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Air Guns The importance of bullet-to-barrel alignment and fit: Part One

The importance of bullet-to-barrel alignment and fit: Part One

This report covers:

  • Why go to the effort?
  • Ten shots in 0.2-inches at 200 yards
  • So what?
  • Other airgun breeches
  • The Crosman 3622 breech
  • Always liked single shots
  • Summary

Harry Pope was considered the world’s most accurate barrel maker for more than a century. He made breechloading rifles whose bullets were loaded from the muzzle and rammed down the barrel to the front of the loaded cartridge that was already in the chamber. This process took longer than simply loading a cartridge, but Pope demonstrated that 100 rounds could be loaded and fired for record in 130 minutes. That seems slow today, but bear in mind that the pace of these contests was not rushed.

muzzleloading breechloader
If you ordered a Pope muzzle-loading breech loader you got this bullet starter with it — along with a false muzzle, a bullet mold, a bullet lubrication pump and a powder flask. From the book,
The Story of Pope’s Barrels, by Ray M. Smith copyright Stackpole, 1960.

Why go to the effort?

Why go to all the effort of doing this when breech-loaded cartridges were available? The answer? Accuracy. We still do things like this today. When a field target competitor loads a pellet and then discovers some damage to the skirt, he doesn’t shoot it for record; he blows it off by firing it into the ground, after announcing his intentions. That is the same level of fanaticism that’s found in any target-shooting competition.

Ten shots in 0.2-inches at 200 yards

Sometime around the turn of the 20th century Harry Pope shot ten shots with his personal muzzle-loading breech-loading .33 caliber rifle at 200 yards that measures 0.20-inches between centers. It stands as the best group ever shot at that distance, but the actual target was blown into a river and never recovered, so it never made it into the record books. Harry Pope measured it when he retrieved the target, before it blew away. He was so respected for his honesty that the group was accepted by shooters, nevertheless.

200-yard group
Yes, that is the trime coin next to what I believe is the smallest 200-yard 10-shot group ever shot from a conventional rifle. From the book,
The Story of Pope’s Barrels.

There has been no record 10-shot group shot at 200 yards since that time that exceeds this one. According to the National Bench Rest Shooters Association the record for 10 shots at 200 yards is 0.202-inches, shot by Fred Hasecuster on September 16, 1989.

So what?

The title of this report is The importance of bullet-to-barrel alignment and fit, so what does muzzle loading the bullet have to do with that? A lot, as it turns out. Pope knew that the base of the bullet was the most critical area because of the gunpowder’s gas acting on it unevenly as the bullet left the muzzle. A breech-loaded bullet has small lead “fins” put there by the rifling as the bullet passes down the bore. Muzzle-loaded bullets have the same fins at the front of the bullet, where they don’t cause inaccuracy problems.

bullet fins
When the bullet starts at the breech, it develops fins of lead at its base from the rifling.

How does this relate to airguns?

It relates in several important ways. First, we know that a pellet that is deliberately (and correctly) loaded into the breech before the shot will almost always be more accurate than one that is blown into the breech from a magazine at firing.

Take 10-meter target pistols for instance. I competed in 10-meter air pistol at the national level for several years with a Chameleon from Aeron. It had two strikes against it. It was CO2 and it blew the pellet into the breech upon firing. It was a single shot, but it was loaded via a swing-out chamber that aligned with the breech like a magazine. I believe my choice of pistol probably cost me 5 points per 600-point match.

My FWB P44, in sharp contrast, is a PCP that has a hollow bolt probe to seat the pellet directly in the breech when the bolt is closed. It may not look like such a big deal, but for ultimate accuracy (and confidence) it makes a big difference.

Other airgun breeches

The RAW HM1000 rifle action has a special bolt probe that aligns the pellet with the barrel as it inserts it into the rifling. It’s my personal belief that this bolt one reason why RAW air rifles are at the top of the PCP heap.

