Headspacing Rimfire Ammo for Accuracy

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  • Headspacing Rimfire Ammo for Accuracy

    So, want better and more consistant accuracy from your Rimfire Rifle and / or Pistol.

    There is a pretty easy way to improve both and that is to sort your ammo by Rim Thickness in various grades of thickness of the rim. A Rimfire is Headspaced on it's rim and as the thickness here can vary somewhat it affects overall accuracy when the cartridge sits in the chamber at different depths and / or position from the Bolt Face. Keeping this constant through a number of rounds helps a lot with accuracy and reducing that crazy wild flier that seems to creep into any group.

    Target shooters do this measurement but it will help with any form of shooting a Rimfire even hunting to know you can be more consistant with your point of impact.

    There are various Gauges you can buy to achieve this measurement and costing upwards of $70.00 or more but one can be made fairly easily from a fired .22 calibre centrefire cartridge. My method is just one and depends on the equipment you have at hand but start by finding a fired .22 calibre case that your Rimfire Ammo will slide down the neck of easily. Do not size the case but remove the spent primer so it doesn't interfere with your measurements.

    I came up with an old .222R Case and a .22 Hornet Case. The later worked out better as it fitted into my Sinclair 30Cal Bullet Comparator Insert and made measurements a little easier and more consistant.

    The first step I did was using my Wilson Case Trimmer to square the Case Neck plus reverse in the Trimmer and square the Case Base so that both surfaces are now parallel to each other. There are probably other ways to achieve the same result and if you don't have anything or are unable to do this step then I would suggest you mark the case so that you are taking measurements from the same position each time.

    Trim Case Neck ....



    Trim Case Base (Head) .....



    The neck now trimmed ....



    The base now trimmed first the .222R case...



    The .22 Hornet Base trimmed and sitting in the Sinclair 30Cal Bullet Comparator Insert....







    The assembly mounted into the Comparator Body and Anvil attached to Digital Calipres with .22 Rimfire ready to measure.



    Zero Calipres on the cartridge case....



    Insert your ammo, take measurement and sort in either 0.001" lots or even better.



    The same proceedure without a Bullet Comparator to use. Just try and be consistant with the position of the measuring case and how it sits between the jaws of your calipres.....







    I use ammo made by Lapua, SK Rifle Match and found that this was the most consistant with headspace measurements, I had about 25 out of three boxes (150) I sorted that were outside the main dimension. The other ammo I had handy was two boxes (100) of SK Standard Plus and out of those I ended up with three main rim thickness sortings. Even though both types of these have the same specifications like velocity I have the feeling that the SK Rifle Match might already have been rough sorted in the factory.



    I have done this to all sorts of rimfire ammunition and achieved the same results, more consistancy and much better accuracy. Even with the likes of .17HMR it was the same.

    Would have shown some targets but during this week it has been blowing a gale with up to 50km winds so I'm not going to attempt a demo with sub velocity ammo under these conditions. Test targets and photos to follow in time.

    Hope this helps someone.

  • #2
    Good info, I use SK Pistol Match Spezial and found by sorting out the different rim thickness I have cut out the odd flyers

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    • #3
      I used the case method. Just tried some old Stirling ammo. About half was 0.052", the other almost half was 0.53", with one single round 0.048".

      Did the same with a quantity of Remington Golden Bullet (percentages rounded a little for convenience):
      10% 0.041"
      10% 0.042"
      30% 0.043"
      25% 0.044"
      25% 0.045"

      Re-checked them and that's accurate enough. Is that right? Seems like it's all over the shop. I'll try some other brands.

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      • #4
        Bushman HP
        15% 0.039"
        45% 0.040"
        20% 0.041"
        20% 0.042"
        I'm sorting into trays as I go. This looks like fun.

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        • #5
          I'll have to get out my rough old CCI Standard and Winchester Power Points (for Hunting) and do those but I don't want to fire any as I'm running this barrel with the SK's for a club competition in a couple of weeks. I don't want to contaminate the bore with a different manufacturers wax coating or I'll be back at square one again after my last bore clean.

