Any Interest in 6mm ? - Build Thread

[rsterne]

Barrel length and pressure are the routes to increased FPE, assuming the caliber is a constant.... I am already at 120 calibers in the 6 mm and 113 calibers length in the .257.... My Monocoque .257 had an even longer barrel, 33", or 128 calibers, and had about 10% more FPE, peaking at 227 FPE at 4000 psi with the 113 gr. BBT at 950 fps.... When I tuned it for a shallow bell-curve starting at 3790 psi, I got 4 shots of 921, 935, 933, 924 ending at 3320 psi with the 109 gr. HP version, averaging 209 FPE at 1.04 FPE/CI.... That tune would allow me to shoot tethered at 3800 psi for as long as the SCBA tank stayed above that pressure....

Although 4500 psi will allow a further increase in FPE, you are indeed faced with how to fill the gun at that pressure, or keep it at that pressure tethered.... You could do that with a 6000 psi welding tank, but at 300 lbs. they are hardly portable like an SCBA tank is.... I don't think it is practical to tune a PCP for over 3800-4000 psi at the present time.... One other problem is that as you go above 3000 psi, Boyle's Law no longer works properly, because the gasses are not longer "ideal".... You have to start worrying about the VanDerWaals correction factor.... Density increases slower with pressure increase than Boyle would predict over 3000 psi in air.... That is the reason that a 4500 psi SCBA tank only holds 88 CF of air, not the 98 CF that Boyle would predict.... While that doesn't affect the FPE, it DOES affect the shot count.... You get about 30% fewer shots from 4500 psi down to 3000 than you do from 3000 down to 1500 if you are shooting a PCP tethered at 1500 psi....

I think your 3 lbs. drag over a 2 ft. barrel being 6 FPE of loss is about right.... I checked it with Lloyd's Internal Ballistics Spreadsheet and it seems to work out that way.... Starting drag is higher, but because it is only applied for a short distance has much less affect on the total losses....

Efficiency is closely related to where the pellet is in the barrel when the valve closes.... Closing the valve after the pellet leaves the muzzle only wastes air, of course, with no velocity gain.... What is harder to understand is that closing the valve when the pellet is 50% of the way down the bore saves half the air, but only drops the velocity about 3-5% from the maximum.... After that, closing the valve earlier starts to drop the velocity much quicker, but saves even more air.... Likewise, with a dump valve (one that dumps all the air inside on firing), using a valve volume greater than half the barrel volume gains little performance for the extra air used.... The smaller the dump valve (or the earlier you close the valve on a conventional PCP), the higher the efficiency.... Really high FPE/CI occurs with short dwell combined with heavy bullets.... All these factors interrelate in a complex way, but that is the Coles Notes version....

Bob

[subscriber]

Thanks for the education, Bob,

It sounds like for everyday PCPs, 3000 PSI is a very good pressure to operate from. 

When it comes to regulated guns, I can see that you don't want the pressure set point too low.  That a small plenum at higher pressure may be better than a large one at low pressure.   

A large plenum would surely have a certain "lost volume":  Similar to "scavenging losses" due to the space above the piston in an air compressor.   

Certainly, because the peak force to drive the projectile is at the start, the regulated pressure needs to be well above that required to "break it loose, or some may stick in the breech. This may not be a factor with pellets, but with tight fitting slugs it might become a significant consideration.

Even if the whole plenum volume is "dumped", it may be easier to create the kind of pressure/distance curve you described for best efficiency, by means of a small plenum.   That is, unless one can engineer a very well behaved valve system - as you seem to have achieved.  Such a well behaved valve would seem to be imperative with an unregulated PCP.

Trying to understand the theory is one thing.  Implementing it in a design, and then making it real by actually constructing your own guns, is at a whole other level.  I have some experience with how much time and concentration it takes to machine parts on a manual lathe and mill (which is what I assume you are using).  So, it is a pleasure to watch you produce these masterpieces.  Even more so, because they work as you intended.

[rsterne]

Hmmmmmmm.... well there is a big difference between what we call a "plenum", which is a chamber holding the regulated air supply.... and a dump valve chamber, where all the air in the chamber is discharged on every shot, like what happens in a typical pumper.... If you are dumping all the air in the chamber/valve, then you don't want to go more than 1/2 the barrel volume, and smaller is a lot more efficient (but loses power).... Here is a chart that shows you what happens in that case.... Note that the 1/2 barrel volume rule of thumb applies regardless of pressure....



