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Air Guns Air transfer ports on spring-piston airguns: Part Two

Air transfer ports on spring-piston airguns: Part Two

Part 1

This report covers:

  • Not that simple
  • Hot topic
  • An experiment
  • A proof-of-concept testbed
  • Air leakage
  • Shapes
  • Public not happy
  • Summary

Today we will explore more about air transfer ports. Remember — we are only discussing the ports found in spring-piston airguns.

This subject was raised by the late Frank Ballistreri, I believe, from our conversations regarding deep-seating pellets in spring guns. Someone asked if the transfer port of the Hy Score 801 was particularly short, which he felt explained why seating pellets deeply would show a velocity increase.

Not that simple

I have to tell you that it isn’t that simple. We learned in Part 1 that the air transfer port conducts the high-pressure air from the compression chamber to the back of the pellet sitting in the breech. While it has a simple job to do, the transfer port is another factor in the overall performance of the gun. In that capacity, the tune of the gun relates directly to the size, length and even the shape of the transfer port. Yes, I said the shape, too. I will get to shapes later. The length we covered in Part 1. For the moment let’s consider port sizes.

Hot topic

Changing port dimensions and shapes was all the rage back in the mid-1990s. Jim Maccari did a brisk business altering ports for customers. And he came up with some observations of his own while doing it. If you want the rifle to continue to function over a broad range of power, based on changing the state of tune, he found it was best to leave the port as the factory designed it. Let me give you an example to illustrate the wisdom of that.

The Beeman R1 used to come in all four calibers (.177, .20, .22 and .25). But when Weihrauch produced the R1, they made the transfer port the same size for all of them. It would have been a costly management nightmare to make a spring rifle with different port sizes according to the caliber. So, all R1s came with a transfer port that’s very close to 0.125″ in diameter. The actual size is metric, but that’s what it measures on an inch scale.

An experiment

Let’s say you’re the owner of a .177 R1 that you want to convert to .22. All that’s needed is a new barrel and cocking link. You don’t really need the cocking link, but removing and replacing them on Weihrauch barrels is a bit of a chore. Everything else on the rifle is the same between the two calibers. But if you altered the port for enhanced performance in .177 caliber, you might find it next to impossible to get decent performance out of it in .22. And, when you altered the port for optimum performance in .177, that was just for one or two pellets. You generally lose performance with other pellets when you make changes to the transfer port.

A proof-of-concept testbed

Because the transfer port is such a permanent part of the spring tube, any changes that are made are usually permanent. There are several ways to bush the spring tube so you can start all over, but is it worth the effort? Jim Maccari apparently didn’t think so, because he donated a ruined spring tube to me for an experiment. Dennis Quackenbush made a set of transfer ports that slid into the hole and were held in place with a setscrew.

transfer port R1
The transfer ports were inserts held by a setscrew.

transfer port set
Dennis Quackenbush made these transfer ports in graduated sizes. He gave me several blanks for further experimentation.

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Air leakage

What about air leakage around the inserts? There has to be some clearance for the inserts to slide into the hole in the spring tube. Does that affect anything? It might at a level I can’t test, but across the chronograph I noticed no difference. However I was testing the modified R1 against a standard R1. What I could not do is test the R1 that accepted inserts with that same spring piston tube that still had a factory transfer port. Because once the port was drilled out there was no going back. It’s sort of a redneck Heisenberg principle.

What I learned is — wonder of wonders — Weihrauch got it right. A transfer port of 0.125-inches diameter is ideal for the .22 caliber R1 and for all pellets. More specifically, this is what I learned about transfer port diameters.

Transfer port sizes from 0.120″ to 0.145″ give the same results for a .22 caliber R1 tuned for higher power — in this gun that  was about 19 foot-pounds. When the port size drops below 0.120″, the velocity slows. When it gets above 0.145″, velocity slows and the gun acts like it’s being dry-fired. Lotsa dieseling, etc.

I’ll now cover what transfer port shapes do — or at least one of the most talked-about shapes. Let’s see what I did.

Shapes

This is another topic in the discussion of transfer ports that never ends. Many people thought an air venturi would improve performance. That would be a smaller hole with a bevel on either side. A properly designed venturi should speed up the flow of compressed air through the port because it’s made to pass through a tunnel that changes shape from large to small. It looks like this.

air venturi
A venturi shape was tried but gave no conclusive results.

Quackenbush did make a venturi-shaped port for me to test but I never recorded any advantage from it. Perhaps the machining was too rough, but I did test it. This is what I learned.

