Make life easier on yourself with a headspace comparator bushing

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  • LRRPF52
    Super Moderator
    • Sep 2014
    • 8569

    Make life easier on yourself with a headspace comparator bushing

    A lot of people have been asking about sizing, how to approach it, do I cam over, smash the shell holder to the bottom of the sizing die, do I use a case gauge, etc.

    I think this method really cuts to the core of what we're trying to do with sizing, namely where we're placing the shoulder so we get consistent headspace that will actually fit in the chamber and allow the bolt to lock over the reloaded cartridge. For the 6mm PPC family and its .350" datum line on the center of the shoulder per SAAMI, we use the .350 bushing provided by Hornady in the kit.

    I thought some pics of how the process works would be worth a thousand words:

    Bushing installed in my calipers, indicated to 0.000" off the 2" mark, which is the length the tool will fill between the blades after being installed.

    L-R: Unfired AA 123gr Scenar load, 1x fired Lapua brass, reloaded cartridge with enough shoulder setback to chamber and lock-up



    Unfired Lapua brassed cartridge headspace measurement: 1.205"




    Fired Lapua brass, unsized headspace measurement: 1.218"




    Sized to fit and lock-up in my chambers with a bolt that has the ejector removed: 1.210" (I can get away with 1.212" and still lock up, so this is a little excessive sized back .008")

    Last edited by LRRPF52; 08-19-2017, 02:20 AM.
    NRA Basic, Pistol, Rifle, Shotgun, RSO

    CCW, CQM, DM, Long Range Rifle Instructor

    6.5 Grendel Reloading Handbooks & chamber brushes can be found here:

    www.AR15buildbox.com
  • wheelguner
    Warrior
    • Oct 2011
    • 407

    #2
    Well done! Very good pics too.

    Comment

    • Confederate
      Bloodstained
      • Nov 2014
      • 28

      #3
      Just what I needed. Thanks for the clarity given!

      Comment

      • grayfox
        Chieftain
        • Jan 2017
        • 4295

        #4
        Excellent writeup!
        Now just use a Frankford with the digital readout for us who are dial-a-metrically challenged !!!hahaha!!! just kidding.

        I had read elsewhere that to minimize brass cold-work you could bump the shoulder back for a given chamber, by 0.004" (that was their stated goal).. I've been a bit conservative and trying for about 0.006. If you're resizing back by 0.010" give or take, is that "too much cold-working" in your opinion?
        Anyone have any thoughts or insights on that theory...
        It does usually mean that if you have 2 or more rifles you may be doing "rifle-specific" brass after the first shooting. Right now that seems ok for me.
        This has all been with Hornady brass by the way for me.
        "Down the floor, out the door, Go Brandon Go!!!!!"

        Comment

        • Kswhitetails
          Chieftain
          • Oct 2016
          • 1914

          #5
          My understanding is that for gas guns, I want to FL size the brass every firing, and trim OCL down to spec when it gets too long to fit the case guage. I think I understand (book knowledge) the headspace measurement, but am wondering if since I now have only one rifle, neck sizing the cases and bumping the shoulders would be a better fit for me.

          I may be misunderstanding "bumping the shoulder back" as well. I understand it like this: In this process, a bushing die is used to adjust the shoulder head space measurement (as measured in OP) a thousandth at a time by changing the bushings. Am I correct?

          All of my Grendel loads so far have been on once fired Hornady brass, and have been FL sized. As such, I have not needed to verify head space beyond a case gauge. Also, I thought FL sizing reset this measurement back to unfired spec, am I mistaken on this as well?

          The other benefit of neck sizing is the extended brass life cycle, which is greatly beneficial. I may be getting a new die soon...
          Last edited by Kswhitetails; 08-19-2017, 09:45 PM.
          Nothing kills the incentive of men faster than a healthy sense of entitlement. Nothing kills entitlement faster than a healthy sense of achievement.

          Comment

          • grayfox
            Chieftain
            • Jan 2017
            • 4295

            #6
            I haven't heard that the bushing die moves the shoulder, only that it re-sizes the neck; and so some brass gets squeezed upward, not much so case length doesn't change much either. The shoulder is assumed to fit ok in the bolt-action since it expanded and then shrank back a tad from the chamber length during the firing process. I will defer to those who know more on that subject however.
            There are 2 important lengths: "datum" length which is a measure of shoulder length, and overall length. I know from personal experience that when the shoulder is 0.004 or 0.005 too long, there is a definite smack of the case against the chamber and nothing will get that bad boy locked and loaded (don't try too hard either!!!!!)
            The standard mfr recommendation on setting the die to meet flush with the case seat will bump the shoulder, resize the neck and squeeze brass upward (thus requiring trimming for overall length) - you feel the little "cam-over" bump at the end of the stroke which is the shoulder getting bumped down.
            I had a few creedmoor cases that separated from the body right at the neck/shoulder line, after 3 firings... I think it may have been due to some overworking during re-sizing (I think I have since solved that issue since it's no longer happening). I'm just trying to minimize my cold-working b/c I want my brass to last as long as possible... I guess someday if I get more time to spend I might look into annealing but for now it's just one step I don't plan to get into. Hence, my interest in re-sizing but only as much as necessary...
            Still a student of all of this, however!
            "Down the floor, out the door, Go Brandon Go!!!!!"

