Testing the effect of barrel length on a precharged rifle

by Tom Gaylord, a.k.a. B.B. Pelletier

This test was done for blog reader GunFun1, who asked to see what effect barrel length has on velocity. Though it appears simple, this test took 2 days to conduct because of barrel changes and other sundry things. But what was learned far exceeded my hopes, so the effort it took was well worth it.

Conduct
I tested with an AirForce Talon SS, which has the facility to accept interchangable barrels. All testing was done with the rifle in .22 caliber, which means every barrel used was that caliber. I used the factory-installed 12-inch Lothar Walther barrel, an optional 18-inch Lothar Walther barrel and an optional 24-inch Lothar Walther barrel.

I fired 5 shots at each power level with each barrel. I could have shot more; but since I know the stability of the Talon SS powerplant, it really wasn’t necessary. And fewer shots made the test go faster. After every power change, I fired one shot before testing to settle the valve at the new level. I know that’s necessary for the Talon SS. Other PCPs may have different techniques after making power adjustments — including no warmup shots at all. But the SS requires one shot after every power adjustment. After testing each barrel, I refilled the gun to 3,000 psi. The pressure remaining in the reservoir when I began each fill was about 2,600 psi. So for all shot,s the rifle was running right in the middle of the optimum power curve.

I started each test on the highest power, then dialed back as the test advanced. That meant that when I got back to power settings 6 and zero, the rifle was in the middle of the power curve and was at its most stable condition.

I used only a single type of pellet — the 14.3-grain Crosman Premier. Had I used additional pellets, there would have been different velocities. The relationships we’re looking for are revealed in this one pellet as well as if I’d tested greater numbers of pellets. There was no need to waste shots or air.

Because I’m testing with a Talon SS, the air reservoir I’m using is the standard air tank that’s found on both the Talon SS rifle and the Talon┬áthat is the lowest-priced model of the line.

I filled the air tank to 3,000 psi, then shot 5 rounds on the highest power, then 5 at power setting 10, then another 5 at power setting 6 and finally the last 5 at power setting zero. After every power adjustment, one dry-fire shot was taken to set the valve at the new power setting.

Talon barrel test

Analysis
First, let’s look at what happened with all 3 barrels at the maximum power setting. The 12-inch barrel gave an average 835 f.p.s. The 18-inch barrel averaged 921 f.p.s. and the 24-inch barrel averaged 1024 f.p.s. That’s all with the same powerplant, the same amount of air being used with each shot — in fact, everything was the same except for the length of the barrel. This is a clear demonstration of what a longer barrel can do for a precharged gun.

However, there’s one thing we need to note. The barrel I used for the 12-inch barrel is not the same 12-inch barrel that was used in the previous test of rifling twist rates. If you look back at that test, you’ll see that this same rifle shot a Premier pellet on power setting 10 (854 f.p.s.) faster that it did in this test (827 f.p.s.). The reason is probably due to small differences in the individual barrels — the smoothness and diameters of the bores and how well the backs of the barrels seal against the air valve in the reservoir. So, one 12-inch barrel can be different than another 12-inch barrel — even when they’re both produced by the same manufacturer! That’s worth noting. I used to see this all the time when I tested guns while I worked for AirForce, and now I’m showing you what I used to see. A lot of shooters don’t understand or even believe this can be possible. They think that one barrel must be exactly identical to another barrel of the same specifications made by the same manufacturer.

A second thing to note is the fact that the 12-inch barrel didn’t get much more from the maximum power setting than it did on power setting 10 (835 f.p.s. to 824 f.p.s.). In fact, that held true up through the 24-inch barrel, which tells me that power setting 10 is about as high as I need to go to get the most from this particular rifle’s powerplant. Other PCPs that have adjustable power will behave differently than this, but they’ll all have settings that get the most effective use of their air. Anything more than that is just a waste.

Now, look at power setting 6 across all 3 barrels. The velocity increase as the barrel lengthens is smaller with this power setting than with the higher settings. Also, look at the 12-inch barrel between setting 6 and setting zero. That’s where the bulk of the adjustability for this powerplant is when that barrel is installed. But with both the 18-inch and 24-inch barrels, the useful power adjustment range extends all the way up to power setting 10.