An air venturi transfer port doesn’t seem to affect the performance within the optimum port dianmeter size range in a .22 caliber.rifle.

Public not happy

People did not like what I told them when I reported this. They had lots of suggestions for me. What about testing with this pellet? What about that other pellet? I couldn’t test them all and of course the ones I didn’t test became the only ones that might have worked. My advice to everyone who wants to know more this subject is to build your own test rig and test it yourself.

People even said they “knew” that airgun companies had reams of secret data on transfer ports they were unwilling to divulge. Airgunners in the UK thought the “good” transfer ports would push their air rifles into the power level that required a firearm certificate (FAC), which is why the information wasn’t shared.

Grab your tinfoil hat! Air rifle transfer ports are like aliens — either you don’t believe in them or you do, and nothing anyone can say will change your mind.

Summary

I will admit that there is a lot I did not test. But what I did test provides the only solid information I can find published about air transfer ports on spring guns. Oh, people will tell you there is huge “black magic” that can be done when a transfer port is changed — they just have no proof. Search the internet and prove me wrong.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

46 thoughts on “Air transfer ports on spring-piston airguns: Part Two”

  1. “Changing port dimensions and shapes was all the rage back in the mid-1990s.”

    BB,
    I remember all the hubbub about that back in those days.
    Thank you for this informative report to set the record straight. 🙂
    Blessings to you,
    dave

  2. Tom,

    The 0.125 inch (3mm) port turned out as the best compromise for the R1 power plant. So do other spring piston transfer ports measure the same? Or is there some engineering work done before to determine the best size prior to manufacture?

    Siraniko

    • Siraniko,

      Spring piston transfer ports all do measure pretty close to the same size. As far as engineering work, I doubt any is being done at all. It like the 1:16-inch twist rate for air rifles. They mostly just do what has worked in the past.

      A few companies like FX have looked at twist rates, but most of them just use the same old rate they have always used. Same, same for transfer ports.

      BB

  3. “Weihrauch got it right. A transfer port of 0.125-inches diameter is ideal for the .22 caliber R1 and for all pellets.”

    I have often heard that .22 R1’s/HW80’s are remarkably un-pellet fussy. Is that less the case with the other three calibres?

  4. BB,

    This is really awesome knowledge. I had never thought of modifying any of my transfer ports before and after reading this, I likely will not. My rememberer, which may be on the blink, recalls you fiddling with transfer ports some with your Whiscombe. I do not recall reading Part 1. I will have to go back and do such.

    P.S.
    Air leakage
    Second Paragraph – Last Sentence
    More specifically, this is what I learned and (about?) transfer port diameters.

  5. I just reread and remember Part 1 now. It is a good thing that all of this “stuff” is written down somewhere. My rememberer does not work as well as it used to. I think it is still in there. it is just not as easy to access it as it used to be.

  6. B.B.
    You made my day!!!

    ” redneck Heisenberg principle.” What a ingenious description.
    Perhaps the subject for a new blog series, or at least the next April1st blog?

    -Yogi

      • B.B.

        So you are saying that the manufacturers got it right?
        What I find interesting is not what the TP size and shape is, but how that got it right?
        Was this just trial and error with prototypes? Was this advanced mathematics? I understand at the moment of air compression, that it acts more like a plasma than just air.

        I wonder how Mr. Q decided on the TP size for his double cocker?

        -Y

        • Yogi,

          “I understand at the moment of air compression, that it acts more like a plasma than just air.”

          Interesting!

          I have had an interest in Plasma in the form of St. Elmo’s Fire (corona discharge) that is commonly observed on the periphery of propellers, turbine blades, wing tips, windshield, and nose of aircraft flying in dry snow, in ice crystals; or near thunderstorms. I did research on the effect(s) it has on our airborne collection receivers and processing equipment.
          I would be interesting to know if the compression of a Spring Piston is enough and fast enough of a pressure increase without the input of, external heat, photon or electrical charge boost to strip electrons and cause ionization.
          The effects on spring piston performance, IF there is in fact ionization, would be quite the science project!

          shootski

  7. BB,

    I have a question concerning the venturi design. My rememberer does not recollect whether the bevel only on the inside of the compression chamber is effective.

    I am also picturing a concave type slope to the end of the compression chamber toward the concentric transfer port with a corresponding convex matching shape of the end of the piston. I do hope my ramblings are making some sort of sense.

    I am aware that most of this relates to the design of the compression chamber instead of the transfer port, but I for one am thinking of this at this moment and would like to know if you have done any research along these lines.

