping the cartridge closed:



Incidentally the Saxby & Palmer cartridge eventually manufactured did use an o-ring seal:



Another thought with more performance potential is the "valveless" design where the pellet itself seals the mouth of the cartridge, which would maximize flow and internal volume with zero "opening time":



This does put a lot of stress on the projectile however which could deform and affect accuracy.  Also crucially in practice were it to be made like this the friction from the balls pushing against the sleeve would make it impossible to move, increasing with pressure.  It would be better to include a slightly unbalanced spool to compensate, although getting the proportions right would be a challenge.  Too little bias and it will still be difficult to move, too much and it will fire as soon as it's pressurized.



The disadvantage of having a large diameter spool is that even though the diameters of the two ends might be identical, there is still a lot of friction that needs to be overcome and it increases with the diameter of the seals.  The Inovairtech Magnum Air Cartridges for example uses a smaller diameter spool design, however this limits flow and dead volume:



My ultimate aim for a cartridge airgun would be to generate enough pressure in the chamber to be able to cycle a semi-automatic action, and crucial to this would be a hammer that needs minimal pressure to actuate the cartridge.  This is why the mechanically locked piston is appealing since the pressure moves the piston, thus negating the need for the hammer to overcome the friction.

The only other option that would have similar low hammer requirements would be an exhaust valve similar to the original subject of this thread, where a small diameter poppet empties a pilot chamber an the main chamber pressure moves a coaxial piston:

« Last Edit: February 10, 2019, 11:18:13 AM by jackssmirkingrevenge »
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Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #23 on: February 13, 2019, 11:28:17 AM »
Some mechanically locked designs I'm considering, drawn to scale:



0.75" OD 3" long cartridge for a 3/8" (0.375") ball bearing projectile



0.75" OD 3" long cartridge for a 3/8" (0.375") ball bearing projectile with spool piston to maximize internal volume



1" OD 4" long cartridge for a 0.875" shot load
  • Quebec, Canada
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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #24 on: February 13, 2019, 03:10:19 PM »
Cartridges are fine, but by definition are dump valves with limited volume.... That limits both the FPE and efficiency.... They have their place, providing your expectations are realistic....

Many very cool designs, BTW.... The one where the pellet is the valve may have problems with the skirt stretching as it passes the ball locks and into the barrel and rip the skirt off, or at least severely distort it....

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #25 on: February 13, 2019, 08:09:23 PM »
Cartridges are fine, but by definition are dump valves with limited volume.... That limits both the FPE and efficiency.... They have their place, providing your expectations are realistic...

I'm with you on the efficiency but don't you think some configurations would have the edge over hammer valves in terms of flow and opening time?

As you pointed out, using a lead pellet as the "valve" sounds good on paper but wouldn't really work in practice.  I would think however that something like this with a sabot would get some good performance:



The dead volume is virtually zero as is the "opening time" if indeed there is any, there is full pressure on the base of the sabot the moment it starts down the barrel and the flow is equivalent to the bore without any asymmetrical turbulence.  I'd wager you'd lose the sabot o-rings with every shot but it would probably be worth the performance.


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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #26 on: February 13, 2019, 10:43:28 PM »
All dump valves have a pressure drop as the pellet moves down the barrel, in inverse proportion to the volume of the air you dump.... If you go larger than 50% of the barrel volume the gains in power are small compared to the amount of extra air ejected from the muzzle after Elvis has already left the building.... In fact, most guns using dump valves (eg. pumpers) have a valve volume of only 25-33% of the barrel volume for this reason....

Let's say your valve volume is 50% that of the barrel, and the valve is filled to 3000 psi.... You start with 3000 psi, and by the time the pellet is halfway to the muzzle you only have 1/2 x 3000 = 1500 psi, and when it gets to the muzzle, the pressure is down to 1/3 x 3000 = 1000 psi.... That means that 1/3 of the air in the dump valve is completely wasted each shot....