Finally, power setting zero had a surprise. The 12-inch barrel was the fastest of the 3. That can be explained by more friction in the other 2 barrels, but it doesn’t explain why the 18-inch barrel was slowest and the 24-inch barrel was faster. That’s one of those anomalies you sometimes see when you test things like this.

I also want to say that the rifle became quieter with the 18-inch barrel, but got noisier again with the 24-inch barrel. You Talon owners don’t have the shrouded barrel the Talon SS owners have, but apparently your barrels are doing a great job of quieting the shots all by themselves.

Summary
Of course, all of this was made possible by the use of a chronograph. No amount of listening to how long it takes the pellet to hit the hickory tree from the back door will give you results like these.

A word from Edith
As many of our readers know, this blog was originally started on the Blogger site/software, and they’re listed in the right-hand column as the Historical Archives. Those blog posts are being moved to this site from Blogger so everything’s in one place. As originally planned, the comments to the old blog would have been lost. Due to the diligence of several of our blog readers who gave me links to help pages that showed how comments could be transferred along with the posts, we’re not losing anything!

The transfer process is almost complete, and the old blogs/comments will soon be available on this site. One caveat: Blogger posts didn’t have categories and tags when this blog first started, and we didn’t start using them when that feature was added later. So, in my spare time (imagine hysterical laughter at this point), I’ll be categorizing the old posts and creating tags that will help you find related items quicker than if you had to do an ordinary word search.

42 thoughts on “Testing the effect of barrel length on a precharged rifle

  1. BB

    Just a friendly reminder. Back in October of 2012 you talked of testing one of Lloyd Sike’s Disco Double air reservoir kits. I have been hankering for that report.


  2. BB
    Thanks for doing the test.
    And I bet your fast at doing those barrel changes on the AirForce guns.

    And one of the things that gets my attention is about the 18″ barrel being quieter.
    It kind of makes me think that the barrel and pellet choice (weight and how good the pellet was sealing to that barrel) helped make the gun be efficient for that tune.

    I don’t know if any body remembers me talking about my .177 Marauder. I was trying to make it quieter and get a reasonable amount of usable shots with good power. That was kind of why I was interested in this test.
    I wanted to cut my barrel off shorter and put more of the factory baffles in. I was worried about if it was going to affect power (fps) and usable shot count.

    Well I got my Talon SS and I learned there was another way to tune guns also.
    Guess what. Cut off the Marauder barrel to add 3 more baffles. And of course re-crowned the barrel.
    Tryed different fill levels and strokes on the striker along with the spring pressure adjusted different ways. And I did still leave the 10# spring in it. And the airflow screw is backed all the way out (I think 4 turns).

    The gun now shoots at 980 fps with only 2 turns on the spring pressure and full amount of stroke on the striker adjustment. I’m using JSB Exact Heavy Diabolo pellets. 10.34 grn.

    And gun is now very quiet. It gets 25 usable shots per fill.
    And oh yeah. 2300 psi fill.

    And I’m still trying to understand why my Talon SS likes the lower fill level.


    • GF1,
      My Talon SS is a Frankenstein version with an old high flow valve and a 26″ .177 HW barrel. It screams 16 grain Eun Jins downrange at around 1100 FPS once I overcome valve lock at about 1800 PSI!

      Right now my SS is back in the lab. I hope that when he sees the light of day again, he will still like only a little air.



      • RR,

        I built that rifle for Mac, as I recall. He wanted the fastest .177 rifle possible, and we used a Weihrauch barrel from Tim MacMurray for the extra length. It is also stunningly accurate. You are right, the valve is an old-style Hi-Flo valve that was built to .22-caliber specifications.

        B.B.


        • It is indeed very accurate after I get the power tuned down! LOL! When I adjust the power to where Eun Jins are at about 800 fps, it will produce 1″ groups at 50 yards. Not bad for a heavy pellet of such poor quality.