  8. This confirms why FM will continue with his philosophy of “leave it alone” and “if it ain’t broke, don’t fix it” when it comes to mechanical and a lotta other things as well. As Dirty Harry famously said, “a man’s got to know his limitations.”

  9. The jist of the transfer port from a compressible fluid stand point, it is a sharp edged orifice. Putting a radius on the inlet will increase flow and reduce pressure drop across the orifice. The radius on the outlet will help in pressure recovery if there is a sufficient straight section after the orifice. At a minimum of 3 times the diameter of the flow, if straight section is short the flow will not recovery any or very little of the pressure drop. At transonic velocity the pressure recovery is essentially zero unless the venturi is very long (10 x diameter) to keep the flow attached to the walls of the transition. The flow is most likely sonic in the orifice / transfer port.

    Now here is where it’s my opinion: The orifice is there to slow the flow down to allow for the pressure behind the pellet to be high while it’s traveling down the barrel until it exits.

    Also my opinion: This reduced flow would allow the spring piston to get to the end of its travel at nearly the same time as the pellet exits the barrel.

    Mike

    • bmwsmiley,

      “Also my opinion: This reduced flow would allow the spring piston to get to the end of its travel at nearly the same time as the pellet exits the barrel.”

      Mike i don’t believe actual flow of the compressed air is all to much of a factor in the spring piston powerplant when compared to the PCP powerplant where most of the power comes from flow velocity and duration. The inertial energy of the well balanced spring piston powerplant is found in the microsecond(s) long “kick in the butt” with the residual flow into the barrel behind the projectile just enough to keep the pressure from dropping sufficiently to slow the projectile before barrel exit in my opinion based on reading the work of others.

      Internal ballistics is INTERESTING in any shooting implement!

      shootski

      • Shootski: I don’t have the academic/engineering credentials to compete here, but what you wrote makes sense to me. I’ve thought, as an untrained shooter (my degrees are in theology and counseling) that the “kick in the butt” is the secret to the spring piston systems. A big blast of high pressure air from a larger cylinder into the “smaller cylinder” behind the pellet would be the kick necessary to move the pellet. There would not be residual gas, or much anyway (unlike a powder burn), to keep pushing much as the pellet travels down the barrel – hence the short barrel length of travel.

        Also, the barrel length would seem to me to be calculated so that there is still enough residual back pressure to slow the piston from its crash into the end of the compression chamber. In other words, a smoothing of the compression chamber opening of the transfer port would be counter to the need of some resistance for the piston at the end of its travel.

        I may be FOS on this whole issue as I am not trained in the engineering and fluid dynamic principles implicit in the whole shot cycle of the spring piston power plants. I go “merrily along” (?) thinking that there has to be a balance between the power impulse to the pellet and the braking effect to buffer the collision of the piston and end of the compression chamber. A certain amount resistance, therefore, it would seem to be necessary in spring piston power plants to preserve the mechanical system at the cost of absolute power and performance of the projectile. It would come down to costs; pellets are cheap, gun parts are not.

        Am I anywhere near on point here, do you think?

  10. BB

    Good report. Springers are and always will be fascinating airguns for me.Just simple pop guns some may think. The Cardews book and your many reports prove their complexity. Perhaps the Sig ASP20 is the high water of springer design excluding the Whiscombe. But as RR suggests, someone may design an affordable regulated air/gas springer.

    Deck

    • Decksniper,

      The SIG ASP20 engineering drawings (seem to be 1:1 scale) that i have for a .177 caliber do not show anything beyond a straight, but well offset, Transfer Port that measures approximately 3 mm (0.125 inches) in diameter and approximately 14 mm (0.55 inches) in length.

      shootski

  11. G’morning,

    Interesting subject and I’ll throw in my 2 cents. I’m not an engineer so my theories are based on discussions with knowledgeable people.

    With a pneumatic airgun the whole thing needs to be considered as a balanced system and tampering with any one part can (will?) throw the whole thing out of kilter.

    The transfer port diameter relative to the volume/pressure/velocity of the air flow is an obvious consideration but there’s many other factors (like barrel and projectile interactions) that affect the dynamics of the air flow.

    As one part of my “super-tuning” of my springers I’d chamfer the piston side of the transfer port slightly (.025″) then polish away the chamfer to create half a venturi as a lead in to the transfer port. The pellet side of the transfer port was lightly deburred (if required) but left at factory size.

    My modifications were not to second guess the engineers doing their job, it was to reduce turbulence at a critical point where hand polishing was too expensive for the manufacturer to do on an interface that was “good enough” for the price point of that product.