Hunting Guide

The Crosman 3622 breech

I didn’t mention this in Part 6 but there is a lip in the Crosman 3622‘s breech that prevents pellets from being pushed straight in. The rifle has to be held down and the pellet coaxed over that lip to enter. That might have shaved lead from the pellet as it was loaded. And the pellet may not have been aligned properly with the bore.

Always liked single shots

I think most readers know that I prefer single shots over repeaters, and today’s subject explains a big reason why. A rifle is not accurate just because it’s a single shot. If you only shoot commercial ammo, a single shot doesn’t give you any advantage over a repeater. But if you reload, a single shot allows you to tailor your ammo so that — wait for it — your bullets are precisely aligned with the bore! I am one of the few people on earth who shoots an AR-15 single shot, and my AR is quite accurate.

Summary

Today we have explored part of the reason why bullet-to-barrel fit is important. There is more to this discussion — the fit of the bullet to the barrel/bore. It’s coming next.

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 “The importance of bullet-to-barrel alignment and fit: Part One”

  1. BB,
    You can be sure I ‘m paying close attention to this Blog.
    I did mention that we don’t usually get to see the pellet after it is shot to see what kind of distortion it has after entering the breach and being forced to rotate through possibly very sharp-edged rifling, but I never considered the displaced led hanging off the back of a pellet. I imagen some could drop off and then some could hang on and totally throw the balance off the pellet or slug. And … would the small shavings, possibly moving faster than the pellet push it out of wack leaving the barrel?
    The pellet to bore diameter match would probably play a big part here also and the rifling engagement design at the breach. Would it squeeze the pellet or immediately shave it.

    I recently had that problem with long heavy pellets folding over when inserted them, or tried to, in the breach of my P5 Magnum Pistol. The rifling was not recessed enough … Pause … Just checked, there is no taper, or recess to the rifling at all. There is just a very light countersink job at the breach, and it is actually off-center so much that it is hardly noticeable at all on the top of the breach. No wonder. By the way the pellet heads were trashed when it cocked over.

    I assumed there was some taper because the heads of most pellets slid right in, and the skirts got really tight.
    Wow, I may be using pellets with too small of a head diameter to even engage the rifling. The reason for on and off accuracy? I will need to do some modification there with a larger drill and pointed countersink to avoid opening the bore too much.

    Started thinking, how can he see a hole smaller than a trime at 200 yards. Doesn’t have to. A large dark round target is what you would aim at and keeping it centered with some cross lines would put the shot in the center. Corrected for elevation I’m sure.

    • Bob,

      Beyond a tiny bit of careful reaming or polishing with the proper equipment (like a lathe and run-out gauges for setup) your chance of making things worse are very high.

      Best to find a pellet that fits well (or get a pellet sizer to adjust the pellet) than trying to modify the breech.

      Removing an obvious burr or adding a slight (.005 inch) radius to break a sharp edge could be ok but I’d be cautious doing that.

      Fixer-upppers can be fun. I enjoy taking an average product and fixing it up to have it perform at a higher level. Amazing what can be done with upgraded hardware, a bit of shimming/fitting and a some spit and polish. That’s fun if you have the tools, knowledge and experience to do so.

      For most people, they are better off buying at the level of performance that they want/need. It’s easier to invest some more money than suffering the frustration. LOL!

      As far as one-hole groups go (presuming that the shooter is capable) there are two main approaches. In fixed factory tuned airguns (like most springers) you have to find the “golden pellet” that suits the (built in) tune and harmonics of the airgun. With user tunable airguns (like some PCPs) you have the option of adjusting the power to suit the pellet and the harmonics of the rifle. Harmonics tuning is an other subject.

      All that being said, I’ve found that most decent quality PCPs will easily shoot 1/4″ groups at 25 yards – straight out of the box, with the factory tune and unsorted good quality pellets.

      Hank

  2. Tom,

    The fact that Pope muzzle loaded his bullets ensured that the resulting projectiles was fitted to the lands and groves of the barrel before it was fired. JerryC’s Pelletgage does work for those looking to get the best fitting selected pellets down their barrel. Next you’ll probably dealing with the fit of pellets and the importance of pure lead in contrast with alloy pellets.