          I did grab another box of SK Rifle Match (50) and got.

          3 @ 0.042"
          42 @ 0.043"
          5 @ 0.0435"

          That is about the numbers or less I get out of the 200 (4 packets) I have sorted so far another 4 to go out of this brick then pick up a fresh batch.

          As I said and the figures indicate I am sure these have been factory sorted.

          Ages ago I did a few packets of Elley Match (Black Box) and found like the SK Rifle Match that very few were out of the main stream measurement but the Elley are nearly three times the price.

          Even cheap ammo like the CCI Standard which have figures like your Stirling benefited in my other Brno 2E (sold it) if batched and fired in their own batch lots.

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          • #6
            I saw a post lst year where a couple of scientific geeks did a comprehensive analysis of the value of sorting 22LR ammo in this manner. After several thousand rounds and 20 different brands, their conclusion was that it made absolutely zero difference to overall grouping accuracy.

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            • Guest's Avatar
              Guest commented
              Editing a comment
              Originally posted by Stan 66" post=3758
              I saw a post lst year where a couple of scientific geeks did a comprehensive analysis of the value of sorting 22LR ammo in this manner. After several thousand rounds and 20 different brands, their conclusion was that it made absolutely zero difference to overall grouping accuracy.
              Not sure I would believe them as accuracy is all about consistency and repeatability, the more variations you have the worse you are going to shoot.

          • #7
            Thanks for the info. Will give it a go next time its rainy and windy.. Probably tomorrow.........
            No coffee no workee

            30-06 . 308 . 223 . 22lr . 177air and now an O/U 12g

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            • #8
              I wouldn't take their statements to mean anything. The system works and is very easy to prove for yourself as long as you have an accurate rifle and as long as your skills match. Any scientific fool can come up with figures they want to disprove.

              For example say one is able to fire a series of 0.5" groups with sorted ammo, then add one for the last shot that is way out of the sorting sequence and it will hit with a POI outside the remaining group. Been there, done that.

              Benchrest shooters worldwide use this method and I know what they would say to the "scientific geeks" but probably not in public.

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              • #9

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                • TwoFourThree
                  TwoFourThree commented
                  Editing a comment
                  Would you not have to sort ALL the ammo into say 5 separate groups and shoot a group with one round from each batch to garrantee that you had a true random batch. Random selection of unsorted ammo could actually be all identical. A slim chance, but possible nonetheless.

              • #10

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                • #11
                  If you have five head space size sorted batches of ammo and shoot one from each it's the same thing as not sorting them in the first place. Each different head spaced one should have a different POI.

                  I think the better test would be to have say 14 of the same batch and one completely different. Test yourself by shooting two five shot groups and they should all form the same small group then shoot the last four of that batch one the odd man out as last and you should see it hit a different POI. You should also have enough experience to know if you pulled a shot yourself as soon as you touch the trigger.

                  I have done this in the past but not recently. I know I can shoot this Brno under ideal conditions and have 5 shot groups at 50m under 0.5" consistently and having tried my test the odd shot out of the batch can be 0.25 - 0.5" outside the main group. I can also have a bad day and most of that fault is me not concentrating on what I am doing and pull shots.

                  The test in my mind is only viable if conditions are perfect. Not the slightest breeze or convection currents in the firing line and a reasonably cool not hot day. The next factor is a correct scope and paralax setting as it's quite common to see bad groups when the shooter is not positioned behind a scope the same way each time when the parallax is not correct.

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                  • TwoFourThree
                    TwoFourThree commented
                    Editing a comment
                    My reasoning is that if the 'unsorted' batch (1 from each group) produce a group the same size as any 'sorted' batch then it means that that ammo is not consistent (inaccurate) or extremely consistent (very accurate).
                    In the first case the ammo is no good, in the second case you are wasting your time sorting it in the first place. The results you obtain will tell you which is the case.

