Pellet velocity versus accuracy test: Part 4

by B.B. Pelletier

Part 1
Part 2
Part 3

Today we lower the velocity of the Whiscombe and test each of the four pellets, in preparation for the next accuracy test.

One reason I selected the Whiscombe for this series of tests is the fact that I can control the power output over a wide range of velocities by installing various transfer port limiters. For those who are new to airgunning, every spring-piston airgun like the Whiscombe generates a brief blast of compressed air by means of a piston racing forward in a compression tube. In the Whiscombe’s case it is actually two pistons racing towards each other. At the exact end of their travel a small air tunnel called an air transfer port conducts the compressed air from the compression chamber to the base of the pellet, where it blows it out the bore.

The amount of compressed air is extremely small, but the pressure is fantastically high for a brief moment — as much as over 1,000 psi. In fact, well over 1,000 psi. Now you probably also know that precharged airguns operate on compressed air that is at an even higher pressure, but the difference is, when a precharged valve opens, it does so relatively slow, releasing air at far below the pressure that’s in the reservoir. But a spring piston gun doesn’t have any valves, so when the air is compressed, it flows through the air transfer port to the back of the pellet like a small explosion. Everything the gun has to give is right there at the same time.

However, if anything gets in the way of this air as it flows, it slows down the flow and the resultant energy output of the gun. The amount of compressed air remains the same, but the rate at which it flows is slower, and the pellet is not moved with the same sudden push.

That’s a fancy explanation for the Whiscombe’s air transfer port limiters, which are nothing more than Allen screws that screw into the threaded transfer port. In the center of each are holes of various sizes though which the air must then pass.


The rifle has a threaded air transfer port. It’s shown here without any limiters installed.


These limiters have various sized holes that reduce the airflow through the transfer port when they are installed. For this test I will install the largest one on the left.


The transfer port limiter is installed with an Allen wrench.

Velocities
Now let’s see what changes have been made by the insertion of the transfer port limiter. I will shoot the same four pellets as before and in the same order.

Beeman Devastator
First I tested the Beeman Devastator pellet. This is the pellet that gave an average velocity of 1,205 f.p.s. in Part 1 without any transfer port limiter. With the limiter I show being installed above the Devastator’s average velocity dropped to 1,123 f.p.s. In the first test the spread was 28 f.p.s. In this test is was 16 f.p.s., running from 1,116 to 1,132 f.p.s. That isn’t a huge drop in speed, but the power that was 23.32 foot-pounds in the first test has dropped to 19.89 foot-pounds. We will see what this does to the accuracy.

Premier lite
Next came the Crosman Premier lite pellet. In the first test they were averaging 1.134 f.p.s. for an energy of 22.56 foot-pounds. The spread in that test was only 12 f.p.s. In this test the same pellet averaged 1,057 f.p.s. for an energy of 19.56 foot-pounds. The spread was an even tighter 6 f.p.s., running from 1,054 to 1,060 f.p.s. That is incredible performance at this power level.

Beeman Kodiak
Beeman Kodiaks came next and they averaged 992 f.p.s. in the first test. That generated an average 22.29 foot pounds of energy at the muzzle. The spread in test one was 22 foot-seconds. After the transfer port limiter was installed the average velocity dropped to 937 f.p.s. for an energy of 19.89 foot-pounds. The total velocity spread was 24 f.p.s. — ranging from 927 to 951 f.p.s.

Eun Jin
The heavy 16.1-grain Eun Jin was the final pellet I tested. In the first test they traveled 726 f.p.s. and generated 18.85 foot-pounds of energy. The spread was 13 f.p.s. With the transfer port limiter installed they averaged 687 f.p.s. for an energy of 16.88 foot-pounds. The spread with the limiter installed was 10 f.p.s. — from 682 to 692 f.p.s.

What’s next?
Next I shoot groups at 25 yards with each of these pellets so we can compare them to the groups fired with no transfer port limiter. That will give us our first look at how velocity affects accuracy.

After that I install another limiter that restricts the airflow even more, so the rifle shoots slower. And then we shoot it at that level for accuracy.

At the end of this test I still have one pellet out of four that’s supersonic. The Devastator, at 1.123 f.p.s. will always be supersonic where I live. The Crosman Premier, however, will be right on the cusp of the sound barrier and whether of not it breaks though will depend on the day.