    Without access to a chronograph or doing elaborate testing I can’t say that polishing the transfer port made a difference but in combination with all the other little tweaks and improvements one of my super-tuned springers shot noticeably faster, smoother and quieter than an untuned one. Back then, I tuned dozens of airguns, and all for the bargain price of 10 boxes of pellets (about $10 in 1960s money). “Boxes of pellets” were the airgun-smith currency back then and I was never short of ammo.

    Cheers!

    • Vana2,

      “With a pneumatic airgun the whole thing needs to be considered as a balanced system and tampering with any one part can (will?) throw the whole thing out of kilter.”

      Boy Hank that’s NO WOOFEN! All too many PCP owners find that out the hard way!

      shootski

  12. The Cardews’ book, “The Airgun, from Trigger to Target” has a chapter (8) dedicated to the air transfer port. They do discuss diameter, length and some discussion as to shape (much like the intake bell of carburetors. One of my many valuable books on airguns (including my signed copy of the R1 book by one Tom Gaylord).

    Fred formerly of the Demokratik Peeples Republik of NJ now happily in chilly GA

  13. I remember reading that Beeman was involved with the first Airgun that was designed using computer simulation. I do not remember whether or not it was the R1 rifle. But I think that at least was in the same time era as when the R1 was first produced.

    It’s interesting to see these experimental tests being done with actual devices. I would guess that if one had access to the appropriate airgun design software (or something similar) and knew how to interpret the results, that a lot of experimentation could be done in a relatively short period of time.

    I think there are a lot of other factors involved and that one of them might involve the gap that the pellet has to travel before engaging the rifling in the barrel. I recently read a book by Jim Owens titled “Sight Alignment Trigger Control and The Big Lie.” This book is about high powered firearms. And he discusses the gap and how it affects accuracy and flyers, etc. Part of this involves how deep the bullet is set in the casing when hand loading ammo. Part of it involves wear and tear on the chamber and barrel. Anyway, I think that this concept might apply to Airguns and pellets. And how deep we set the pellets when loading, the lengths and shapes of the various pellets, etc. Which might help to explain why some pellets are more accurate in certain guns. I think that there are a lot of variables involved. Transfer ports are an important aspect. But not the only one.

  14. I’ve sleeved down a couple transfer ports to .125”. Both were larger and seemed like the piston was contacting the front of the compression tube at moment of firing. One was a Haenel 303 Super and the other was a Hatsan/ Webley .25 Patriot. Solved the noise in both cases

  15. It seems like its a balancing act. You want transfer port airflow, but you also need to create a cushion of air to slow down/stop the piston from smashing into the front of the compression tube.

    It seems like pellet to bore fit also matters, since a tighter fitting pellet would require more pressure to build up behind it before it moves–and that pressure would slow down the piston…

    And I think piston weight (mass) factors in as well. Adding a top hat to the spring would also change how much force/pressure is needed to slow the piston.

    And then how well the piston seal fits the ID of the compression tube matters.

    So is the spring gun voodoo we’re looking for just a combination of several factors that could all be different to get to the same result?

  16. Shootski: I don’t have the academic/engineering credentials to compete here, but what you wrote makes sense to me. I’ve thought, as an untrained shooter (my degrees are in theology and counseling) that the “kick in the butt” is the secret to the spring piston systems. A big blast of high pressure air from a larger cylinder into the “smaller cylinder” behind the pellet would be the kick necessary to move the pellet. There would not be residual gas, or much anyway (unlike a powder burn), to keep pushing much as the pellet travels down the barrel – hence the short barrel length of travel.

    Also, the barrel length would seem to me to be calculated so that there is still enough residual back pressure to slow the piston from its crash into the end of the compression chamber. In other words, a smoothing of the compression chamber opening of the transfer port would be counter to the need of some resistance for the piston at the end of its travel.

    I may be FOS on this whole issue as I am not trained in the engineering and fluid dynamic principles implicit in the whole shot cycle of the spring piston power plants. I go “merrily along” (?) thinking that there has to be a balance between the power impulse to the pellet and the braking effect to buffer the collision of the piston and end of the compression chamber. A certain amount resistance, therefore, it would seem to be necessary in spring piston power plants to preserve the mechanical system at the cost of absolute power and performance of the projectile. It would come down to costs; pellets are cheap, gun parts are not.

    Am I anywhere near on point here, do you think?

    • LFranke,

      I think folks that know way more than i do on the topic might agree with your conclusions. Many things trend in and out of favor in the human experience but as Goethe said “it all depends…”

      shootski

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