When you have a regulator between your main reservoir and the valve, since the regulator cannot even come close to topping up the valve in the 1-2 mSec. it is open.... you get a pressure drop in the valve during the shot cycle that is MUCH worse than before you installed the regulator, when the entire reservoir volume was available to keep the pressure up in the valve (assuming the valve inlet passages are larger than the outlet).... Here is an example of what happens as you change the plenum volume....



That chart is based on a .25 cal MRod that is shooting 45 FPE at an efficiency of 1 FPE/CI (which was typical a couple of years ago, before we all got on the efficiency bandwagon).... The red diamond shows what happens when instead of having the entire 215 cc reservoir available for the shot, you only have 45 cc.... where the plenum size is 1 cc per FPE.... You lose just over 2 FPE, or about 5%, because the smaller volume in the plenum causes a greater pressure drop at the valve seat.... The smaller you make the plenum, the greater the FPE loss, unless you increase the pressure to make up for it.... which in turn reduces the "headroom", or the difference between the fill pressure and the setpoint pressure.... and loses shot count.... Note in the chart that if the plenum was only 1/2 cc per FPE (23 cc), you have lost nearly 5 FPE or more than 10% of what you had at the same pressure.... You would have to increase the setpoint pressure 10% to make up for the loss in FPE....

As I said, that chart was before the big increase in efficiency we have seen in the past few years.... If the efficiency is higher, you use less air per FPE, so the pressure drop in the plenum is less.... That means that you can use a smaller plenum for the same power loss.... Many guns are modified by installing an "in-tube" regulator, where you lose reservoir volume from the space the regulator occupies, and any plenum you make (usually with a spacer between the regulator and valve).... In that type of installation, you have to come to a compromise on plenum volume, between making it bigger (with a lower setpoint but less reservoir) or making it smaller (with a higher setpoint with more reservoir).... There was a lot of discussion about this, and it seems that most have agreed that 1/2 cc per FPE is a good size to optimize shot count, even though it requires about a 10% higher setpoint than if the plenum was 1 cc per FPE....

When you have a separate bottle for a reservoir, as I will be using in the regulated 6 mm build, then your plenum volume is independent of the reservoir volume.... Therefore I designed this gun to use the 1 cc / FPE plenum size, because no matter what size I make it, I will still have 500 cc of 4500 psi air to feed it through the regulator.... In fact, the plenum will be about 150 cc, and the gun will only be shooting 120-140 FPE, so I am a little over what I "need" (although you can never be too big, IMO).... That is the advantage of using a reversed bottle under the main tube (which becomes the plenum).... finding space for a large enough plenum is no longer an issue.... and the bottle serves as the forestock....



Your comment about "scavenging losses" (if I understand you correctly) applies more to the "wasted volume" between the valve seat and the base of the pellet, which in Lloyd's spreadsheet we call the "Transfer Port Volume".... That volume should be kept as small as possible, because the air in the reservoir/plenum must expand to fill it before the pellet sees any pressure to start it on it's journey.... That process takes about 0.05 mSec if the distance is 1", because that is the average random molecular speed (1650 fps) of the air molecules.... If that "lost volume" is 5% of the plenum volume, you have lost 5% of the pressure before any force can be applied to the bullet.... On the other hand, you want the passages to be as large a diameter as possible to eliminate restrictions to the flow.... so another compromise....

I actually don't own a Mill, all I have is a milling attachment for my old Atlas 12 x 36 manual lathe.... The milling attachment has a very small vice (1" x 2.5") and a vertical travel of just over 3".... and the crossfeed on the carriage is just over 6".... so doing large parts is a challenge, and often requires accurate repositioning of the work.... However, with persistence, it is amazing just what can be accomplished if you have the time and patience....

One other thing.... you are 100% correct that a "well behaved valve" is critical for all PCPs, and is the key to power with efficiency....

