Any Interest in 6mm ? - Build Thread

[rsterne]

Yes, Paul, that is the tank I bought.... 500 cc, 4500 psi, 18 mm x 1.5 mm threads, 60 mm diameter (actually a bit over where the label is, call it 2.4"), 21 oz. in weight, and the length is 11.5" without regulator....

Bob

[lloyd-ss]

I swapped the 6 mm upper onto the unregulated gun tonight and filled it to its MSWP of 4200 psi.... I set the SSG to just a whisker of gap and fired one shot to settle things, topped it up and fired two shots with the 73.4 gr. bullets.... 1028 & 1029 fps (173 FPE).... Then I did something I have just been dying to try ever since I came up with the idea of building a 6mm.... I loaded up a 1.8 gr. Airsoft BB, topped it up to 4200 psi and took a bead through the Chrony.... 2093 fps.... repeated it.... 2097 fps.... once more.... 2087 fps.... That gives a three shot average of 2092 fps (17.5 FPE) with a basic PCP, just shooting very light ammo.... No special equipment, just a standard SS style valve, porting that turns through 180 deg., and a rifled 29" barrel that is a poor fit on the BB.... it will actually roll out the muzzle if you hold it pointing downwards.... Running air at room temperature (the pressure in the 300 cc reservoir dropped ~ 250 psi each shot), not Helium.... Lloyd still holds the record at 2162 fps.... but he better be looking over his shoulder.... 

Bob

Bob,
I am looking at this again and the wheels are turning and questions are coming out, LOL.

Full bore porting?
How much clearance on the bb? Maybe a thou or 2?
250 psi drop for the shot. Do you think the valve was open for the full barrel length?
Really loud? Did you have the chrony clamped down, ha, ha?
What secrets are you holding back, LOL?

[rsterne]

Yes, full bore-area porting, with retractable bolt.... The diameter of the BB is about 0.002" less than the lands, and about 0.010" less than the grooves.... The valve was definitely open until long after the BB left the muzzle.... The best model I had using your J4 spreadsheet for the 73.4 gr. bullet which I also tested at 4200 psi was a dwell of 0.0036 sec @ 67%.... just shy of a dump shot.... 1029 fps with ~250 psi drop in pressure = 0.55 FPE/CI.... Since I was using my SS valve, when I used the 1.8 gr. BB the valve may not have had the same dwell, even though it did have the same hammer strike.... the dwell may have been a bit less, or more?....

I can't swear if the pressure drop was still 250 psi when I used the 1.8 gr. BB, and I was only using the small 1" gauge on the gun anyways.... maybe it was closer to 300 psi?.... I admit to being more interested in the velocity and how consistent the 3 shots were.... No, the Chrony wasn't clamped down, but I didn't blow the diffusers and lights off it.... the key is that it was so consistent I have no reason to doubt the results....

No secrets, WYSIWYG.... 

Bob

[lloyd-ss]

Bob,
Given the fact that you were using a 29" barrel and I was using a 46" barrel, if we extrapolate the velocity in your gun to 48", the velocity is about 2305fps, quite a bit faster than my 2162fps. But it also seems that the difference in projectile weight, 1.8gn vs 7.7gn could account for that difference in velocity. In other words, well done Bob.

It appears that your 1.8gn BB might actually be leading the front edge of the expanding air in the barrel, almost like surfing.
All of these velocities are well above Mach 1, so what about choked flow? How is it limiting the velocity... or is it limiting the velocity?

After some serious reading, I believe that, just as the 1650fps  velocity limit was the result of the misapplication of a physics principle, we have been misapplying the idea of choked flow inside an airgun. Please correct me if I am wrong, BUT: We have been saying that if the velocity through the most restricted passage within the flow path exceeded Mach 1, the flow would become choked.  Actually, that is false.  What really causes choked flow is the pressure drop across the restricted section, and a pressure drop of almost 50% is required to cause choked flow. I believe that the only time there will be a pressure drop that great is the instant (a few micro seconds) the valve poppet lifts off the seat, and the final instant as the poppet closes.