            Comment

            • LRRPF52
              Super Moderator
              • Sep 2014
              • 8569

              #7
              Originally posted by grayfox View Post
              Excellent writeup!
              Now just use a Frankford with the digital readout for us who are dial-a-metrically challenged !!!hahaha!!! just kidding.

              I had read elsewhere that to minimize brass cold-work you could bump the shoulder back for a given chamber, by 0.004" (that was their stated goal).. I've been a bit conservative and trying for about 0.006. If you're resizing back by 0.010" give or take, is that "too much cold-working" in your opinion?
              Anyone have any thoughts or insights on that theory...
              It does usually mean that if you have 2 or more rifles you may be doing "rifle-specific" brass after the first shooting. Right now that seems ok for me.
              This has all been with Hornady brass by the way for me.
              Yes, I personally think .010" is too much cold-working of the case.

              I've read where others state an arbitrary number they want to move the shoulder back, but for me, even .005" simply will not allow the bolt lugs to rotate inside the barrel extension (bolt with ejector removed). I keep a spare bolt stripped down for this purpose that has the same face depth and lug length as my others. If it does not allow the lugs to rotate around inside the chamber with the cartridge chambered, I can't mass-produce it, pure and simple-not if I want the rifle to actually chamber rounds.

              I thought the bolt gun would be more forgiving in this regard since it allows you to cam down with a large lever (the bolt handle), but nope. Even when I tried aimed brass from the AR chamber with the shoulder moved back .005", no amount of force would close the bolt, until I went to .006", and I had the same results with the AR barrels. I don't know what my headspace growth on brass will be like in the bolt gun yet, so don't extrapolate this to mean anything about the bolt gun fired brass conditions.
              NRA Basic, Pistol, Rifle, Shotgun, RSO

              CCW, CQM, DM, Long Range Rifle Instructor

              6.5 Grendel Reloading Handbooks & chamber brushes can be found here:

              www.AR15buildbox.com

              Comment

              • LRRPF52
                Super Moderator
                • Sep 2014
                • 8569

                #8
                Originally posted by Kswhitetails View Post
                My understanding is that for gas guns, I want to FL size the brass every firing, and trim OCL down to spec when it gets too long to fit the case guage. I think I understand (book knowledge) the headspace measurement, but am wondering if since I now have only one rifle, neck sizing the cases and bumping the shoulders would be a better fit for me.

                I may be misunderstanding "bumping the shoulder back" as well. I understand it like this: In this process, a bushing die is used to adjust the shoulder head space measurement (as measured in OP) a thousandth at a time by changing the bushings. Am I correct?

                All of my Grendel loads so far have been on once fired Hornady brass, and have been FL sized. As such, I have not needed to verify head space beyond a case gauge. Also, I thought FL sizing reset this measurement back to unfired spec, am I mistaken on this as well?

                The other benefit of neck sizing is the extended brass life cycle, which is greatly beneficial. I may be getting a new die soon...

                No, we only full-length size in gas guns since gas gun chambers have to be more generous to have reliability. We just try to minimize how much we move the shoulder back so that we don't overwork the brass when it's cold. With a comparator, you can find out exactly where your shoulder setback window is like described in the OP.

                Measure your unfired headspace to see where it's at just as a reference.

                Measure your fired headspace position (it will be much larger/more forward).

                In steps, find out what it required to provide adequate lock-up in your rifle.

                I prefer not to ride the fine edge of a perfect fit because we need a bit of slop in a gas gun for that bolt to rotate, especially in a dirty chamber.
                NRA Basic, Pistol, Rifle, Shotgun, RSO

                CCW, CQM, DM, Long Range Rifle Instructor

                6.5 Grendel Reloading Handbooks & chamber brushes can be found here:

                www.AR15buildbox.com

                Comment

                • Klem
                  Chieftain
                  • Aug 2013
                  • 3507

                  #9
                  I would like to add something here, and feel free to correct or discount it.

                  The term 'Headspace' is the gap between the shoulder and the chamber when the cartridge is in the chamber. i.e. the space at the head of the case; 'headspace'. The amount of gap is a result of the length of the cartridge from the shoulder to the base. We influence the length of our cases with our dies and so the gap or 'space' (Headspace) varies. Too much space and the case is expanding more than it needs to, to the point that it will eventually rupture because it is not supported by the chamber. No space and the bolt simply won't close because the length of the case from the shoulders to base is too long to fit into the chamber when the bolt is closed. These two ends of the continuum can be managed with measuring instruments and ready-reckoner field gauges called 'GO' and 'NO GO'. The critical end of this length continuum is when there is too much space, because this risks the more dangerous outcome, case rupture. The other end of the continuum is nothing more than an inconvenience because the bolt won't close. So, if you're going to make mistakes it's best that your cases are too long than too short.