Tomorrow
Normally I would run the accuracy part of this report immediately after the velocity report, but something wonderful happened at the range on Wednesday. So I am making a special report on Friday to show you something very amazing — at least in my eyes. And no, it isn’t the Ballard. Not yet.

41 Responses to “Pellet velocity versus accuracy test: Part 4”

  • flobert Says:

    Wow this is a spicy gun you’re testing here.

    I suspect that there’s a point where pellet instability decreases, but piston and barrel time increase, and there’s some velocity where the negative effects of both are at a collective minimum.

  • Wulfraed Says:

    something wonderful happened at the range on Wednesday.

    The Great Pumpkin did a pre-Halloween inspection?

  • JTinAL Says:

    My guess is:the only big change will be with the Crosman Premier
    since it’s so close to the sonic region.I’m betting the small velocity
    change in the others won’t be enough to make a noticeable difference.
    But I’m usually wrong lol.Yeah I’m used to it :)

  • twotalon Says:

    B.B.

    Looking at that selection of transfer port plugs, I have to say that you are going to be working a long time if you are going to use them all.

    twotalon

  • CowBoyStar Dad Says:

    Harking back to yesterday and measuring groups.
    I just realized that no one seems to have touched on the electronic scoring methods used in modern Olympic matches where the shot is measured by electronic triangulation.
    I would surmise that using this equipment it would be possible to score group sized to thousandths of an inch?

    • B.B. Pelletier Says:

      CSD,

      Yes, one-thousandth of an inch is the resolution/accuracy of the sound-scored system.

      B.B.

    • Herb Says:

      No, you could not measure group size using the electronic system. At least not the way groups are traditionally shot. After you chew a hole in the paper there isn’t any paper to hit so you’d not get any sound!

      For the paper scoring system to work you’d have to shoot one shot at each bull, and have multiple bulls on the target. Which of course is the way that such tournaments are run. The big advantage with this is that you could also get a x-y displacement of each shot. So you could do a much better statistical analysis.

      For a 10 shot group, group size is based on the the two widest shots. Using sound scoring you could register all 10 shots. Maybe one of those outliers is a “flyer” and could be discarded using appropriate statistics.

      • B.B. Pelletier Says:

        Herb,

        Perhaps without being aware of what you did, you described how the sound-scored target works. It actually doesn’t have a real target. There is just a hole that has a roll of black paper winding continuously behind it. So the pellet always tears fresh paper.

        As for the white part of the target, I’m not sure of how that is handled. It is seldom needed but every once in a while even a world-class shooter will pull the trigger too soon, so there must be a way of scoring it.

        The shooter has a video screen at his position that shows a conventional target and the pellet hole is overlaid on it. One hole at a time — with the ability to scroll around the shots that have been recorded.

        B.B.

        • Herb Says:

          BB,

          Thanks for the correction!

          I didn’t realize that there was a roll of paper behind the target. I ASSUMED that the sound was registering from hitting the target itself.

          With microprocessors today, I wonder how much it would cost to build such a system registering from the target alone? The mechanical frame to hold the target and microphones would cost more to build in small quantities than the electronics needed. I’m thinking a frame with holes punched into the target to register the target correctly in the target holder. I think the propeller microprocessor would be great for multiple microphones. The most expensive electrical part would be the circuit board.

          Even measuring to 0.01+ inch would be fine for most of us.

          Herb

          • PeteZ Says:

            Herb,
            There are some non-acoustic systems using beam breaking, etc. Would you really want to subject your electronics to the repeated vibrations from pellets hitting a frame or back plate?

            Molecular average speeds run around 410 – 500 meters/sec, so outrunning the gas is plausible for a springer. Assuming nitrogen = air and T = 300K, and depending on what’s your measure for v – mean, median, etc. That’s surely low for T, but if you raise it the molecular velocity goes up too.

            Good analysis!

            pete

  • pete zimmerman Says:

    Actually, I doubt the electronic scoring measurements are as accurate as CBS-Dad suggests. They don’t need to be. They really only need to split a pellet diameter in 10 parts, so half a millimeter. To be safe, you might want to reduce the uncertainty to .25 millimeter.