With a conventional timed PCP valve, and let's say a reservoir of 10 times the barrel volume (quite common), starting at 3000 psi, if you keep the valve open until the pellet reaches halfway to the muzzle (which releases the same amount of air as your dump valve contained), and close it at exactly that moment, the pressure profile would be 3000 psi at the start, 10/10.5 x 3000 = 2857 psi at valve close (compared to 1500 for the dump valve at half distance), and 2857/2 = 1429 psi at the muzzle (compared to 1000 for the dump valve). Obviously you will get more power from the higher pressure, and since you are using the same volume of HPA, you will greatly increase the efficiency compared to the dump valve....

In order to get similar power from a dump valve compared to a timed valve, you must dump more air.... If you dump 100% of the barrel volume, you will have 2000 psi at mid barrel and 1500 psi at the muzzle, still less average pressure than the timed valve, and now using twice the air.... and you are now blasting 50% of your air out the muzzle after Elvis has departed.... Double it again to 200% of the barrel volume, you will have 2/2.5 x 3000 = 2400 psi at mid barrel, and 2/3 x 3000 = 2000 psi at the muzzle.... This may get you close to the timed valve in power, but you are now using 4 times the air of the timed valve.... It is this combination of factors that drastically limits the balance of power and efficiency achieved by dump valves.... What the cartridge idea does have is a certain "cool factor", and of course the satisfaction that you managed to make it work....

You mentioned the "opening time", and whether that may gain you power, but consider this.... The wasted "transfer port volume" in a typical PCP is quite small, and calculations show that it fills so quickly that the pellet has barely begun to move by the time the pressure at its base is nearly reservoir pressure.... There is no real "flow volume" early in the valve cycle, only a sharp pressure rise at the base of the (nearly stationary) pellet.... The real "airflow" that occurs takes place late in the shot cycle, as the pellet velocity is higher.... Yes, there may be a slight advantage to having full pressure at the base of the pellet when it is released.... but nothing like the gains from the much higher barrel pressures available from a conventional PCP.... unless, as I showed above, you use a HUGE dump reservoir.... with the resulting horrible efficiency....

I don't have all the numbers in front of me, but one of the problems with the MAC cartridges is that the internal volume was insufficient compared to the barrel volume of the large calibers (eg. .357) they were trying to make work, and in addition the internal ports were too small.... barely OK for a .25 cal.... I can tell you that the internal volume was only 8 cc.... This severely handicapped the performance in the larger calibers.... They weren't even a decent performer in .257....

Bob
« Last Edit: February 13, 2019, 10:48:21 PM by rsterne »
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #27 on: February 14, 2019, 06:23:13 AM »
Appreciate the detailed response.

Quote
With a conventional timed PCP valve, and let's say a reservoir of 10 times the barrel volume (quite common), starting at 3000 psi, if you keep the valve open until the pellet reaches halfway to the muzzle (which releases the same amount of air as your dump valve contained), and close it at exactly that moment, the pressure profile would be 3000 psi at the start, 10/10.5 x 3000 = 2857 psi at valve close (compared to 1500 for the dump valve at half distance), and 2857/2 = 1429 psi at the muzzle (compared to 1000 for the dump valve)

I find it hard to imagine pressure behind the pellet equals chamber pressure before it starts to move, in fact being a dynamic system it seems dubious maximum barrel pressure would be anywhere close to the full chamber pressure.  In practice this is what the barrel pressure vs projectile position curves would probably look like, assuming a "valveless" configuration for the dump valve setup:



As you say however while the peak pressure is lower, the average pressure would be higher for the timed valve resulting in greater acceleration and ultimately more power unless the dump valve chamber is unreasonably large.

Quote
I don't have all the numbers in front of me, but one of the problems with the MAC cartridges is that the internal volume was insufficient compared to the barrel volume of the large calibers (eg. .357) they were trying to make work, and in addition the internal ports were too small.... barely OK for a .25 cal.... I can tell you that the internal volume was only 8 cc.... This severely handicapped the performance in the larger calibers.... They weren't even a decent performer in .257....


Doubtless one of the reasons that Inovairtech seems to be in trouble as a commercial venture.  Beyond the novelty factor of shell ejection, a cartridge system cannot compete with a conventional PCP in terms of ballistic performance and efficiency and that novelty factor will soon wear off when you're on the range being outperformed by fellow shooters with more traditional setups.  As a platform therefore, a bolt-action rifle is likely the worst possible choice.