                • #12

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                  • #13

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                    • TwoFourThree
                      TwoFourThree commented
                      Editing a comment
                      Of course you have to sort a batch of the ammo once. If the results are no better than unsorted (and the ammo is very accurate) then there is no need to sort that brand/batch of ammo at all.

                      What I should have said before was " In the first case the ammo is no good, in the second case you need not bother to sort any more of that ammo, to do so would be a waste of time"

                  • #14
                    I heard that at a rimfire shoot last weekend or the one before that a bloke shot pretty good for 2 cards then shot the best card
                    of the match on the last then picked his rifle up and noticed the wobble between the action and barrel!
                    I wonder how consistent the headspace was on that thing!

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                    • Guest's Avatar
                      Guest commented
                      Editing a comment
                      OP has overlooked that rimfire headspace is achieved by the force of the firing pin pushing the case forward into the chamber - as far as it can go until it is firmly seated against the camber wall recess into which the rim fits - and it is only at that point the case rim containing the primer is crushed sufficienly to cause the primer compound to ignite - therefore every 22 rf cartridge no matter what its rim thickness variation is will be at its most forward point possible in the chamber at time of ignition

                      if anything might make a difference in accuracy with a rimfire its the chamber diameter (tighter chamber the better)

                      measuring rim thickness is an imaginary improvement and waste of time

                  • #15
                    Originally posted by cosmo" post=14060
                    OP has overlooked that rimfire headspace is achieved by the force of the firing pin pushing the case forward into the chamber - as far as it can go until it is firmly seated against the camber wall recess into which the rim fits - and it is only at that point the case rim containing the primer is crushed sufficienly to cause the primer compound to ignite - therefore every 22 rf cartridge no matter what its rim thickness variation is will be at its most forward point possible in the chamber at time of ignition

                    if anything might make a difference in accuracy with a rimfire its the chamber diameter (tighter chamber the better)

                    measuring rim thickness is an imaginary improvement and waste of time



                    How can the Firing Pin possibly push the Case Rim forward. When the Firing Pin is Cocked it is recessed well inside the Bolt behind the Bolt Face.

                    It's the Bolt Face that pushed the cartridge into the chamber and the Bolt Face does not apply any pressure to the case rim, if it did there would be something drastically wrong.

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                    • Guest's Avatar
                      Guest commented
                      Editing a comment
                      what i was saying was when you fire the rifle the firing pin will push the rim of the 22 ctg against the chamber face - my description used the minimum amount of words possible , and I presumed that it should have been very obvious to readers that the firing pin moving forward during firing was what was being described

                      in most 22 rf rifles the chambering is very 'loose tolerance" (mostly done on purpose so as to allow variations in brand dimension variations in different 22 cases) - and the result is the bolt face does not apply any pressure to the cases - we both agree on that - which then proves my point that the final position of the ctg at point of firing is determined by the strike of the firing pin which pushes it as far forward againt the breech face as it can go as it crushes the primer compound and sets the charge off

                      hence thickness of ctg rim is virtually irrelevant as at the time of ignition regardless of minute variations in rim thickness the cases are all in the same position - pushed hard againt the chamber wall by the firing pin force

                      btw any valid test to develop a theory about rim thickness causing variations in accuracy would require all the possible variables (other than rim thickness) to be eliminated during testing - eg shooting would need to be done from a machine rest, same type and weights of powder charge, same projectiles etc etc

                    • Guest's Avatar
                      Guest commented
                      Editing a comment
                      How can the Firing Pin possibly push the Case Rim forward. When the Firing Pin is Cocked it is recessed well inside the Bolt behind the Bolt Face.

                      It's the Bolt Face that pushed the cartridge into the chamber and the Bolt Face does not apply any pressure to the case rim, if it did there would be something drastically wrong.
                      There must be clearance for the rim, that is the headspace; because there is clearance then the firing pin will push the rim the distance of such clearance. in theory at least.
                      Insufficient headspace on rimfires can lead to premature ignition and I have seen a newly set-up semi-automatic pistol fire the entire mag full in one burst.
                      I have been present when a breakopen .22rf went off on closing simply because of a build up of dirt in the rim recess.
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