    BB,

    What are the actual pressures behind the pellet when it starts to move with each of the transfer port apertures? I don’t think it’s simple to calculate. Is the integral of pressure vs time about the same for all ports? In other words, how much if at all do the ports reduce the total energy available in the system?

    • B.B. Pelletier Says:

      Pete,

      I can’t calculate what you ask, but here is a simple thought experiment that might clear things up for you. Imagine a stadium filled with ten thousand fans and one exit. If the exit was the size of the average garage door the exit rate would be one rate, but if the exit was the size of the average interior door the rate would be slower. All the people would still leave the stadium, but it would take longer.

      B.B.

      • Alan in MI Says:

        It is actually much more complicated than that ;-)

        One of the big differences between PCPs (including MSPs) and spring guns is the thermal energy. The PCP air reservoir is at ambient temperature, or at least not much above it because the compression occurs slowly and long before the valve opens to let the air flow. When the valve opens and the air flows, there is actually a drop in temperature in the air flow, and thus a reduction in the absolute energy in the air pulse.

        But in a spring gun, the pressure rise is rapid, and the temperature rises rapidly as well – so much so that we get autoignition of oils that raise the temperature and pressure even further. So there are not a lot of molecules, but they do have significant amounts of thermal energy.

        So in PCPs the energy is mostly a function of the number of molecules of air used to generate pressure, and in spring guns (“magnum” ones, anyways), the energy is mostly a function of the heat developed to generate the pressure with a much more limited number of molecules.

        So I think the analogies have to be different. I’m thinking a PCP is a the stadium full of people walking casually and patiently through an exit of different sizes (representing the transfer port), but the spring gun is more like the start of a marathon, with the runners patiently but quickly running across a start line of varying widths (representing the transfer port), all triggered off the same signal.

        This even works well from the standpoint of the stadium being full but contained behind a closed door that must be opened for the flow to start (PCP valve), while the mart hon has a smaller reservoir of people but a wide open door with no flow until a signal creates the pressure to start the flow (piston in the cylinder) . . . .

        Alan in MI

      • pete zimmerman Says:

        BB,

        I certainly understand how throttling works in principle, but the thermodynamics is a bit tricky in a truly dynamic system, which a springer is. You cannot think of it just as a compressed air tank, a transfer tube/port and another chamber, where the pellet is. At the very least it is two pistons, the pellet being the second piston. There’s no equilibrium state, and there are plenty of losses to friction and turbulence as well.

        My thinking starts like this: to first approximation, the force that it takes to overcome static friction, engage the rifling, deform the pellet as needed, and getting it moving is likely the same, regardless of the rate of increase of pressure behind the pellet. So maybe regardless of transfer point size the pellet starts to move at the same position of the piston moving in its chamber. Now, with a small transfer port, the piston will stop its forward motion earlier in its trajectory because the air in front of it will compress more, as the flow outward is restricted. We can see this by imagining the transfer point is vanishingly small, and then by imagining that the transfer port is the same diameter as the cylinder. In the second case the air gets out of the way of the piston easily and quickly, and there’s almost no pressure buildup in the cylinder. In the first case, the air cannot get out of the way, and the pressure buildup is quite large.

        The air flow through a smaller aperture is slower than through a large tube, but the pressure built up in the cylinder reaches a higher value. I wonder if the Cardews did the thermo; I’ll have to look later in the day.

        • Herb Says:

          Pete,

          I don’t remember the Cardews doing any really detailed thermodynamics. They sort of waved their hands at it. They were avid experimentalists not theoreticians.

          I totally agree that a spring air gun is a very dynamic system. There are numerous interactions and it is virtually impossible using any equipment that airgun enthusiasts have to measure enough of the factors to start to solve the equations. It of course could be done.

          It would be cool to have high speed x-rays to look at piston position relative to pellet position. But who has such equipment in their basement?

          As far as analogies, they are always imperfect. As the piston starts forward gas pressure increases, temperature increases, and gas starts to flow through the transfer port. As the piston reaches its most forward position, the temperature is at a maximum, and the pressure in the piston chamber is at its highest. (The pellet may or may not have started moving by this point…). After the piston reaches its most forward position, it starts to recoil. The pressure drops, the temperature drops.

