What is accuracy?: Part 2

by Tom Gaylord
Writing as B.B. Pelletier

Part 1

This report covers:

  • Pellet head size
  • Barrel crown
  • BSA Meteor .177 caliber
  • Gun vibration
  • A case for the PCP
  • Barrel size
  • Choke
  • The right pellet
  • Summary

This report started in the historical section, but today I’m moving it into the mainstream reports. Tuesday I talked about why some new air rifles don’t shoot well. Today the topic is broadened to all airguns.

In Part 1 we learned about all that Dr. Mann did in his 37-year quest to discover why all bullets don’t go into the same hole every time. In that report we learned that things people think work for accuracy, like clamping a gun in a vise, don’t always help.

Today I’m going to talk about accuracy with pellet guns. Dr. Mann showed us that pellets (bullets) will never go to the same place every time, no matter what you do. But what helps bring them together? Why are some airguns accurate and others are not?

Pellet head size

I’ve known for decades that pellets came in different head sizes, because 10-meter ammunition is sold that way. I always assumed the number on the tin was the exact head size, and I had no desire to measure pellets because of the difficulty. Then Jerry Cupples came up with the Pelletgage idea and the rest is history. I will be honest about this. At first I thought Jerry was being too anal with his idea that pellet head size is critical. I say that because when a pellet passes through a barrel, the barrel resizes the head and skirt automatically. But the first test I did in Part 2 of the report showed me just how important head size is. If you follow that link and look at the targets, you’ll see what I mean.

Since testing the Pelletgage many times (now in all 4 smallbore calibers), I no longer have the shadow of a doubt that pellet head size is significant, as far as accuracy in concerned. If you want to be as accurate as possible, you need to shoot pellets with the proper head size — and you can’t rely on the pellet tins to tell you which ones they are.

Barrel crown

Most shooters believe that the finish of the crown or end of a barrel is important to accuracy, but I was a scoffer. I had read where Dr. Mann purposely damaged bullets with pointed screws through the sides of his Pope barrel near the muzzle, so they exited the bore with deep scars along their sides, yet they suffered no accuracy loss. What I failed to realize was that was the side of the bullet and not the end. It turns out when a bullet or pellet exits the muzzle with uneven gas (compressed air) pressure against its base because the crown in uneven, it really matters!

BSA Meteor .177 caliber

I learned this lesson when testing the .177 BSA Meteor rifle that I rebuilt in 2014. If you read Part 8 of that report, you’ll see what I did to perfect the crown of that rifle. Part 9 then shows the results of that recrowning.

Until I did that test I don’t think I was really convinced that the crown was that important. But if you go on You Tube and watch high-speed film of a firearm being fired, you will see the gasses from the burned powder exiting the muzzle before the bullet. If the crown is uneven, those gases will push against one side of the bullet (pellet) harder and start it wandering off course — however slightly.

Gun vibration

I don’t have a film or photo for this one, but my experience tells me I am right, nonetheless. When an airgun shoots smoothly, it tends to be more accurate than when it vibrates and recoils violently. In fact, I have never seen a violent airgun that was accurate. I can tell by the firing cycle if a gun is not going to be accurate, and I think you will see it, too.
That doesn’t mean that a smooth-shooting gun will necessarily be accurate. Often they are, but I have seen it go both ways. I’m just saying that a violent gun will never be accurate.

A case for the PCP

If you buy into that, then you immediately understand why pneumatic guns, especially PCPs, tend to be accurate more often than springers. It’s not a guarantee of any kind, but it is a trend. Guns like the Air Arms TX200 and the Walther Terrus are accurate springers, but please notice that they are much smoother than most spring rifles.
On the other hand, finding a PCP that’s inaccurate is a shock to most veteran airgunners. Such guns do exist, and they are always guns where quality has been sacrificed for price. And yet there are PCPs like the Benjamin Maximus that are inexpensive and yet still accurate, so don’t make decisions on price, alone.

Barrel size

You guys always want to peek behind the curtain. Here is your chance. I won’t reveal who the manufacturer is, but many years ago I was contacted by an airgun maker who wasn’t getting the accuracy they expected from a PCP they had developed. We had several conversations and two things came up. First, the manufacturer was not aware that airgun barrels are different sizes than firearms barrels of the same caliber. In this case, it was a .22 caliber that was in question. The maker wondered why he wasn’t getting any accuracy from a barrel he made using a .22 caliber barrel liner used to refresh .22 rimfires that have been shot out. This was back in the 1990s and I wasn’t aware of the difference, either, but when I slugged the bore (used a lead form to measure the inside of a barrel) I learned that airgun bores of .22 caliber guns are really 0.2165” to 0.218” in diameter. I knew that .22 rimfire barrels measure 0.222” to 0.2235” inches, so the difference is significant. No way will a .22 firearm barrel give acceptable accuracy with a .22 pellet — not even with 5.6mm Eley Wasps.