Bob

[rsterne]

I spent the day today working on the forestock and BiPod mount for the unregulated gun.... The stock is a 10" long piece of 1-1/4" ABS plastic pipe.... It fits loosely over the main tube, and at the back fits under a notch in the receiver designed to clamp it in place against the top of the tube.... That leaves a gap at the bottom of just under 1/8".... At the front there is a curved, taperered shim inside the bottom of the tube, made from a piece of aluminum barstock.... It was quite complicated to machine, because the inside fits the 1.25" OD of the reservoir, and the outside fits the 1.36" ID of the plastic pipe.... and of course the two radii are offset.... I started with a short piece of 1.5"D bar stock, drilled a 1.25" hole 0.055" off center, then chucked it in the lathe and turned the outside down to fit inside the plastic pipe.... I ended up cutting over half the circle away, leaving a crescent shaped piece less than 1.25" wide by 1" long.... I took a short piece of Picatinny rail, milled a concave in the bottom to fit over the outside of the plastic pipe, then drilled and countersunk two holes in it for a pair of 6-32 SHCSs.... These went through two holes in the plastic pipe and were tapped into matching holes in the crescent, so the whole thing went together like a sandwich, with the plastic pipe the filling.... I had made the crescent just a bit too tight a fit, so a drum sander relieved it enough so that the complete forestock would slide over the main tube and back to fit into the notch in the front of the receiver.... Two setscrews tapped through the plastic pipe and crescent, one either side of the Picatinny rail, secure it against the reservoir, pulling it down tight against it, just like the receiver does at the rear.... It looks nice, and feel right, and is very solidly mounted.... I then attached the BiPod to the Picatinny rail, and mounted the scope my wife gave me for Christmas.... This is what the finished rifle looks like, complete with the Harmonic Tuner as well....



The scope is a BSA 4-14 x 44 Side Focus FFP on medium height Hawke mounts.... The gun as shown s 44.5" long, and weighs 9 lbs. 5 oz. and without the BiPod it is 8 lbs. 14 oz.... Without the Tuner, it would be 8 lbs. 6 oz.... and the bare rifle, as I said before, only 6 lbs. 9 oz....

Bob

[subscriber]

Thanks for taking the trouble to explain that, Bob.

The fact that you are using relatively primitive machines to make your airguns, makes this all the more impressive.

[Rob M]

looks incredible Bob..cant wait to see some long range results.

[subscriber]

Your comment about "scavenging losses" (if I understand you correctly) applies more to the "wasted volume" between the valve seat and the base of the pellet, which in Lloyd's spreadsheet we call the "Transfer Port Volume".... That volume should be kept as small as possible, because the air in the reservoir/plenum must expand to fill it before the pellet sees any pressure to start it on it's journey....

Not to belabor the point, Bob; but with a dump valve firing from a regulated plenum, the plenum volume would be added to the Transfer Port Volume.  I am looking at it from not just the perspective of the pressure front meeting the pellet base in the chamber, but the whole pressure profile as the air in the barrel expands behind the pellet, all the way to the muzzle: 

Simply put; the greater the expansion ratio, the better the efficiency:  So, a small plenum "wins", assuming a dump valve.  Now, you are using a sophisticated valve that acts very quickly; so perhaps what I am saying is irrelevant, rather than incorrect.

I key pointer to this seems to be:  When using a plenum sized to .5 CC per FPE, that requires only 10% greater pressure to get the same performance as a plenum sized to achieve 1 CC per FPE.

Either that, or I am more confused than I thought

[skorec]

looks incredible Bob..cant wait to see some long range results.

+1

[rsterne]

On almost all PCPs, only a small fraction of the air in the plenum is used during the shot, because the dwell of the valve is on the order of 1-2 mSec, while the bullet spends 2-4 mSec in the barrel.... If the valve is open for more than half the pellet travel in the barrel efficiency suffers bigtime.... I do not have a handle on the shortest dwell time usable (because there are too many varilables), and if you go too short (for example if the bullet has only moved 1% of the barrel length when the valve closes) the drag on the bullet will slow it down before it reaches the muzzle.... It is very common for the efficiency to peak when the valve is closing at 15-18% of the barrel length (a dwell of well under 1 mSec), and I have seen the efficiency still rising when the pellet has moved less than 10% of the barrel length (dwell of about 0.5 mSec)....

To understand what is happening to the pressure in a plenum where the valve is closing with the pellet only part way down the barrel, perhaps an example of what is happening to the pressure in the barrel would help.... Let's look at two examples, with a gun intended to shoot at 100 FPE, and having plenum volumes of 50 and 100 cc, both fed from a regulator at 2000 psi.... We will assume an efficiency of 1.0 FPE/CI, so the volume of air (in std. CI at atmospheric pressure) required is 100 CI.... At 2000 psi (138 bar) that 100 CI of air occupies (100 / 138) CI x 16.4 cc/CI = 11.9 cc.... so that is the volume of HPA required to create the 100 FPE shot....