I'm after something a little more "fun", ideally self-loading firing a larger projectile (such as 3/8" bearings in a smoothbore barrel, with sufficient accuracy for plinking at short range) and sub 500 feet per second, that will probably look more like a movie prop:



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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #28 on: February 14, 2019, 03:20:09 PM »
I didn't say "before it starts to move", I said "barely begun to move".... Perhaps I should have said "before it moves very far".... Let's say the air molecules inside the valve are vibrating at a speed of 1650 fps, as they would be at room temperature.... When the valve cracks just a few thou (which is MUCH larger than the diameter of a molecule), some of those random movements are now past the seat, and the pressure in the exhaust port starts to increase.... The least time for them to reach the pellet base, for a 1" long port system, would be (1/12/1650) = 0.05 mSec (ie 50 uSec). Let's say it takes 4 times that long, or 0.2 mSec. before the pellet starts to move at all, and 0.5 mSec. for the pressure at the base of the pellet to rise to "almost" reservoir pressure.... The pellet starts from rest, and with 3000 psi pushing on it, in that 0.3 mSec. it will have only moved about 1/4-1/2" (for a typical example). light pellets a bit further, heavy bullets not as far, of course.... Certainly, not far enough to cause much of a pressure drop from it's movement along the bore.... and more importantly, in that brief time there will be little expansion of the HPA, and therefore little pressure drop, providing the reservoir is large (but a lot more drop for a small dump chamber).

My point is that the pressure available at the base of the pellet is the most important thing in the acceleration process.... Yes, the first part of the acceleration is the most important, which is why high pressures are more efficient.... Your proposed model of the barrel pressures for a timed valve vs pellet position is not possible, or we could not achieve the performance we do with the short dwell time and high efficiency of typical PCPs.... Remember, many PCPs have the valve close before the pellet has even moved 20% of the barrel length, some 10% or less.... Here is a more likely representation of what is happening inside a PCP.... compliments of Lloyd's Internal PCP Ballistics spreadsheet....



While this is a representation of what happens with a small dump chamber.... I used a chamber volume of 1/2 the barrel volume, and had to increase the pressure to 3600 psi to get the same FPE.... even with zero "wasted transfer port volume" for the dump valve setup....



I'm not trying to convince you that you can't build a dump valve that will work and provide good performance, far from it.... I'm just trying to point out some of the pitfalls along the way to consider, and build around, in your design.... Some people won't care about the efficiency, figuring "air is free", and I can understand that.... Others, such as those hand pumping, will have a completely different attitude to keeping the valve open after Elvis has left the building....  ;)

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #29 on: February 14, 2019, 05:27:54 PM »
Thanks for that, I suppose I was visualizing it incorrectly - mainly not considering that the pressure rises behind the pellet at a much faster rate than one would think because of the limited volume and the relative speed at which the gas propagates.

Quote
Lloyd's Internal PCP Ballistics spreadsheet

Is this available for download?

I'm not trying to convince you that you can't build a dump valve that will work and provide good performance, far from it.... I'm just trying to point out some of the pitfalls along the way to consider, and build around, in your design.... Some people won't care about the efficiency, figuring "air is free", and I can understand that.... Others, such as those hand pumping, will have a completely different attitude to keeping the valve open after Elvis has left the building....  ;)

Just to add a bit of background, my experience in tinkering is mainly with dump valves.  I've owned several PCPs throughout the years and did little in the way of optimization, keeping the commercially made airguns more or less separate from my other experiments.  I have to say the past months looking further into hammer valves have been quite eye-opening and your data has been very helpful, I had definitely been harboring some misconceptions in this area.  In this case though, efficiency is not a primary concern and indeed might prevent the blowback function I have in mind from working, there must be excess energy in order to eject the cartridge and give enough energy to the bolt to chamber a new round.