Choke

At the same time I learned that the manufacturer was not choking their barrels, and I knew that would affect accuracy. Harry Pope always choked his barrels 0.0005-inches about two inches from the muzzle. I told them and they tested a barrel that was choked (and also the correct diameter). The results were astonishing! Yes, choking barrels does work — just don’t go off the deep end! A thousandth of an inch is enough.

The right pellet

I’m not talking about head size now. I’m talking about the entire pellet. It’s length, skirt, where the waist is located, the shape of the head, the center of gravity and even the hardness of the lead all play a part in why a certain gun “likes” a certain pellet. And some guns like more pellets than others. You only learn this through experimentation. The right pellet makes all the difference in the accuracy of an airgun.

I have a small selection of pellets that I trust to usually be accurate. But every once in awhile I am surprised by something. Most recently I was surprised by the accuracy of RWS Superpoints and also by the accuracy of vintage Eley Wasps. But there is more to this than just guessing. If your airgun was made in Germany then it’s probably going to like German-made pellets. But if it’s from the UK and made between 1905 and 1960, it’s probably going to do better with larger pellets like the .22 caliber Wasps (the .177 Wasp was never that large). If it’s a powerful springer, look for a pellet that has a thicker skirt, but if it’s a taploader, it probably likes thin skirts of soft lead. PCPs nearly all like pellets made from softer lead, except some, like the Crosman PCPs, that seem to like hard pellets. These are some of the many things you learn about which guns may perform best with which pellets.

Summary

I promised this report back in August and I’m delivering it today. I don’t see a Part 3, but perhaps your comments will inspire one.

35 thoughts on “What is accuracy?: Part 2

  1. BB,
    Nice report, i agree with all you said but i have a doubt with the example you gave for the barrel size. After i read the post i searched the lothar walther site and for the .22 airgun barrel the specs are: lands dia .215 grooves dia .221 twist 17.7″. For the .22lr these are: lands dia .216 ,grooves dia .221 ,twist 16.4″ and for the .22short are: lands dia .208 ,grooves dia .215 and twist 9.8″ . For the other .22 calibers they give lands dia .219″ and grooves dia .222″ to .224″. I don’t know if other manufactures have different specs on their barrels but as for the LW barrels this is what they advertize.
    Any thoughts , thanks


    • The SAAMI spec for both 22 Long Rifle – Sporting and 22 Long Rifle – Match barrels is .217 / .222, which is a thou larger in diameter than what you quoted. You can search and download the print from the SAAMI site for free. In slugging and sleeving barrels myself, including a 22 cal. air rifle barrel, I too found the differences to be significant. Furthermore and L-W barrels notwithstanding, many air rifle barrels have shallower rifling than .22 LR barrels–sometimes too shallow (Benji 392) in my opinion!



        • B.B., I was just looking at the .22 LR numbers in Bullseye’s post, which put the 22LR diameter to be identical with the L-W airgun barrel spec (.221). If we want to compare an airgun barrel diameter to a 22LR diameter, we must either slug a large number of samples or compare some published specs. The L-W spec was a good choice, I think (though comparing specs can often be “apples and oranges” for a variety of reasons too) and the SAAMI specs seem like a good starting point to me too. The small number of barrels and sleeves I’ve slugged have correlated well with SAAMI.

          The SAAMI diameter is larger (.222) than the L-W diameter and your .22LR numbers too. (“I knew that .22 rimfire barrels measure 0.222” to 0.2235” inches.”) I agree with you that the larger 22LR diameter has significant accuracy implications.


          • In summary, I meant to say that both your number and the SAAMI spec are larger than the L-W published spec, whereas Bullseye’s number was equal to the L-W spec. In truth, 22LR diameters ARE larger than AG diameters and I agree with you that the difference is significant.





  2. B.B.,

    Very nice addition to Part 1. I even went back a read the supplied links, less 1or 2,… and even searched things like “chokes” to further refresh my memory. This,.. along with “Why won’t my new air rifle shoot well?”,.. just about covers it all.