Here is my reasoning. Before the shot, the bullet is wedged into the breech end of the barrel just like a cork. The bullet has mass that must be accelerated out of the barrel. As soon as the valve opens, the air blasts past the valve  poppet and starts to stack up behind the bullet. As soon as the air starts to stack up, the pressure differential within the system becomes very low (less than 10%, maybe?), and certainly nowhere near the 50% needed to cause choked flow. As the bullet starts to move, it is still acting like a cork for the expanding air, and the pressure differential is still very low because the air is building up faster than the bullet is accelerating.  That magical 50% pressure drop never occurs within the system, and therefore choked flow does not occur, even though the velocity of the bullet and a major portion of the air column greatly exceed Mach 1.

Consider this as additional proof.  Given a gun operating at 3000psi with .25 cal and .25" passages, a 20% pressure drop across the valve could produce a flow rate of 2000scfm, which is 57,600 standard cuin/sec, or 57cuin/millisec. Considering the fact that most of our guns will produce about 1 fpe/cuin, and the valve open time might vary in the 1.5 to 5 millisecond range, that 20% pressure drop across the valve is probably quite adequate for a .25 cal system. Now, if the pressure drop goes to 40% (as with a very light projectile) the flow rate increases to about 74 cuin/millisec. So even though the pressure drop doubled, the mass flow did not. But the real point is that we are almost always, IMO, operating safely below the that 50% pressure drop that would initiate choked flow. Therefore, no choked flow!

Is this what is really happening? It certainly makes a lot of sense and explains what our empirical data demonstrates.
Lloyd

[rsterne]

WOW, JUST WOW !!!

just as the 1650fps  velocity limit was the result of the misapplication of a physics principle, we have been misapplying the idea of choked flow inside an airgun.

Lloyd, I think you may have a SOLID REASON for the lack of choked flow.... insufficient pressure differential.... This may be a chicken-and-egg problem and we have been looking at it from the wrong end of the chicken !!!

We have been saying that if the velocity through the most restricted passage within the flow path exceeded Mach 1, the flow would become choked.  Actually, that is false.  What really causes choked flow is the pressure drop across the restricted section, and a pressure drop of almost 50% is required to cause choked flow.

We have been saying "well the flow is supersonic, so it must be choked, which causes a 50% pressure loss".... when in fact we SHOULD have been saying "nowhere in the system (with one exception below) is the pressure differential approaching 50%, so even though the flow is supersonic, it is NOT choked".... BRILLIANT, MY FRIEND.... SIMPLY BRILLIANT....

The exception I mentioned above is when the poppet is CLOSING only, just before the poppet touches the seat.... At that point the flow rate is high (as is the velocity of the pellet and air column), but the area under the poppet is decreasing, and the pressure differential increasing.... When it reaches 50%, the flow chokes for a few microseconds before the valve shuts.... At the other end of the shot cycle, when the valve is opening, there is no comparable choking because all the air is doing is filling the dead space between valve seat and pellet (which is stationary), so there is no real flow volume or air velocity, beyond the random vibration of the air molecules, so no choking....

I was playing with your two spreadsheets last night (the unchoked and choked versions), and they both underpredict the velocity of my shot, with the choked version predicting a lower value.... I think you are correct that in the case of this ultralight BB, it is "surfing" the front of the moving wave of air, which is expanding away from the valve instead of being contained inside the barrel by the mass, inertia, and friction of the projectile.... There is no question in my mind that the difference between my 1.8 gr. BB and your 7.7 gr. "pellet" is the big reason for the ease with which I broke 2000 fps.... Let's face it, it doesn't get any easier than what I did.... 

Bob

[lloyd-ss]

I think I have to revisit the math behind my internal ballistics spreadsheet now, LOL.  The choked flow part in the spreadsheet is definitely wrong, but I think the VanDerWaals pressure/density enhancements need to be incorporated.  I was noticing, like you did, that the eff in the spreadsheet had to be above 1 for the numbers to work.

The lively discussion that you and subscriberand MJP were having on Travis's wing-ring thread really pushed me into trying to make sense of this air flow. Stuff just seemed intuitively wrong, and that always bothers the heck out of me.  So even though I was mostly just lurking on that thread, POW, it turned the light bulb on!

This forum is great!