If you want to be horrified at the depths of inefficiency I have plunged to in the past, here's a design that just dumped a constant flow of air into the chamber, relying on a tight seal that the breech to act as a detent allowing pressure to build up in the cartridge:



I made an extremely primitive prototype (this was before I had a lathe and mill) to try out the concept and it worked pretty well:



It took 850 psi to work however and with all that it could barely put a 0.177" BB through a soup tin, but at the end of the day I was more interested in the mechanism cycling that making something efficient.







 
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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #30 on: February 14, 2019, 06:27:46 PM »
That is a really cool video.... and I certainly understand the joy of making an idea come to fruition....  8)

Lloyd's spreadsheet is proprietary, and he has only given out a couple of copies of it.... I was one of the fortunate few, and of course cannot share it further.... Sorry....

I used to think there could be choking of the flow at both ends of the valve cycle, both opening and closing, when the poppet was close to the seat.... However once you realize that the airflow cannot exceed the pellet velocity.... and the molecular speed is much higher than that when the valve first opens.... it becomes obvious that choking of the flow, or indeed simple resistance to the flow, once the valve is open just a few thou, cannot be occurring on valve opening.... The delay is simply that of the random motion of the molecules becoming organized (from not bouncing off the poppet but passing through the gap between it and the seat). and a fraction of a mSec. before the barrel pressure reaches nearly the reservoir pressure.... during which time the pellet position has not changed much....

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #31 on: February 15, 2019, 08:17:58 AM »
Lloyd's spreadsheet is proprietary, and he has only given out a couple of copies of it.... I was one of the fortunate few, and of course cannot share it further.... Sorry....

That's fair enough, especially if it's part of his bread and butter.

I did however plug some numbers into GGDT and got similar results.

0.25" airgun with a chamber volume 10x the barrel volume at 3000 psi, firing a 31 grain pellet through an 18 inch barrel.  I assumed valve opening time and transfer port volume to be zero, with the valve closing when the pellet is approximately half-way down the barrel.



With a chamber 0.5x the barrel volume and dump valve there is a 12% drop in muzzle energy:



In order to get the same energy, we either need to up the pressure by 22% to 3650 psi...:



... or increase the chamber volume by 130% to 1.15x the barrel volume:





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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #32 on: February 15, 2019, 03:37:39 PM »
Interesting internal ballistics model.... I wonder why the barrel pressure at valve close is so low compared to Lloyd's?.... Perhaps because Lloyd calculates the pressure and then after that applies a correction (efficiency) factor, which takes into account things like the mass of air being accelerated.... whereas Hall applies those corrections before he plots the pressure curve?....  However, the results are similar, so it doesn't really matter....  ;)

Interesting to note that if you use 3650 psi, and the same 1/2 barrel volume, you are using 22% more "std. CI" of air.... and if you use 3000 psi but 1.15 barrel volumes, you are using 130% more air.... both to get the same FPE.... Pretty much as I would expect....  8)

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #33 on: February 15, 2019, 06:57:11 PM »
Hall had made the point that GGDT goes a little funny at extremes, certainly the pressure curve looks a little odd for the timed valve, and when I added a bit of dead volume the FPE actually rose a little.

Here are some of the variables GGDT is supposed to take into account:

Quote
- Valve configuration and opening times. In fact, GGDT models four different types of valve: chamber sealing pilot, barrel sealing pilot, burst disc, and "generic." Each of these valves have different behaviors and GGDT accounts for these behaviors (more on that below).
- Pressure drop across the valve oriface.
- Temperature (and thus pressure) increase in the valve pilot due to work performed by gun gases on the valve piston/diaphragm.
- Gas leakage from the main valve body into the upper valve chamber (pilot).
- Performance differences due to different gases.
- Temperature effects on gas properties (and thus, performance).
- Performance limitations due to flow choking in the valve or the barrel.
- Valve effective oriface increases due to lowered valve throat Mach number.
- Temperature (and thus pressure) drop in the barrel due to work performed by the gas accelerating the projectile.
- Gas leakage around the projectile in the barrel.
- Compressibility (Mach) effects on air pressure both in front of and behind the projectile to include the creation of shocks.