For the 100 cc plenum.... Volume of HPA before shot = 100 cc.... The shot uses 11.9 cc, leaving (100 - 11.9) = 88.1 cc, which expands to fill the plenum, dropping the pressure to (88.1 / 100) x 2000 = 1762 psi.... so the average pressure during the shot was 1881 psi.... If you compare this to an infinite plenum, where there is no pressure drop, you will generate about 4% less FPE in the shot.... 

For the 50 cc plenum.... Pressure after the shot is (50 - 11.9) = 38.1 / 50 x 2000 = 1524 psi, and the average pressure is only 1762 psi.... about 12% less than with an infinitely large plenum..... Obviously, we would need to start with more than 2000 psi to get the same FPE as with the 100 cc plenum.... If we increase the pressure by the 10%, to 2200 psi, the pressure after the shot is now 1676 psi, and the average is 1938 psi, so the 50 cc plenum at 2200 would likely produce about 3% more FPE than would the 100 cc plenum at 2000.... However, if you were feeding these plenums from a fixed volume at 3000 psi, you would only have (3000 - 2200) = 800 psi of air supply, instead of 1000, yielding only 80% of the shot count....

I will include one more calculation, so that you can see what happens with a dump valve, which is still open when the pellet leaves the muzzle, where the valve volume is equal to the barrel volume.... Let's choose a barrel that I know can produce 100 FPE at 2000 psi, a .30 cal that is 24" long.... The barrel volume is (0.15^2 x PI x 24) = 1.7 CI = 28 cc.... so we will use the same volume (28cc) for the valve.... Starting with 2000 psi, and dumping all of it, the pressure when the bullet leaves the muzzle is 1000 psi (the volume has doubled from 28 cc to 56 cc at the instant the bullet reaches the muzzle).... Therefore, the average pressure for the shot is only 1500 psi.... To get back to the FPE of either of the above examples, we would need to increase the pressure to maybe 2500 psi, to get 1250 psi at the muzzle, and an average of 1875 psi.... Not only that, but since ALL the air in the valve was dumped, we actually used 28 cc of 2500 psi air to create our 100 FPE shot.... That works out to (28 / 16.4) x (2500 / 14.5) = 1.7 CI x 172 bar = 293 std. CI of air.... which drops the efficiency to (100 / 293) = 0.34 FPE/CI, or about a third the efficiency of the above examples.... This is exactly why dump valves are NOT the way to go for efficiency....

I hope the above examples help you to understand why larger plenum volumes increase the efficiency, because they have less pressure drop during the shot.... and also why dump valves are not used in PCPs, because they are so inefficient....

Bob

[gabi.nechita]

Hi.
You say for a good eficiency to close the valve when the bullet is in middle of the barrel.
If the barrel is 24 inch long half of barrel is 12 inch . If we use a barrel  with 12 inch length and close the valve near the muzle how much speed we can obtain? Dont mind eficiency.
It is hard to get 90 percent from 24 inch barrel speed ?

[rsterne]

You will get much less velocity and power with a 12" barrel, closing the valve at the muzzle, than you will with a 24" barrel, closing the valve when the pellet has only travelled 12".... This is because in the 24" barrel, you have all the air trapped in the first half of the barrel continuing to expand as the pellet travels through the second half, and it continues to add energy to the pellet that entire time....

As an example, if you have a .25 cal PCP shooting a 34 gr. pellet at 900 fps (60 FPE) from a 24" barrel, at 1.00 FPE/CI efficiency, with the valve closing at 12".... and you shorten the barrel to 12" (the valve still closing there) but make no other changes.... The velocity will drop to about 735 fps (41 FPE) while still using the same amount of air, so the efficiency drops to 0.68 FPE/CI.... With the 24"' barrel, the residual air pressure at the muzzle was 540 psi, but with the 12" barrel it is 1360 psi, so the report would be much louder as well.... The extra energy from the expanding air that gave you 900 fps in the 24" barrel now just gives you more noise and only 735 fps....

Actually, I stated that I would not use MORE dwell than that required to have the valve close with the pellet half way down the barrel, because the efficiency drops quickly when you do that.... Most of the time you want the valve to close when the pellet is only 20-35% of the way down the barrel....

Bob

[gabi.nechita]

Thank you.
I  understand now.

[subscriber]

I hope the above examples help you to understand why larger plenum volumes increase the efficiency, because they have less pressure drop during the shot.... and also why dump valves are not used in PCPs, because they are so inefficient....