          The problem is as you said, the valve is meant for .22. The 12″ Eun Jin barrel I got from you is too short to use what the valve will put out. That is why I am planning to drop an 18″ .22 barrel in it. If you happen to have one you would not mind parting with, bring it to the Roanoke show and maybe we can do the Tarantula Dance. ;o)


    • Maybe I do understand now why my Talon SS is getting good results with the fps at the lower fill pressure.

      Maybe I got lucky and got a good barrel ???


    • GF1

      It’s pretty much normal for AF rifles to work better with less than max on the fill. Some will like some surprisingly low fills. All depends on the rifle.
      You can go through a lot of air and pellets trying to find just the right setup, or do what I did…got it to a place where it worked “good enough” and left it there.

      twotalon



      • BB
        Cant you over come that with the adjustment of the striker and spring.

        I guess the spring in the air valve is more like a regulator if I’m thinking right though.


        • GF1,

          Yes, adjusting the valve return spring will change the behavior of the gun. But so will changing the valve port diameter, the hammer weight, the hammer spring tension and the length of the hammer stroke. It’s all a balancing act.

          No, a valve return spring isn’t quite the same thing as a regulator. But you do get more consistent shots from a gun when it is adjusted to the optimum.

          B.B.


          • BB
            Just one more question regarding a regulator for a air gun. (one quick note, I work with hydraulic systems at work and air systems) I make things in the tool room to automate machines. We even have air and hydraulic systems working together with one PC and even multiple PC’s.

            When we regulate the system pressure for the final out come of the device we are powering. We still have full system pressure (basically like a airguns fill pressure say 3000 psi).
            But when we put the regulator in the system we still have full pressure but now we can control the flow of the air or hydraulic fluid with the regulator for what ever device we are powering.

            Is that how regulating a air gun works? And well two questions. Do you know where a person could get a regulator for a air gun?


  3. B.B.

    Glad you mentioned that first shot to settle the valve after a power adjustment. I could just see someone trying to set up their rifle if they don’t know this. A military expression used by Clint Eastwood in Heartbreak Ridge comes to mind. Can’t repeat it here.

    twotalon


  4. B.B.

    Great figures and great text as usual!
    I may have overlooked that in your blog, but I must also notice that PCPs tend to give more efficiency as a powerplant with heavier pellets.

    Just another accuracy observation. My C62 is converted to HPA (300 bar in reservoir, 110 bar after air pressure regulator, a _tremendous_ number of shots per fill before going direct-flow) and it changed behavior concerning pellets. Before conversion, feeding on CO2, she liked JSB RS pellets, but after conversion RS suffered a considerable decrease in accuracy. However, JSB Exacts and Heavies became laser-accurate both at 25 and 50 m. I believe the reason for that is that lighter pellet tend to be destabilized inside the long barrel sleeve by more escaping gas (air) and heavier pellets seem to be unaffected by that.

    duskwight


    • duskwight,

      Yes, a heavier pellet would give better performance in this rifle — especially with the 2 longer barrels., But this was just a comparison between the barrels — any pellet would work.

      Tour C62 sounds wonderful! A 300-bar fill! Where do you find air at that pressure?

      B.B.


      • B.B.

        It’s just my hands :) A little bit reworked Hill Mk.3 with improved seals.
        Of course, there are tanks – but I don’t like to keep a 400-bar capable tank at home, it’s expensive after all + filling is not this easy, you need to be attentive to gauges. And HPA is quite available and cheap, e.g. in diving shops or firemen stations.

        duskwight


  5. Edith,

    Thank you, thank you, thank you, for all the time and energy that you have spent and will spend enabling the old blogs/comments to be available on this site and categorising and tagging the old posts to make searching that magnificent data base a successful venture for dummies like me.

    Bruce


    • Yes, thank you Edith and the blog readers who assisted. Since things are supposed to live forever on the internet, that would be kind of hard if we were the only exception.

      Matt61


  6. BB and RR

    Sounds like the gun you made for Mac would be a cool gun to have.

    And RR why is it back in the lab ? Sounds like it is a great gun to me the way it is.