    Siraniko

    • Siraniko,

      I’m thinking that sharp lip at the breech is a real problem. I intend trying at least one more pellet and a different loading technique to test my theory. This rifle may be accurate yet!

      BB

    • Elmer,

      These are “standard” on these airguns, but not quite as long. They can also be used in the Discovery / Maximus / Fortitude / 3622 platforms. There are also other aftermarket manufacturers of this product. The longer probe is to ensure the pellet / “slug” is pushed beyond the transfer port.

      These help, but more attention needs to be paid to the breech / rifling taper. Another issue that many fail to heed is the transfer port. This can have a “burr” on the inside of the barrel.

      • RR, I currently have the barrel out of the 3622. I am looking at the breech and see what you mean about a longer probe. The factory probe appears to only push the pellet about half-way past the transfer port. The product I linked to also has a hollow probe and a hole that lines up with the transfer port (see video). I do see a shiny place that could be a burr on the bolt side of the transfer port inside the breech. I am planning to look at it closer. So far it doesn’t appear to have affected the pellets. But if it is sticking up into the transfer port I plan to do something about it. Is there a tool or method to smooth a burr down that you would recommend?

        • Elmer,

          A transfer port burr is caused by the tool bit breaking through the side wall of the barrel pushing up a skim of metal. It’s typically very small and will break off after a couple of shots, getting more polished as the airgun is broken in.

          If the tool bit is dull, spinning too slow, or being fed too fast, you can end up with a burr that is pronounced.

          You can check for transfer port burrs by chambering a pellet, then pushing it back out with a cleaning rod, and inspecting for damage/scarring. Blackening the pellet with a felt marker makes shiny damaged areas easier to see.

          It’s best to remove the barrel (as you have done) for deburring because you want to avoid getting bits of metal or abrasive grit into the valve system. On barrels that can’t be removed, I fill the transfer port with a “plug” of thick grease and shoot the grease out (safely) afterwards.

          If you have the barrel off and can access the outside of the transfer port, depending on the size (diameter and depth) of the hole, you can often remove the burr by polishing the inside of the transfer port and (slightly: .005″) rounding the edges that the pellet might contact.

          You can often just use a wood dowel to remove a burr. You will feel the sharp burr dig into the wood and working it back and forth will break it off. You can also use the flat end of the dowel to feel if the burr is gone or not. Sometimes you have to shoot the airgun to raise the burr before pushing it down again to break it off. You may need to do this a couple of times.

          I also use fine (400 grit) emery cloth (a type of sandpaper) rolled into a tube or wrapped/glued to a suitable (wood, plastic or insulation covered electric wire) round rod to polish off the burr. You have to be careful when using an abrasive – it’s easy to remove metal from where you don’t want to.

          Hope this helps!

          Hank

            • I second what Hank has to say about removing said burrs. While you have the barrel off you may wish to countersink the breech end of your barrel some. It will make the loading process much easier and reduce the chances of damaging the pellet / “slug” upon loading. It will not take much to do such. The smoother, the better as any abrasiveness can cause damage to the projectile, although with use the metal will be polished or filled smooth.

    • Elmer,

      I’ll second what RR said about burrs around the transfer port in the barrel.

      Seen plenty of them an make a habit of checking the breech of every new PCP with a bore-scope.

      Most burrs are small and wear off as part of the break in.

      Hank

        • Thanks RR!

          Sorry, but that warm weather deal with my Australian friend fell through (shipping issues) – looks like we’re in for some cold temperatures for the next couple of months! 😉

          And I was hoping for a January thaw to get some outdoor trigger time to sight in the new PCP. 🙁

          Hank

          • Hank,

            I am saddened that the shipping issues were a problem, but I am not surprised.

            You can always shoot from your basement to outside. I myself intend to do such once my new garage is built, scaredy cat dog or not. That is of course after I experience a bit of thaw around here.

  3. B.B.

    I hope BobM realizes that FX has moved away from the smooth twist barrel liners?
    Also, please pay attention to the variation-however slight- in the velocity of the projectile.
    Even small variations lead to inaccuracy at distance.