          After the pellet starts moving the volume past the transfer port starts to increase. The gas past the transfer port cools more rapidly than the gas in the piston chamber because the volume of that gas is increasing. By the time the pellet is about a foot down the barrel it is traveling on its kinetic energy and not receiving any additional significant energy from the gas. At that point the piston has retracted so far that gas isn’t flowing from the piston chamber to the barrel.

          BB has discussed the transfer port before. It is a marvel at balancing the need to get gas through the port slowly enough to let pressure build, but not so slowly that the piston retracts so much that the maximum pressure pulse is lost.

          The pressure build up behind the pellet in a springer is typically much greater that the pressure behind a springer. It is common for a springer to blow out the skirts of a pellet, but uncommon for a PCP to do so. So there are real differences in the dynamics.

    • /Dave Says:

      Pete,

      If the pellet never moved and the system stayed sealed, the gas pressure would equalize regardless out the port size. But, since the pellet begins to move down the bore once a specific pressure has been reached, the volume of the chamber grows, effectively lowering the pressure rise per time period. During this time frame the pressure can only increase in proportion to the size of the port. The smaller the port, the higher the pressure drop across it and the lower the flow, as BB stated. This reduces the acceleration and final muzzle velocity of the exiting pellet. A combination of Boyle’s Law and Bernoulli’s Principle. Clear as mud? ;-)

      /Dave

      • pete zimmerman Says:

        Yes, yes. See above. I don’t see Bernoulli getting into the act, however. And Boyle’s law is for equilibrium systems, which this ain’t in spades. And the gas flow probably is turbulent.

        • /Dave Says:

          When you consider the thermo dynamics of the system, the heat rise affects the rate of pressure rise and thereby the flow through the port. Bernoulli gets into the act there, affecting the rate of flow through the port and subsequent pressure rise on the other side of the port and the rate of that pressure rise. The fact the the port is rough only affects its effective size versus its actual size due to turbulence.

          Correct me if I’m wrong, but I seem to remember that Boyle’s Law also states that in a closed system, pressure acts equally on all sides of the container. This is a dynamic system, but it’s also closed until the pellet leaves the barrel. So, Boyle’s Law would apply in my mind as trying to take effect while the system is in motion. To my understanding, this is what causes the transfer of gas and pressure from the piston movement to the gun’s chamber. The system is trying to reach equilibrium but has not got enough time to do so before the pellet leaves the bore. I know this description is an oversimplification, and there are many other factors to consider such as inertial energy, system efficiency, inertial losses and so on in order to fully describe and predict the system, but for me, that’s where experimentation comes in. I love experiments!! :-) (maybe I should have paid closer attention in school to the details……)

          /Dave

          • pete zimmerman Says:

            As you say, Boyle’s Law is trying. But it’s not quite succeeding. If it did, then the system would be more like an unthrottled hydraulic system where the pressure is at all times equal everywhere. But during the movement of the piston there are distinct variations in pressure between the main cylinder and the barrel of the gun. I don’t think transfer tube roughness is a first order effect. I may take an hour or two and see if I can set up the equations.

            I did pay attention in school, but that was 48 years ago, and I haven’t had to use thermo and gas flow since my qualifying exam 44 years ago.

          • Herb Says:

            Dave,

            The static application Boyle’s law doesn’t apply. Boyle’s law is for systems at equilibrium. The overall firing sequence is dynamic not static, hence it is not at equilibrium.

            Let’s do a different thought experiment. I have a 1 cubic meter reservoir to behind a valve to shoot a 0.177 inch pellet down a 20 inch barrel. When the valve pops open it stays open.

            For a PCP the average molecular velocity also provides a limit on how fast a pellet can be propelled. (The absolute limit doesn’t apply to springer since a springer also heats the gas.) If Boyle’s law were the controlling factor with a springer, then I could keep using a bigger and bigger reservoir, and a longer and longer barrel to get any pellet velocity that I wanted. But that of course doesn’t work.

            Now as the pellet start to accelerate down the barrel it picks up speed. It gets going so fast that the pressure behind the pellet is lower than the pressure in the main reservoir. This is so because the pellet is traveling at velocities cost the average molecular velocity of the gas particles. So one gas molecule couldn’t bounce off the pellet, then bounce off the transfer port and bounce back to the pellet before the pellet’s position changed.