  7. BB do you think a Crosman 2240 co2 would perform about the same (or close to) the zero power setting on the AirForce Talon SS with different barrel lengths or would co2 act different.


    • James,

      I have actually done tests on a CO2 rifle to find this out. I used a Quackenbush .22 rifle that started with a 24″ barrel that I cut off one inch at a time to see how the power went. When the barrel got to around 16 inches the rifle was at its most powerful. When the barrel was longer and shorter than that, it slowed the gun down.

      I think the Crosman pistol would perform similarly.

      B.B.





          • BB and james
            I have got a full length Disco barrel and receiver on a 2240 along with the same setup on a 1377 with the 1399 stocks and both guns shoot great.

            But I can see where cutting the barrel down a bit at a time would be good or maybe bad ???

            But how did you know when to stop. I would guess there would be no other way to do it but with a Chrony. But really how did you know when to stop other than cutting one or two more inch’s of till you see your fps start dropping off ?
            Then I guess you get another barrel and cut it off were the test barrel performed the best ?

            I’m interested in this also. I probably got lucky with my .177 Marauder that I modified to get quieter by cutting the barrel and adding the 3 more baffles.

            So I think what you explained may be only the best way to do it.


            • GF1,

              What I did was a test, only. It was not the way to build an airgun. I simply started cutting off the barrel an inch at a time to see what would happen.

              The problem is, the optimum barrel length will differ with each valve. So there is no cutting to a certain length and being sure the length is right until you know the valve very well. But there is probably a range of barrel lengths around 14-18-inches in which many pistol and not-so-powerfiul CO2 rifle valves will be optimum.

              B.B.



              • BB
                I just tryed to post a comment and it directed me to a different screen saying it couldn’t be posted.

                So I may double post here. But I wanted to say thanks good info to know about the 14-18″ barrel lengths for the lower power co2 guns.


                • Gunfun1,

                  Your comment posted properly and just one time. We’ve been having some issues, and I’ve reported them to Pyramyd Air’s IT department.

                  If you or anyone else has a problem posting a comment, please email me about it (edith@pyramydair.com): When it happened (date, time & time zone) and what happened after you hit the SUBMIT button. If you can, send me a screenshot of the page you got or the message you got when trying to post. Another blog reader has told me that he’s occasionally redirected to Pyramyd Air’s website when he submits a comment. That’s been reported to IT, and they’re working on fixing that.

                  Thanks,
                  Edith


  8. I don’t know that I can keep track of all the variables in this test. In terms of the principles, my understanding is that greater barrel length gives you more gas containment and more velocity while the lesser length reduces friction of the pellet. Then you adjust some more, probably to make the barrel shorter, for ergonomics or whatever your shooting purpose might be. But one thing I get out of the test is that additional variables will complicate things even more.

    B.B., you’re right. My mental construct of follow through is probably physically based on what you described. Sort of like the way Jack Dempsey advised people about punching four inches through their target. He wasn’t punching four inches through someone’s body. It was just a description of what was happening at the moment of contact.

    Wulfraed, I didn’t quite follow your drawings about swordmaking. An edge converges metal into a line. How can you insert a groove into it? My understanding of the Viking swords is that they twisted bars of steel. Then they laid three bars on top of another three bars to form of the core of the sword to which they attached more pieces for the double edges. Whatever that is, it is not monolithic by definition. I also read an interesting article about Damascus blades. I don’t know if that really counts as a Western technology although it is certainly not Japanese. My sense is that it was used in the Crusades and while the Crusaders may have used it, the swords originated with the Arabs. Anyway, I believe that process has been lost. Then some fanatic in an obscure group called something like the Knifemaker’s Guild was experimenting around and claimed to have rediscovered the lost process. The distinctive patterning was different from other attempts, having very subtle gradations of color, almost milky in appearance, and the performance was phenomenal.