    The “take away”?,……. there is a lot involved to accuracy. Some things you can change,.. and others not. Sometimes there will no definitive answers. Don’t give up. Utilize this blog and all that is offered in the archives. Surely something here will help.

    Most of all,…. when you have an air gun that is accurate,….. hang onto it! A lot of factors have come together in one place and time,.. just right,.. to make it that way.

    Chris


  3. Pingback: What is accuracy?: Part 2 | Airguns: Air Rifles and Pistols

  4. BB

    Must read for anyone interested in accuracy and who isn’t? The mind boggling number of variables involved is what makes the pursuit so fun. Then there are the human variables, some mental some physical. My HW30S .177 cal. is accurate with a number of different pellets and head sizes, but a 4.51 mm will have a different POI vs a 4.53 mm (same pellet type). Someone shooting brown box Premiers without measuring head diameter is not going to be happy and may think these pellets aren’t accurate. Well they are accurate when sorted in many airguns.

    Another: My Hatsun 95 Vortex .177 cal. is surprisingly accurate with 10.5 grain pellets. But if the POI starts wandering up and to the right I know from history that it is time to tighten the forward stock machine screw. It is not loose, just wants a 1/4 inch turn.

    Decksniper


  5. BB, what makes a good barrel? I mean in all smallbore air rifles the twist rate is the same, then why is the lothar walther barrel so much more coveted than a chinese barrel ? What does lothar walther do different from the others that their barrels are so accurate?



  6. B.B.,Another very interesting blog, as usual ! By the way,I picked up a Markmans 2070 from PA because of a two-part review I read. Maybe you could do a blog on this very inexpensive .177 Break Barrel.
    PZ1


  7. Hi BB, very interesting observation on barrel crown condition. I have noticed “good” crowns have crisp profiles of each land showing, correct? if a well used barrel crown is not showing much of the lands, does accuracy suffer then? what about recrowning and what does that involve?

    Regards,
    Peter


    • peter,

      Yes, recrowning does improve accuracy sometimes. It involves cutting the crown evenly, as you observed.

      I am not an expert on doing this, other than to say what I have already said, but there is a “redneck crown” that seems to get good results. I would sure like a guest blog on that!

      B.B.


      • Recrowning ideally needs a lathe and a knowledgeable operator, but can be done with hacksaw, files, ball lap, etc. The redneck crown job just eliminates burrs at muzzle, but since that is probably #1 issue, it is often successful and rarely hurts. I would almost swear someone (maybe Vince or Derrick or Fred) did a blog on crowning, but maybe it was in comments.

        Cleaning/polishing barrels and recrowning barrels (outside of center-fire) mainly should be reserved for barrels that almost group well but throw fliers regularly. I don’t think it helps barrels that throw big round groups, something else going on…

        Always look for star on muzzle! Even airguns show a pattern with the star limbs extending from the grooves. Should be clean and symmetrical.

        Just some random thoughts:)!



  8. Dear BB, I would also be very interested. Moreover, a blog about this topic would be of great interest, I believe, for many people shooting air guns here in Argentina. We have very active discussions about this point within the context of the possibility of manufacturing here barrels which could or could not be comparable to those imported. You know, as in any “third world” country, here we oscillate between two extremes: 1) low dollar but terrible bureaucracy issues making almost impossible to import anything, and 2) freedom to import, but high dollar, and the consequent almost impossibility (for most people) of buying anything from other countries. This pathology of our economy obviously translates to air guns and more specifically to the question about manufacturing or not barrels.



  9. B.B.,

    I too would be interested in a barrel exclusive article. I know from reading past articles,… that you have covered most,.. if not all aspects,.. at one time or another. To have 1 article, in 1 place, would be a real treat. Maybe even worthy of it’s own category in the “categories” archives. One suggestion,… link it heavily. That way you could hit all of the main/important points and for those that are interested,… can read on. Also,… link to the (latest) article even it is part 7 in a 9 part review. That way everything on the subject can be read without searching for parts 8 and 9.

    Maybe easy, maybe not. Surely you have some method of referring back to past topics in some orderly fashion.

    At any rate,…. just a few ideas and will be looking forwards to the article when you can fit it in.