[oldpro]

 This is very interesting!!! So how do we test this theory? Do we put the parameters to be tested into a spreadsheet and adjust the constriction of the TP or air column and measure speed and air used or Not sure how to go about proving or dis proving.

[lloyd-ss]

This is very interesting!!! So how do we test this theory? Do we put the parameters to be tested into a spreadsheet and adjust the constriction of the TP or air column and measure speed and air used or Not sure how to go about proving or dis proving.

I think it is all in the math, and that we just weren't applying the physical laws correctly. I don't think physical testing is required to prove or disprove.

Take a look at this lively discussion, which is where my research started.

https://www.physicsforums.com/threads/velocity-of-air-coming-out-of-a-nozzle.694656/

If you branch out from this Physics Forum thread, all the information supports what I stated.
Lloyd

[rsterne]

Travis, we pretty much already have the proof.... Lloyd and I have been trying to incorporate choked flow into his spreadsheet for a few years, and every attempt has failed and we didn't know why.... NOW we have the answer.... It is very much a case of the facts didn't fit the theory, so it was time for a new theory, or at least a fresh LOOK at what we thought the theory was saying.... exactly like the "1650 fps limit" which Lloyd blew out of the water, and we spend the better part of a year trying to figure out why.... The two effects are somewhat related, with the "cork in the bottle" effect of the pellet keeping the pressure high in the barrel during the time the valve is open (avoiding choking), while the volume under pressure increases as it is "topped up" from the reservoir at the breech end (pushing the air already in the barrel along in "packets", with the molecules in each packet vibrating at 1650 fps) and so allowing the muzzle velocity to exceed that proposed 1650 fps limit....

This, for me, was a true "lightbulb moment".... and thanks to Lloyd I now have a much better and more complete understanding of what happens inside a PCP.... and why we can do things some said were impossible.... I agree with Lloyd that we need to drop the idea of choked flow from the spreadsheet, but we do need to incorporate the VanDerWaals effect on the mass of air used per shot, instead of just using the Boyle's pressure/volume relationship.... which breaks down over 2500-3000 psi.... It won't make any significant difference to 90% of our calculations.... but it will show up for those few high-power, high-pressure applications.... At least we won't need to use the Speed of Sound calculations in your spreadsheet any longer.... 

BTW, Lloyd, I saw a tank fill calculator the other day which has the VDW correction in it.... it uses the air MASS leaving the source tank to top up the air MASS in the receiving tank/reservoir.... It doesn't tell you it does that, but the results indicate that to be the case....

Bob

[oldpro]

Travis, we pretty much already have the proof.... Lloyd and I have been trying to incorporate choked flow into his spreadsheet for a few years, and every attempt has failed and we didn't know why.... NOW we have the answer.... It is very much a case of the facts didn't fit the theory, so it was time for a new theory, or at least a fresh LOOK at what we thought the theory was saying.... exactly like the "1650 fps limit" which Lloyd blew out of the water, and we spend the better part of a year trying to figure out why.... The two effects are somewhat related, with the "cork in the bottle" effect of the pellet keeping the pressure high in the barrel during the time the valve is open (avoiding choking), while the volume under pressure increases as it is "topped up" from the reservoir at the breech end (pushing the air already in the barrel along in "packets", with the molecules in each packet vibrating at 1650 fps) and so allowing the muzzle velocity to exceed that proposed 1650 fps limit....

This, for me, was a true "lightbulb moment".... and thanks to Lloyd I now have a much better and more complete understanding of what happens inside a PCP.... and why we can do things some said were impossible.... I agree with Lloyd that we need to drop the idea of choked flow from the spreadsheet, but we do need to incorporate the VanDerWaals effect on the mass of air used per shot, instead of just using the Boyle's pressure/volume relationship.... which breaks down over 2500-3000 psi.... It won't make any significant difference to 90% of our calculations.... but it will show up for those few high-power, high-pressure applications.... At least we won't need to use the Speed of Sound calculations in your spreadsheet any longer.... 

BTW, Lloyd, I saw a tank fill calculator the other day which has the VDW correction in it.... it uses the air MASS leaving the source tank to top up the air MASS in the receiving tank/reservoir.... It doesn't tell you it does that, but the results indicate that to be the case....

