Any Interest in 6mm ? - Build Thread

[MJP]

You'd be surprised how hard bigger balanced valve slams wide open. I have seen gouges on aluminum valve body for it hitting the balancer cylinder sleeve.

Marko

[YEMX]

Not trying to thread-jack, but I've changed my plans slightly for my 6mm build Bob.

I'm going to work with Jim at WAR to build a 6mm Flex.  I've finally come to the realization that going this route will produce a better airgun than I can build.  Plus I would have had to farm out most of the machining work anyway.  I'll keep you posted Bob, just not on this thread...  I want to keep this open for just your 6mm build.

Awesome work on your build so far Bob!!  Sad about your valve...  But you'll be back up and shooting soon, I have no doubt!! 

[rsterne]

Travis, that photo of the failed retainer certainly proves that the force on the thimble is towards the valve seat.... *LOL*....

Bob

[oldpro]

Travis, that photo of the failed retainer certainly proves that the force on the thimble is towards the valve seat.... *LOL*....

Bob

Yes Sir the force has to go somewhere you dont get something for free right, every action has and equal and opposite reaction and that energy is throwing the thimble towards the seat.

[rsterne]

I had trouble wrapping my brain around the forces on the thimble, but I finally understood it (I think) by looking at what happens in the closed position, and considering the poppet as a fixed portion of the valve, and what the forces would be on the thimble if it were the part free floating.... Since the air pressure inside it is atmospheric, but all other areas of the valve are HPA, there is a net force on the thimble towards the seat of pressure times area.... the area is the area of the ID of the large part of the thimble minus the area of the small part.... In the case of my version of the ART/SS valve, that is a 3/8" bore (0.110 sq.in) minus a 1/4" bore (0.049 sq.in).... a total effective area of 0.061 sq.in.... At 4200 psi, that would be (0.061 x 4200) = 256 lbs.... quite a load on the tiny webs of aluminum I left in that wheel.... Well, time to hit the shop and make a stronger one....

Bob

[lloyd-ss]

It seems that most of these "opportunities for improvement" never show up until you increase the pressure, or increase the power, or increase the caliber. Basically, they don't don't show up until we start getting greedy. But that is part of the fun, and it makes us stop, and think, and learn.  Nice observations.

[oldpro]

 Could you imagine a world without failure! How boring would that be!

[lloyd-ss]

Could you imagine a world without failure! How boring would that be!

Absolutely! You would never learn anything new! Or dream up a solution to problem. A day without learning something new, is a day wasted. Even if its just that M&Ms have some new colors.

[rsterne]

SUCCESS !!!

I made a new wheel to mount the front of the valve thimble, with eight 5/32" holes (a total area twice the valve throat), and with the center twice as thick and the webs between the hole more than twice as long, and twice as many of them.... I don't think I will have any more problems.... I put the gun back together and filled it, and my Great White is now down to about 2900 psi.... I installed the 6mm upper instead of the .257, using the same transfer port, and it sealed great.... I sized some of the 61.3 gr. Bowman bullets (a Lyman 257420, scaled down to 6 mm) to 0.243", and replaced the hammer spring with the weakest 3" long spring I have, one of 0.055" wire with a rate of 11 lb/in.... I set the preload to zero, the SSG gap to 1 turn, and shot the first 6 mm bullet.... It ripped through the Chrony at 1007 fps (138 FPE).... 

The gun was pretty loud, so I increased the SSG gap 2 turns as a time, recording the velocity as I went.... It didn't start to drop significantly until I was 7 turns out, and then it was still 990 fps.... I changed the gap only 1 turn at a time after that, recording the velocity until 14 turns out from zero gap, at which point it did not fire.... Turning it back in just one turn, the gun shot at 800 fps.... So, I had an adjustment range of about 200 fps before the valve fell off the cliff.... I now have a digital pressure gauge, so I put that in between my tank and the gun, so that I could measure the pressure drop per shot to determine the efficiency.... I started at 12 turns of gap (0.428"), filled the gun to the pressure remaining in the tank (2840 psi), closed the valve, let the pressure settle, and then fired one shot, let the pressure stabilze again and recorded the difference in pressure, and the velocity.... I reduced the gap a turn, and repeated the procedure, recording the velocity and pressure drop at each setting until I was back to 1 turn of gap.... Here are the results....



I can't believe how easy the digital gauge makes it to accurately measure the pressure drop for just one shot.... The hardest thing is to try and get the reading at the same temperature each time.... The gauge is soooooooo sensitive that after a shot (which cools the reservoir from expansion of the air during the shot), that you can watch the gauge rise slowly as the reservoir returns to room temperature.... The best way is after you close the tank valve, before the shot, wait a minute or so for the temperature to stabilize, record the starting pressure, take the shot, and then after the shot wait until the gauge has stopped going UP and record that as the end pressure.... and then subtract to get the drop.... From there, my spreadsheet calculates the FPE and FPE/CI, with the only two inputs needed the velocity and pressure drop....