    -Y

  4. BB,

    I have always liked single shots myself. This allows for at least a cursory inspection and orientation of each round, whether you do such or not. My dad and I were also very anal when reloading.

    Much importance is placed on the crown of the barrel, as it should be. Though the Avenger / AvengeX lack such, a true, clean cut will do just as nicely, as has been demonstrated by Nova Vista. As Bob M has pointed out, not much effort has been put forth with the countersinking of the breech and just as importantly, with the ramping of the rifling at the breech. The Discovery / Maximus / Fortitude / 3622 would all benefit from a countersink at the breech of the barrel. They would also benefit from tapering of the rifling. Unfortunately, those are steps which can and will add greatly to the cost of these airguns.

    This may be why Bob M and others prize the FX so highly. Once upon a time, after much experimentation, they utilized “smooth twist” barrels. These were smooth bore until the last couple of inches of the barrel. Near the muzzle, the barrel was machined thinner to allow rifling to be “pressed” into them. This would cause a taper in the beginning of the rifling and provide the spin to stabilize the pellet.

    Now they use a two-piece barrel where the shooter selects the type of rifling to be inserted into the outer barrel tube. The rifling is “pressed” into the thinner walled insert. The rifling of each insert is designed to provide the optimum performance for a particular pellet or “slug”. Talk about anal. Talk about expensive. 

  5. BB,

    I’ve long since believed that misaligned projectiles (the axis not parallel to the bore) is the root cause of random fliers.

    The projectile starts off on the wrong foot and it gets worse when it gets put into motion. The higher the velocity, the bigger the problem.

    Now, I just need to do some testing (in warm weather) to see if I can deliberately/consistently create fliers.

    Hank

    • Vana2,

      The static misalignment is certainly to be avoided because when the next shoe drops it gets much worse for potential accuracy and repeatability. That next shoe is the dynamic effect of misalignment which you note as being effected more by velocity increases which is caused by the axial center of Mass and center of Form axis being compromised. IF the absolute Center of Mass is aft (behind) the center of pressure then the wobble will be dampt over time…IF NOT (reversed position of Cp and Cm) it will increase with time until balance is achieved often with the projectile flying sideways into the target.

      The problem we have is that we typically think about each of the variables as an individual THING to fix instead of realizing that some variables are INDEPENDENT and others are DEPENDENT to smaller or larger degree. Although you often hear the quip that shooting airguns is NOT ROCKET SCIENCE it is however Ballistics; Internal, External, and Terminal!

      Ballistics are actually far more complex than ROCKET SCIENCE will ever be…in my opinion…
      and then there is the WIND. No wonder Bob M is frustrated and wants a CHECK LIST of how to go about banishing the ACCURACY DIABLOS of diabolo shooting.

      shootski

  6. Since the subject is barrels, bullets and pellets, sharing this example of pellet impact, penetration and trajectory captured by the sardine can subjected to the test; this was no FM Trick Shot – he could never duplicate it in a billion years.

    The pellet was a Crosman .177 7.9-grainer fired thru the resident HW30 at 25 yards. That was a fun can-killing plinking session yesterday – as usual that little rifle shines even with FM’s sub-optimal eyeballs.

  7. ” I am one of the few people on earth who shoots an AR-15 single shot, and my AR is quite accurate.”

    BB,
    Now that is quite interesting; I hope you’ll elaborate on that in a future report. 😉
    Blessings to you,
    dave

      • Thanks, BB!
        I couldn’t get part 2 to load, but I checked through the comments on parts 1, 3, & 4 and there is nothing from me, so I must have missed that set somehow; thank you. 😉
        Blessings to you,
        dave

      • I too noticed several broken links in old blogs. AR-15 Part 2 is another example:

        404 Error: page not found
        Oops! That page can’t be found.
        It looks like nothing was found at this location. Maybe try searching?