            Hope that helps,
            Herb

    • BG_Farmer Says:

      Pete,
      I tend to think the piston (or pistons) always sweeps the full volume before the pellet exits (especially with the Whiscombe); also, the mass of the pellet is trivial relative to the piston, as is (as a result) the pressure past the transfer port relative to the cylinder. So, I think the transfer port is the main limit, as we can see empirically on the Whiscombe as well as numerous other experiments — i.e. changing the port changes the output. Vince once analyzed the springer in terms of efficiency (i.e. potential of mainspring versus kinetic energy of pellet). Similarly, I think it would be interesting to simplify the system and calculate the maximum kinetic energy of the piston as a product of transfer port flow and the resultant pressure in the cylinder, i.e. how fast can the piston travel, thus how much energy is available to the pellet. At that point, we would have some idea of how efficient the transfer of energy is to the pellet — if the efficiency is high, we could safely assume that fluid flow and thermodynamics, while interesting pursuits are somewhat negligible; if the efficiency is low, then we would need to look farther. Not arguing with anyone here, just voicing my thoughts on this interesting problem, which realistically is not critical enough to get the resources it would require to investigate thoroughly :).

      • /Dave Says:

        Thanks, Herb, Pete and BG Farmer!

        I should have said Pascal’s Law, not Boyle (names and me just don’t get along….. and with such confusion, have led everyone down the wrong path). He’s the one who said pressure acts equally in all directions of a container (but of course, I had already taken off for the range by the time I remembered correctly who it was). I understand the crowding of molecules being the ultimate limiting factor, and that the system is dynamic, but in the end, it’s still just pressure and flow against a resistance. Just thought it would help anyone else to add to BB’s explanation with ‘moment in time’ snapshots of why the reducing ports would reduce the velocity (Pete’s original question, “In other words, how much if at all do the ports reduce the total energy available in the system?”) and I pretty much blew it. One would think I’d know better by now… (One would be wrong…. :-) ) Too many years, since not enough school, have gone by for my memory to keep up. However, since we’ve already shoved Bernoulli, Boyle, and Pascal into the relatively small rifle, who else can we fit in there? I think Newton deserves a place along with lots of other noted scientists!

        That’s why I love to experiment! Sometimes it’s cheaper both in terms of time and resources, while being loads more fun! For instance,…. My cocking arm can tell you that my springers generate much more in terms of ft/lbs to the target than my pumpers for the amount of physical effort I have to put into shooting each one. :-) But, I also like to understand where the mechanics of that efficiency come from. In a word, “dieseling”, but we know there is much more involved than just a word, and mental exercises can be fun too…

        I did manage to find a workable load for my Mosin Nagant 91/30 at the range today! 48.8 gr of H4895 topped with a 150 gr Speer Hot-Cor at 2.880″ COL and W-W large rifle standard primer shoots to pont of aim at 25 yds with the rear sight set at the optimistic “5″ (500 meter mark). No signs of overpressure, so I’ll need to test it at 100 yds and chrony it. That’s my kind of experiment. Where something goes bang! :-D

        /Dave

        • /Dave Says:

          I forgot the disclaimer for the 7.62x54R reload….. “This load is safe in ONLY my rifle. Use at your own risk”

          /Dave

          • Mr B. Says:

            /Dave,

            Got October’s “GUNTESTS”in yesterday’s mail. On the cover is a pictures of a couple of Moisin Nagants, one of which they said was a best buy! They were impressed with the way the gun shot nice little groups (measured by the “string” method) shooting Serbian Prvi Partizan 150-gr SP BT’s.

            Bruce

            • /Dave Says:

              Bruce,

              I have a 91/30 and an M44. Both have “the nicest trigger I’ve ever felt in a military rifle”, as stated by a friend of mine. Of course, I stoned them both, but they were really god to begin with. Both rifles shoot about 4″-5″ groups at 100 yds with surplus fmj’s with the iron sights. While that’s plenty good for me, I hope to tighten that up some.