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Offline rsterne

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #34 on: February 15, 2019, 07:59:06 PM »
Yes, I saw that list, but of course it doesn't say how he accomplishes that.... Lloyd's spreadsheet also has a long list of "losses" that he allows for, some of which you can toggle on an off, with quite amusing "what if" results....

My guess is that some of the GGDT spreadsheet does not apply well to our very "small" caliber spud guns.... but then that is not what it was designed for.... In some of our highest powered PCPs, the air has more mass than the bullet....  :o

Bob
  • Coalmont, BC, Canada
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), .22 QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Bench PCP, 6mm Regulated PCP and .257 Unregulated, Three BRods.

Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #35 on: February 17, 2019, 04:30:48 PM »
My guess is that some of the GGDT spreadsheet does not apply well to our very "small" caliber spud guns.... but then that is not what it was designed for.... In some of our highest powered PCPs, the air has more mass than the bullet....  :o

No doubt, GGDT was written with a much more general scope.

I finally did some work today on this cartridge malarkey and it worked out pretty well!

Here is a youtube video with some testing, even without a barrel it packs a punch...



Loud too, hearing protection was a must.

Here is the cartridge disassembled:



Everything held up well except the firing pin that took a beating and started to jam, it's the reason the piston can be seen to pop out in the last two clips in the youtube video.  Needs to be made out of steel rather than brass.



My crude fill rig consisting of a probe that is placed in the mouth of the cartridge with the assembly then being clamped in a vice for filling.  It took 9 pumps from my cheap-o chinese pump to reach 1500 psi.

Filling and firing cycle animated exactly as built, scale is in centimeters:







« Last Edit: February 17, 2019, 05:31:21 PM by jackssmirkingrevenge »
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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #36 on: February 17, 2019, 10:21:55 PM »
Very Cool....  8)

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Offline MJP

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #37 on: February 18, 2019, 04:07:54 AM »
Using steel rod for the valve stem will give you some more volume.
Looking good, interestin approach using ballbearings for locking the valve.

MAC cartridge was doomed from the start, they made a cool concept but the cartridge valve was very poorly designed, very inefficient, two small holes to feed the barrel, when it could have dumped like the Lane cardridge from the case mouth.

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Offline jackssmirkingrevenge

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #38 on: February 18, 2019, 06:51:45 AM »
Thanks guys.

Quote
interestin approach using ballbearings for locking the valve.

Not an original idea to be sure, almost all airsoft "40mm grenades" use this system.  This example reverses the dynamic, with a central chamber and multiple "barrels" around it:



The disadvantage of this configuration is that it lacks a balancing element so is only suitable for low pressures otherwise it is impossible to open.

Quote
Using steel rod for the valve stem will give you some more volume.

It can certainly be optimized, I'm thinking the piston made in two parts linked with a threaded rod.  This is more of a proof-of-concept to answer some doubts that I had:

1) The effort needed to actuate the valve.  The difference between spool seal diameters is 8.5mm vs 9.0mm, so the net pressure imbalance at 2000 psi is only 21 lbs, keeping the friction from the ball lock to a minimum and making it very easy to actuate.  This should be well within the capabilities of a direct blowback mechanism.

2) Using a floating o-ring to fill the cartridge.  This seems to work well, and it avoids the extra complication of installing a separate fill valve.

3) O-ring retention on firing.  Having the front o-ring blow out with the flow was a concern but it did not happen at all during testing.  My technique here to mitigate the potential issue is to cut a smaller o-ring groove and use a slightly undersized o-ring that is stretched in place.

Quote
MAC cartridge was doomed from the start, they made a cool concept but the cartridge valve was very poorly designed, very inefficient, two small holes to feed the barrel, when it could have dumped like the Lane cardridge from the case mouth.

I suppose it was a compromise in terms of ease of manufacture which is an important consideration for a unit that you have to make multiple examples of at relatively low cost.  The spool is also relatively small diameter which reduces friction from the o-rings which means that you don't need a very strong firing pin, but it comes at the cost of air flow.


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Offline SilentMatt

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Re: Air cartridge prototype - ".338 Lupra Magnum"
« Reply #39 on: February 18, 2019, 10:42:06 AM »
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