          • Sorry BB, I’m still getting that error 404. I went and cleared the cache on my browser. Nothing. I did a search for part 2 and got links to part 3 and 4 only. I will try on a different computer.

            I hope this is not a problem with the blog, because it is such a valuable resource to me to be able to search the archives.

          • I started at part 3 and then used the linked at the bottom of each blog post (just above the comments) to try to go sequentially back to part 2. I found the following posts missing (and I accounted for weekends):
            March 15, 2013,
            March 11, 2013,
            March 6, 2013,
            February 25, 2013,
            February 19, 2013,
            February 15, 2013,
            February 11, 2013,
            February 8, 2013

            I stopped when I reached February 1, 2013, because by looking at your link, Part should be somewhere in February 2013.

            I plan to try again on a different browser when I have time. But what would be important is to preserve a searchable archive of this blog. It’s an invaluable resource of airgun information, not to mention a lot of other interesting stuff.

              • No problem, as long as we can determine the scope of the problem and get it fixed. I’m happy to help if I can on my end by letting you know how things look from my end. First I want to try looking from my laptop (rather than from my “smart” phone). I’ll let you know.

            • Quick update #2 for you, B.B., and IT:

              I tried from another computer with both Chrome and Edge browsers, and I can’t get to the pages for the dates listed above by starting at the AR Part 3 blog and using the links just above the comment section to go sequentially back through March and February of 2013. It is as if your blog entries for those dates simply don’t exist, and the links take me to the next prior blog entry. Let me know if you have any other questions or if I can help in any way to restore those pages or test that they are visible one again.

          • B.B.,

            In Part 1, Where no man has gone: Para. 3 of your AR-15 blog series in the comments Edith, your very capable Editor, discussed not changing your Bass Catcher mix-up because she found it funny.
            I had no idea it was an inside joke as well!
            My spouse loved to catch Sea Bass in the Bay of Cadiz she typically returned with two to three three footers weighing twenty or so pound each; the rest she returned to grow a bit more.

            God willing you will see her forever in the future.
            It will seem as in no time at all compared to the time you two will share rejoined in the beyond!

            shootski

    • I am considering removing some of the abrupt rifling at the breach with a No2 EZ out. If it works, it will taper it.
      If not, I will drill it out enough to permit the pellet to straighten out before it engages the rifling. Not much to lose here.

  8. Bob, my particular pistol also has the breech bore off set and it shoots high and to the left no matter how I try to fix the rear sight.

    However, it can shoot a tight five shot single hole group at ten meters if I do my part.

    • Kevin,
      When I first got this pistol, it shot accurately. I must have used a pellet that passed through that breach without much harm. I have since found others are too loose and some are too tight, and accuracy disappeared with them. Thought it was me until I just looked close at the breach and found the offset taper or countersink job and sharp, uneven, rifling at the very beginning of the bore.
      No telling how much that breach distorted pellets in an uneven manner when inserted. The bore and rifling look good and straight Just need to clean it up some if I can. Hate to get locked into one pellet, if I can find that one again.
      Can’t find the key to my big roll-a-way tool chest with special tools in it. I don’t fix aircraft anymore. Been months since I opened it.
      Seems like everything I want to do these days requires three other things getting done first. Mostly finding where I left something months or years ago. Too much stuff.

      • Bob, the muzzle end is slightly choked and if the crown is still in good condition, it’s not necessary to fix the off setted breech. I would just polish it with a little bit of flitz.

        The pivot screw nut has to have the correct tension allowing the barrel to stay in place in the cocking arc without falling by itself when closing the barrel.

        • Keven,
          My plan might be a bit of overkill. I will move slowly and see how things go. Barrel is snug.

          The P5 Magnum has a removable over-barrel extension that holds the front sight. Inner barrel muzzle is flat and untouched, but I extended the over-barrel out a bit for more cocking leverage and have a 2″ void the diameter of the barrel before it reaches the perfectly formed second outer muzzle.
          It did not seem to affect the good performing pellet I used first.
          Air guns on hold till Feb. Need to move a disabled friend to AZ from CA and set up his life there in a memory care home.

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