Bob

Bob I think you guys are right now I have to go retest some flow dynamics in my CFD program Ill be right back.

[oldpro]

Heres a link to the opensource CFD I play with If you want to Geek out with me LOL.
https://www.openfoam.com/download/install-binary-windows.php
 this will help also

[rsterne]

I don't know of any CFD program that allows you to "put a cork in the barrel" and keep adding air from one end while partially restricting it at the other.... but then I haven't played with them much either....

Bob

[AmBraCol]

Yes, Paul, that is the tank I bought.... 500 cc, 4500 psi, 18 mm x 1.5 mm threads, 60 mm diameter (actually a bit over where the label is, call it 2.4"), 21 oz. in weight, and the length is 11.5" without regulator....

Bob

Thanks, Bob!  I've still not managed to pull the valve or whatever it is in the end of the tank so am not sure on the threads.  The rest?  Looks VERY interesting!  The factory bottle is about 12 inches long, it's just a hair over 60 mm diameter and weighs 31.2 oz. The rifle is rated at 250 bar.  So now I'm REALLY cogitating.     Shaving 10 oz off the rifle would be welcome indeed.  OK.  I'll quit hijacking your thread.  The info is GREATLY appreciated!!! 

[oldpro]

I don't know of any CFD program that allows you to "put a cork in the barrel" and keep adding air from one end while partially restricting it at the other.... but then I haven't played with them much either....

Bob

Thats exactly what I went to the garage to try with OpenFoam. I sent in a Email to the developers to help me out I couldnt figure it out on my own But I dont give up that easy so as soon as I get some help I will give it a go

[lloyd-ss]

The choked flow discussion that I was pushing seems to be hijacking Bob's thread a bit too much, so I have started its own, totally fun, totally geek, topic. If you just shake your head an look away from the new topic, I understand, it's kinda sick. 

Choked flow discussion new location
https://www.gatewaytoairguns.org/GTA/index.php?topic=142095.new#new

[rsterne]

I wanted to add a note to this thread, to give you an idea of what I have in mind for future development of these two PCPs.... If you have followed along, I have two complete lower assemblies, one regulated with a 150 cc plenum at about 3000 psi, with air supplied from a 500 cc, 300 bar bottle.... and the other unregulated with a 300 cc reservoir with a 4200 psi MSWP.... It would normally be tuned to be tethered at about 3800 psi, with a few shots off bottle....

I have two interchangeable uppers for these guns, with single shot actions which accept a 14mm OD barrel with a 20mm carbon sleeve, one in 6mm (0.243) and the other in .257 cal. and both 28" long with a 10" twist.... I am in contact with Mike at TJ's about the possibility of getting two more 28" barrels that I can fit to these uppers.... One will be his 0.172" barrel in the 9" twist, to shoot the 26 gr. NOE bullet, like Cedric is using.... The other will be a .224 cal in a fast twist, either 8 or 10", to shoot the longer FB bullets and hopefully my BBTs, in the 55-75 gr. range.... It is likely the .17 cal will go on the regulated lower, and the .22 cal on the unregulated, but since the receivers are interchangeable, they could fit either....

I don't know when this project will see some action.... but I just wanted you to know I am exploring the possibility of using smaller calibers on these two PCPs.... Whether this proceeds at all depends on what Mike can supply that can be fitted into my existing receivers.... as I really don't want to make two new receivers for the new calibers.... Any serious testing of the existing 6mm and .257.... and of course anything new.... will still have to wait until we close the Motel in 2021.... but it would be nice to have them ready for testing that summer.... 

Bob

[skorec]

Bob, I am really happy that you  also planning to taste lower calibers.  Which is your grouping with .243 and .257 slugs at 100 yd with 10” twist ?
Do you have some ( 1-10 %) flyers  or no any  ?


You  probable have notice that I am  awaiting for  FX STX .177 slug liner "P"  usable for 21gr.  Piledriver.
I believe them a lot.

[rsterne]

Peter, I haven't had a chance to even shoot either gun outside.... The Motel prevents me from having time to shoot in the summer, and we have snow on the ground all winter.... Testing will likely have to wait until 2021 when we close the Motel....

Bob