The plateau, at 2800 psi (which is what I had left at the end of the testing), is basically 1000 fps, and the useful knee of the tuning curve runs from 7-12 turns of gap.... With this 61.3 gr. bullet, it looks like about 9 turns of gap (about 5/16") is the sweet spot, giving 960 fps (126 FPE) at an efficiency of 1.24 FPE/CI.... The pressure only drops 80 psi per shot on the 300 cc reservoir, so it will be about twice that on the 150 cc plenum of the regulated version.... That is only about a 6% drop, so I should be able to regulate at the 2800 psi I ran these tests at, at least with this bullet, maybe 100 psi less with the 58.3 gr HP version.... More testing will produce similar information for the other bullet weights I have, but first I have to refill my Great White.... I have a 3000 psi regulator I can tether to, it will be interesting to test the 73 gr. bullet at that pressure.... I will need to get a weaker hammer spring in order to reduce the SSG gap to a more reasonable 1/8" or so.... The cocking force the way the gun is tuned now is less than 8 lbs.... MAN, do these ART/SS valve ever work to reduce that, I'm VERY impressed, and happy with my first effort building one.... and only 2 failures along the way.... 

Bob

M&Ms have new colours?.... Count me IN !!!

[lloyd-ss]

Bob,
Nice work on getting the broken part re-designed and built.
The gun looks nice and tunable, with good efficiency. Your graphing methods make finding the sweet spot pretty obvious and it sounds like the digital gauge makes that easier and more accurate.
This project is producing a versatile airgun package.

[oldpro]

 With such light cocking one could easily vent shroud back pressure and make a semi auto(not in Canada). Just a few design changes. Bravo Bob you really out did yourself with this build.

[rsterne]

I put the numbers into the spreadsheet, and for the 960 fps (126 FPE) tune at 1.24 FPE/CI I have to put 87% in (M3) to balance it.... Dwell time is 0.0022 sec. and the valve closes when the bullet is 35% of the way down the bore.... I haven't actually kept track of that for my other guns, and of course the exponents for adiabatic or isothermal expansion, and when you use them, change it a lot.... but I don't remember having anything that high before.... I am using my standard conditions, isothermal when valve open and adiabatic when closed.... but now that I can observe the reservoir pressure (temperature) recovering after I shoot, I may change my opinion about that.... What do you think, Lloyd?.... Should we be using adiabatic all the time?.... Hmmmmmmm.... just answered my own question, I tried it, and had to use 1.05 in cell (M3), and 105% is impossible, of course.... 

In terms of my "lofty goal", this gun has full bore area porting, it is .243 cal, 29" barrel, and currently at 2800 psi, so the prediction is....

(0.243 x 0.243 x PI/4) x 2800 x (29 / 12 / 2) = 130 lbs. x 1.21 ft. = 157 FPE....

At the plateau I am getting 136 FPE with a 61.3 gr. bullet at 1000 fps.... It will be interesting to see what I get with the 73 gr. bullet....

Bob

[subscriber]

136 / 157 = 87% of the theoretically maximum FPE.  That is pretty darn good, Bob!  Especially, considering you are not wasting air.

[skorec]

Bob

Amazing power, efficiency and also   accuracy  - I am sure  ,  because the barrel  is  stiffening free floating and harmonic adjusted .
The design of guns is   veeeery  practical however higher scope rings will be needed.

[rsterne]

Well the 157 FPE is NOT the theoretical maximum FPE, that would be twice that.... My "lofty goal" numbers are already derated by 50% to allow for the weight of the air accelerated (the largest single loss, 31 gr. of air per shot at the current tune, 1/2 the bullet weight), drag on the bullet, and the fact there is a pressure drop during the shot because the reservoir is not infinite and there is wasted volume before the bullet in the porting system.... Using 50% s still a VERY hard goal to attain, if you are using a bullet that weighs 1/2 the FPE (ie at 950 fps).... although you can reach or exceed that number with a very heavy bullet.... In a dump shot (valve closes at the muzzle) there would be 77 gr. of air in the barrel when the bullet exits.... so you can see why a heavy bullet helps you achieve 50%.... while light bullets don't have much of a chance to get there....

Actually, medium height scope rings still have tons of clearance with the 44mm AO scope I have, and even more with the side focus one.... I don't think a 50mm objective would be a problem, even with the scope tilted down 22 MOA relative to the barrel....

Bob

[rsterne]

I had a slight leak, and the reservoir was empty this morning, so while I was filling my Great White this morning I pulled the gun apart and replaced all the O-rings.... Afterwards, I found there was still a slight leak through the valve.... Soapy water on a fingertip pressed to the end of the barrel (with the bolt closed) was met with a steady stream of small bubbles.... I don't know if it is the O-rings in the valve, or the poppet not seating perfectly, but after my tank was filled and I tethered it to my 3850 psi output regulator and fired it a few times the leak is almost gone.... I think with time and use, it may seal up OK.... I sized some of the 82.5 gr. bullets to 0.257", set the SSG gap to 1 turn and fired the first shot at this pressure.... 1010 fps for 187 FPE !!!