              /Dave

              • B.B. Pelletier Says:

                /Dave,

                My experience shows that a 2-inch five-shot group should be possible at 100 yards in the 91/30. If you shoot ten shots, better go for three inches.

                I have no experience with the 44. I have avoided it because of the recoil and muzzle blast. In fact, if I were reloading for that rifle I would look at using 3031 powder that is somewhat faster burning.

                B.B.

        • B.B. Pelletier Says:

          /Dave,

          That load for your Mosin is almost identical to one for a 30/06 made for a Garand. It’s a good load in that rifle.

          B.B.

  • Matt61 Says:

    Even though I would never use the Donaldson loading technique, it is certainly food for thought about accuracy. Together with the importance of headspacing, it tends to make me thing that accuracy is very closely tied to the initial conditions of the shot which is to say the placement of the projectile in the chamber as snugly as possible. Close to that would be the next layer of things connected to initial conditions like a floating bolt head in firearms and then a good trigger with a short lock time. Everything else that is so trumpeted like fire-lapping barrels, modifying barrel harmonics, bedding actions, crowning muzzles can minimize the degradation of accuracy but it can’t make up for accuracy that isn’t there with an initial good fit. Maybe a lot of work that goes into custom accuracy jobs is a waste of money!? What if some airgun manufacturer produced a cheap rifle with resources poured into a sort of target chamber with a specially designed brand of pellets to fit the chambering just perfectly. Most people just use a favorite pellet anyway, so this would make things easier for them. I wonder if we would see a new level of accuracy.

    Jack Dempsey claimed that boxing was more gentlemanly and technically much better when it was at an informal amateur level. With the professionalization of boxing which he helped initiate with his million dollar gates, he claimed that it went downhill with low-class people rushing through minimal skills to make money. I wonder if that’s true of other sports.

    The Mr. Hyde in me cannot help but observe that Little League and martial arts events are the sources of some of the most hilarious scenes on YouTube. There was a Karate match where the judge didn’t seem to like what one competitor was doing. Perhaps he was ignoring the judge’s commands. So without warning, the judge comes up behind him and drops him with a forearm to the back of the neck?!

    And for weird, how about this? A news story reports that some guy was very unhappy that his daughter ran away from him. So, when he got her back, he sat on her with his 300 pounds and beat her with willow branches. Then, as a medieval enthusiast, he suited her up in one of his leather and chain mail suits and made her fight him with a wooden sword for two hours until she collapsed(?) I would like to see the 300 pounder who can sword fight for two hours….

    Matt61

  • Joe B in Marin Says:

    Hi B.B.

    Off-topic…

    Do you, or does anyone, know if there is a blue book of values for airsoft guns out yet? Or for that matter, ANY books (not periodicals) about airsoft?

    Thanks,

    Joe B in Marin

    • B.B. Pelletier Says:

      Joe B,

      Steve Fjestad (publisher of the Blue Book) asked me if an airsoft book was needed several years ago. I answered a cautious,” Yes, but not until the Asians get more configuration control over their products.” Right now it’s anything they can slap in a box and sell. The companies use each other’s parts and create “new” guns at such a blinding rate that a reality TV program couldn’t keep up.

      I think that is what keeps everyone from attempting the task.

      B.B.

  • /Dave Says:

    And again, it goes to the spam filter. Maybe I should take the hint….

    /Dave

  • Alex R. Says:

    Off-topic

    I read an earlier blog, the one on the 1938 red ryder, and I was wondering if I could buy one of those parts. I do not now the exact name of the part but it is a 1938 red ryder. I was hoping you could contact my email and tell me the part and the cost.

    thank you
    Alex R.

  • Toby T. Says:

    Another off topic question. I spotted some JT 90 gram Co2 cylinders yesterday. They appear to be pretty much the same thing as the Crosman Air Source cylinders. Will they work in my 850 Air Magnum? Thanks! Toby

    • B.B. Pelletier Says:

      Toby,

      The AirSource cylinders are 88-gram, so it is doubtful that the 90-gram cylinders you found will work. No way of knowing except to try them.

      B.B.

      • Toby T. Says:

        BB, Thanks for the response. I will take one of the 88 gram cylinders to the store for comparison before purchasing. I will post my findings on the blog. Thanks again. Toby

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