    As the Good Book says, “Ye know not the day nor the hour…” I think I had a real breakthrough in shooting last night. If I were to put it into words, I would say that I have gone from seeing the “squeeze” as a trigger technique to the whole shooting cycle–the squeeze event! Before my trigger squeeze was foundering on moving from the lighter trigger of my IZH61 to the harder trigger of my B30. I would overstare and get hung up on the trigger. Now, I imagine the movement as a blob in space and time. The blog can deform a little for different guns but not too much. Once the trigger movement enters the blob, it cannot emerge from the blob without the gun discharging. So now my finger naturally compensates for harder and softer triggers within the confines of the blob. I also combine the deliberation of the trigger squeeze with the decisiveness of my old method with what PeteZ was describing as the courage to the take the first shot. Before, I was thinking too much in terms of separate focuses for the shooting technique, but with the blob, I see it all as one. Well, well, I think you would be more impressed with my groups from last night. Anyway, it certainly worked then, so we’ll see if the new approach continues to hold up.

    Here’s a complication on the follow through procedure that may help with my adaptation of shooting to tennis and miniature golf. In shooting, your eye looks at the sight which coincides with the target. You may be focusing on the front sight in preference to the target but at least they are lined up. With tennis, you definitely have to look at the ball when you hit it, but how do you aim precisely for where the shot is going to go? When Monica Seles made her rapid rise in the tennis world, she did a demonstration where she ran back and forth on the court, hitting balls into these ridiculously small areas. (A professional women’s player watching this said it was time for her to retire.) It’s not uncommon for elite players to hit balls with amazing accuracy within inches of the lines. The same problem exists for putting in golf. You can look at the ball or you can look at the hole but you can’t look at both. My shooting analogy is compromised.

    Matt61


    • The key facet that I get from your description of “viking” swords is that they may have been built up from pieces — but the pieces are all similar construction. (Note: none of my books describe such. My most detailed book on sword/knife making does include such oddities as forging a blade from things like steel cables — which results in a fairly “spongy” pattern when acid etched)

      The book I have showing various periods of Japanese swords show two to five /different/ grades of steel/iron that are NOT twisted together — they may have been folded a few times but they remain different. At the simplest, think of taking an automobile leaf spring (a common source, BTW, for SCA “dress” swords), heating it up, and using a chisel to put a groove on one edge. Now inlay that groove with a long tool-steel (say a large metal file). Now forge weld the two pieces together and taper the edge… Coat the spring with clay leaving the file edge exposed and heat treat/temper. Finally polish/grind the cutting edge.

      European swords tended to be the equivalent of just using the spring without the much harder file. The entire blade was then tempered as a unit. European swords tend to be “cold chisels” in terms of edge [after all, they were meant to hit chain and plate armor], while a Japanese sword was closer to a knife [armor being made from bamboo]. I suspect the classic “Damascus” blades were somewhere in between.



  9. Interesting to see shorter barrels have less velocity impact than firearms. I have read .357 Sig reports of a 3.5″ barrel shooting a 125 gr bullet 1250 vs. a 4.5″ barrel at 1350fps+ in my Sig P226. I wonder what velocity change would occur with a .25 Benjamin Marauder with CO2 vs HPA tuned for max velocity. Mine still has factory tune which someone told me was 850 fps with a 25 grain pellet. Mr. BB I would like to see a .25 Marauder specific chart on velocity changes using CO2 vs. HPA and a 16″ vs. 20″ barrel. So far the .25 Marauder is the fastest and most accurate air rifle I own in PCP, 2nd would either be my RWS Meisterschutze or Gamo CFR nitro piston.




      • Mr. BB sounds good, .25 Marauder is a great air rifle it will be interesting to read results. Mine shoots sub .25″ groups at 35 yards with Benjamin domed. It outshoots some of my firearms which is awesome as report is virtually nonexistent in comparison.


  10. That 18″ barrel is severely choked: only 1fps more from the extra power steps? And the 24″ barrel gets 18fps over the same range? It should avg ~ 14fps if the 12″ & 24″ gains were interpolated. So something is afoot. Very draggy, either from extra choke or insufficient break-in or ? Regardless, your observation that there really isn’t anything to be gained from the higher power settings besides wasted air is very much taken to heart: thanks for the data!


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