I followed the same procedure as yesterday with the 6 mm, gradually increasing the SSG gap until the valve quit opening, and recording the velocity at each setting.... Since the regulator was topping up the reservoir after each shot, I was unable to record the pressure drop to figure out the efficiency.... However, I did establish the tuning curve for the .257 when tethered at 3850 psi....



As you can see, with less than 5 turns of gap, the gun is operating on the plateau, just above 1000 fps.... The usable part of the knee is from 7-11 turns of SSG gap.... Surprisingly, the sweet spot was at 9 turns of gap, exactly the same as the 6 mm at 2800 psi.... I would have thought that the hammer strike required would have increased a lot with the 1000 psi increase in pressure (certainly in a conventional valve that is the case).... but it didn't, so one more thing I have learned about (my version) of the SS valve.... I then set the gap back to just 1 turn, to put the gun back to maximum power, and tried all the different bullet weights I have in .257 cal.... Here are those results at 3850 psi....



It was driving the 66 gr. 257420 HPs nearly at Mach 1 (VERY loud), and the 73 gr. 257420 solids at 1060 fps (182 FPE).... Bullets in the most likely weight range for this gun (82-93 gr.) were shooting 970-1010 fps (186-196 FPE), and the heaviest BBT I have, the 113 gr. FN, was able to reach 895 fps (201 FPE), with the 100 gr. BBT hitting 952 fps (201.3 FPE).... Drop about 4% off those velocities to allow for tuning down to where the efficiency is acceptable.... and I have a .257 shooting at 170-180 FPE.... This is about 20 FPE less than my Monocoque, which has a 5" longer barrel, at the same pressure.... My .257 Hayabusa, running the same barrel length but at only 3000 psi, peaked at 160 FPE and was tuned to shoot at 130-135 FPE.... If I want to tune this gun down to use the 257420s (FN or HP) all I need to do is regulate it to a lower pressure.... At the other end, I can drive my heaviest two BBTs fast enough to find out how they work in the 10" twist of this barrel.... so this gun will eventually prove invaluable for bullet testing.... exactly what I wanted.... With a MSWP of 4200 psi, I will be able to shoot the 113 gr. BBTs in the mid 900s....

I did one other test today.... After filling with the regulator to 3850 psi, I disconnected it from the tether and shot 3 shots, allowing the pressure to drop, just to see what happened.... I was back to using the 82.5 gr. bullets, and I had the SSG gap set back to 9 turns.... The velocities ran 952, 953, and 940 fps, averaging 165 FPE.... According to the small gauge on the side of the reservoir, the pressure dropped about 400 psi, which would indicate an efficiency of about 1.0 FPE/CI.... not a very accurate determination.... but shows me I am in the ballpark at that power....

Bob

[Rob M]

excellent numbers on such a small bore gun.. Do you think or know if the theoretical limits have been reached ? Ok , I think subscriber pointed out what I missed.

[subscriber]

Well the 157 FPE is NOT the theoretical maximum FPE, that would be twice that.... My "lofty goal" numbers are already derated by 50% to allow for the weight of the air accelerated...

Thanks.   That makes perfect sense.  I had not looked very hard at your calculations; so did not notice the 50% derating. 

There seem to be a number of "natural limits" along 50% efficiency.  Another would be the overall efficiency of diesel engines.  The best ones only just get to 50%.  These tend to be be large and very slow running; as used in ships and trains.

I have long thought of guns as "disposable piston engines"; so they would work along similar principles...

[rsterne]

Theoretical maximum FPE for .257 cal, 29" barrel, at 3850 psi is 482 FPE.... so my "lofty goal" would be half that, or 241 FPE, or more properly 950 fps with a 120 gr. bullet.... The bullet my gun shoots the closest to 950 fps is the 100 gr., which hit 201.3 FPE.... or 83.5% of the target.... Believe it or not, that isn't actually that good, the best PCPs hit about 90% of target when shooting a bullet at 950 fps.... If the barrel was 24", the "lofty goal" would have been 200 FPE.... It took me a 20% longer barrel to get there.... The 6 mm at 2800 psi came closer to its target FPE....

However, a 6.5 lb. .257 cal PCP that breaks 200 FPE is still a pretty pleasing accomplishment.... 

Bob

[subscriber]

Bob,

The power and air deficiency you are getting this early with your home designed and home made air rifle are very impressive; regardless of the numbers.

Now, to maximize power and efficiency even further, would you:
1. Go longer on the barrel; or
2. Keep the barrel length at about "100 calibers", but raise the operating pressure significantly?  I recognize that HPA at over 4500 PSI is a very scarce animal.
3. Or does starting at a higher pressure not only enable longer barrels, but requires it; assuming the goal is very high power and efficiency?

Higher pressure air would be denser, so even a short barrel full would weigh a lot.  However, the flow losses attributed to pushing air down the barrel would be reduced; or am I on the wrong track?   Then again, am I correct to assume that the "viscosity"of air goes up at higher pressure too; or is this a negligible factor?

Do you have any idea what the "pellet" friction losses are?  If you had an average pellet friction of 3 lb over 2 feet of barrel, that would be 6 ft.lbs, right?

Thanks