    Chris


  10. You’ll find that British airguns, right into the late 70’s were all 22 rather than 5.5, some even into the eighties.
    I keep banging this definition home, the Wasp 177’s were never large in the same way as the 22 because they were 177 (or rather they were always the continental 4.5mm size and 177 is merely the imperial measurement of that figure)
    When mainland European manufacturers started to adopt the 22 as a calibre, it made sense for them to metricise the British standard 22 to 5.5mm, it was with great and costly reluctance that BSA and Webley followed suit, and that was initiated, mainly by the far superior German pellets that were coming on to the market.
    The Wasps were far and away the best British pellets of the era, and were very light too, but sat next to a H&N in 1982 they looked like they had been made by an idiot, the thin skirts always meant one out of 30 was a throw away and, en masse the British airgunner went for the German ammo, despite the loose fit.
    The genesis of both calibres differ, the 4.5 came from continental gallery shooting, and the British pellet manufacturers fell into line and produced that size, 22 was always the preferred British calibre because the main use for airguns was a bunny for the pot
    When the Germans started to make 22’s it is, somewhat stereotypically German that they efficiently and rather arrogantly metricized the calibre instead of accommodating the British rifles, in precisely the opposite way to the British accommodation of the 4.5mm
    This is why we have to declare war on Germany every now and again (and France, just because they are nearest)
    Amongst other more trivial reasons.


  11. By way of introduction, I am a retired engineer with over four decades of experience in mechanical and electrical engineering. I have read numerous posts on the subject of pellet variability and how it relates to trajectory variability. I have never seen anyone give the accuracy of their measurement instrument, the details of their method, the number of measurements per item, calibration, error determination, or statistical analysis of the data. Measuring differences of 0.01mm is 10 microns, requiring a measurement device AND method accurate to better than about 3 microns. Do you have any idea how small that is? Its on the order of 1/10,000 inch. Sizing with a gage will only address a maximum dimension, and say nothing about cylidricity.
    But pellets get sized in the breech. Closing a bolt without a pellet gives very little resistance, but insert a pellet and it requires some force. I find that most pellets require about the same force to close the bolt, but occasionally one is easier than typical, or harder. Experiment: use a force gage (like for trigger pull) to measure the bolt closing force on a couple dozen pellets.
    You used a PCP rifle, which has a know variation in MV with shot sequence, and variability withing the sequence number. I have calculated that the approximate sensitivity for MV is about 8%/MOA, that is a MV variation of 2% will result in a drop variation of ~1/4 MOA. In order to hold a 2% MV variation over ~98% of shots fired, you really need to demonstrate better than 0.7% sd in MV, or about 6 ft/s.
    It is possible to statistically extract MV variability from shot sequencing effects from pellet diameter effects in a carefully structured experimental plan. I haven’t seen anything remotely like this posted anywhere.
    My sensitivity analysis of pellet mass indicates that you would need mass variations on the order of 3.5% in order to see a 1/4 MOA variation. Mass sorting is a complete waste of time.
    I investigated pellet geometry briefly. I took a few 0.177 pellets and filed the skirt or nose, fairly aggressively. I shot these with the filing located right, and didn’t see a large deviation to the side. I was in a seated position, not bench rest. I expected to see a major deviation after reading about exceedingly small mass and dimension effects. I can’t distinguish an effect. At some point more filing would show an effect, but a careful study would be required.


  12. BB…though I am not very experienced, I enjoy target shooting. I enjoy your blog and use it as a training tool.

    Regarding accuracy, I am hoping you can give me some advise relating to my short range. I have a target setup that is only 25′. I am using the NRA official 25′ air rifle target. My two favorite rifles are the CZ 200T PCP and the Weihrauch HW 30. Both have open sights.
    With the PCP 10shots, my best has been 5/16. I was doing 5 shots until recently when I had my best at 1/16. As this is such a close target, I am not sure what he expected outcome should be. I do not use a rest but I am sitting and have a elbow on a table. I find that there are 3 pellets that seem to have the same range…the meisterkugln 8.2, premier super match and R10 7.0. The best group was using the Super Match.
    With the Weihrauch HW30, being a spring piston and single load, I am still only at the 5 shot level. My best is 4/16, but most are 6-7/16.
    I would appreciate some bench marks for the short range and any advise you might pass on…. Thank you…..Katie


    • Katie,

      I’m sorry I took so long to get back to you.

      Okay, for 5 shots at 25 feet, 1/16-inch is about as good as it’s ever going to get. That’s 0.0625-inches. That is what top 10-meter rifles will do at 33 feet. I would say you are there — especially since you aren’t shooting off a sandbag!

      Your HW30 is getting an average of 0.375-0.5-inches. You might try the artillery hold and see if that can’t go a little tighter. I would think 5 shots in 0.25-inches (2/8-inches or 4/16) is about as good as it will get at 25 feet.

      B.B.


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