Got it.  Thanks

[rsterne]

Here is an additional chart that may interest you.... This is an output from Lloyd's Internal Ballistics Spreadsheet....



The pressure curves (pressure and velocity on the right axis) show clearly the point where the valve closes, I used the same dwell for all three plenum sizes, no other changes, just the plenum volume.... The gun was a Disco, tuned for 30 FPE with the full 135 cc reservoir on tap (FPE on the left axis).... With a 30 cc plenum that dropped to about 28 FPE, and with a 15 cc about 26 FPE.... Even though all shots started with 1800 psi, you can see that the pressure in the smaller plenums dropped more at valve opening, because of the wasted transfer port volume.... with the 15cc plenum already down by about 50 psi just to pressurize the space between valve seat and pellet.... After that, as the pellet moves down the barrel, and the air expands to fill that volume, the pressure drop is much more rapid with the smaller plenums.... When the valve closes, the pressure is about 1710 psi with the full 135 cc reservoir, about 1470 psi with the 30 cc, and only about 1250 psi with the 15 cc.... After the valve closes, the pressure drop is more rapid in all cases, as the air trapped between the valve and pellet expands to fill the barrel as it travels towards the muzzle.... Another interesting thing is that you can see what effect shortening the barrel would have on the velocity, energy and residual muzzle pressure (which shows up as report) with this modelling....

This spreadsheet is proprietary to Lloyd Sikes, and developed by him (with a bit of input from me).... The main variables are caliber, bullet weight, pressure, valve dwell, reservoir/plenum volume and barrel length.... There are additional losses calculated which are from including the air mass (the greatest loss) and you can choose air, Nitrogen or Helium.... transfer port (wasted) volume, bullet drag and starting friction, the type of expansion both valve closed and open (adiabatic, isothermal or in between), and lastly a "system efficiency factor" to make the results of the spreadsheet balance with empirical results for the gun in question.... Once you have all the "known" variables input, then you adjust the valve dwell and the system efficiency until both the velocity/FPE and the efficiency in FPE/CI match your test results.... There is only ONE combination of those two remaining variables that will match your testing, and the results are charted as above, plus tabulated.... Outputs include.... barrel volume, FPE/CI efficiency, air volume and pressure used per shot, muzzle velocity and FPE, bar-cc/FPE, bar-cc per shot, ending reservoir/plenum pressure, residual muzzle pressure, "lock time" (time bullet moves down barrel), % distance down barrel at valve close, % of muzzle velocity at that point, and grains of gas in the barrel at pellet exit.... It is continually evolving, future changes will include corrections to the air mass (and accelerations) due to the VanDerWaals effect (deviation from Boye's Law at very high pressures), and the effect of the smallest transfer port diameter.... The goal is to eventually get to the point that the system efficiency factor is close to 100%, by including all other losses in the calculations.... I don't think we will ever get there, but we are learning all the time, and improving the model as we go....

Bob

[lloyd-ss]

Bob,
Very nice write up on the internal ballistics that you and I have been working on for ... golly, 7 years?  Just shows how much you can do, even without any research grant money, LOL. The modeling is becoming more accurate with each major revision we make.
The VanDerWaals pressure modification that you suggested has proven difficult to incorporate, but it is important, and I am almost there. The modification for T-port dia should be a much easier change to make to the spreadsheet, and will have potentially greater impact.

This is one of those situations where we were probably 80% there in the first couple of years, but this last 20%has proven very challenging. But we will get there!
Lloyd

[rsterne]

Indeed we will.... and the journey itself has been worth it, IMO....

Bob

[subscriber]

Thanks, Bob.

I had to stare at that chart for two minutes for it to soak in.  The density of profound information contained in it, had to overcome my own density 

[oldpro]

 All this information will someday be put into and app and allow the user to virtually build their gun down to the last FPE and CC. Now If I could just find someone who knows how to make apps....

[Machinist]

It just dawned on me that you or Lloyd could write a book on this stuff. 

"Everything You Always Wanted To Know About Pellet Guns"

The theory, practical modifications, tuning, troubleshooting etc. Charts up the ying yang, pictures.

I have no idea how many would sell, but I'd buy one.

Or.... "Air Guns For Dummies"?

Steve

[rsterne]

Too much yet to do and learn to have time to write a book.... and I'll be 70 this year.... 

Bob