                  I have had two cases rupture in an AR in my early days from being too short; too much headspace. Apart from potentially taking your face off the cases split in half with a nice neat line at the weakest point almost exactly halfway. The concentration of hot gas as the case splits cuts a small trench in the chamber which is visible with a torch and you can feel it with a paper clip. An indelible reminder of how close you came to reading Braille for the rest of your life.

                  Full Length (FL) sizing means you size the full length of the case, not just the neck. Neck sizing (NS) means you only squeeze the neck. In bolt guns there is considerable compound leverage as you close the bolt so you don't have to FL size the case between firings, just squeeze the neck to hold the bullet. This then means a good fire-formed fit in the chamber which increases accuracy and extends brass life. Not always however because when a case expands on firing it never fully retracts to its pre-fired dimensions. This then creeps up after multiple firing to the point that you might have to do a FL size for your bolt gun to get he bolt closed. Another exception is when you are loading hot or have large cases like the .338L. it might not retract very much at all, and you will feel this with a 'sticky' bolt having to put some effort into unlocking it. With gas guns all you have is the energy of the gas system and recoil spring to do what bolt gunners take for granted with their hands. Gas guns have less energy to cycle so we have to be more generous with our headspace to ensure reliability. And as 52 says, if it gets dirty we get unwanted friction robbing precious energy from the system.

                  Neck bushings in dies enable you to vary the diameter of the case neck. Dies without bushings means you only get one neck diameter set by the factory. Neck diameter translates into how much neck tension holds the bullet. This is important in gas guns because cartridges get slammed around a lot more than a bolt gun. When cycling is occurring it's like those kinetic hammers we use to pull bullets. The case goes in slams against the shoulders of the chamber and the bullet wants to keep going. Your carefully measured OAL might end up longer by the time you pull the trigger. They might even be touching the lands on firing. Or, the first round chambered by hand ends up a different OAL and the subsequent rounds that are loaded by the cycling action yet another OAL. This means you get flyers on the target and they are vertical (ask me how I know this!...). In the neck diameter of a auto Grendel I would suggest at least .003 neck tension. By that I mean you measure the diameter of the neck after sizing and then again after seating a bullet. The difference should be at least .003. If it's a bolt gun you can get away with less neck tension. Neck tension is something I think is a little overlooked in gas guns as far as accuracy is concerned - maybe because it doesn't relate to more obvious outcomes like cycling and case rupture. I often record the neck tension as I have two dies and different manufacturers bullets have slightly different diameters. Between batches of bullets from the same manufacturer the length, width and weight might change also. Not so important for plinking but at the level of precision this forum is at we need to consider it.

                  FL sizing for gas guns is industry-recommended for reliable cycling, however...as Whitetails correctly says, that implies squeezing the cases all the way back to SAAMI. But, we want our cases to last and we want as much of the previous firing's fire-forming retained as possible, for accuracy. That's what 52' is referring to when he talks about using a FL die but not screwing it all the way down so that it returns the case to SAAMI. The only time you return a case to SAAMI specs is if you are handloading for guns that you don't know the chamber dimensions of. With only one gun (your gun) you fireform the cases with the first firing and from then on only need to bump the shoulders back as much as you think will give you reliable cycling in the conditions you expect to be shooting. Any more and you cold work the brass more than it needs to. The consensus on the forum is that a .003" bump is the minimum for reliable cycling so you screw down your FL die only enough to get this. Sort of like a half-full-length size if you know what I mean.

                  I tend to include the bump and the neck tension measures on the batches I load, but that is just me. In the photo I know all the other dimensions of a favourite load so I just record the neck and bump.
                  Last edited by Klem; 08-20-2017, 06:46 AM.

                  Comment

                  • bj139
                    Chieftain
                    • Mar 2017
                    • 1968

                    #10
                    I loaded my last 20 Hornady cases by unscrewing the FL die about one turn. This partially sizes the neck and body and does not bump the shoulder back. These fired fine from my two Grendels. They must have had similar sized chambers. I sent one barrel back to BCA for the striped ridges which formed on the cases. I tried the fired cases in the replacement barrel and they did not easily eject -needed mortaring so this chamber must be slightly smaller than the other. I FL resized these and shot them. I partially resized these but have not shot them yet. I am not recommending any one else do this but I got a 0.902 MOA five shot group from that inaccurate 16" BCA barrel by doing this before I sent it back. I believe I read about partial resizing in the Lee reloading handbook. There is also information about this on the internet I have seen. One thing this does is leaves a part of the neck, near the shoulder, unsized which holds the bullet concentric to the neck and not just to the shoulder.

                    Comment

                    • Mad Charlie
                      Warrior
                      • May 2017
                      • 827

                      #11
                      I have been using the "partial sizing" technique since the early '80 s, but only in bolt rifles, I also use Forster sizing dies.
                      I have had Forster hone some sizing dies to a custom neck dimension for some of my bolt rifles and I only use the same brand of
                      cases for those rifles, I get great "concentricity" with those loads, and great accuracy.

                      When I first started loading for the Grendel (recently) I was kind of amazed at the difference in shoulder length between factory unfired ammo and
                      the cases fired in my chamber (.010-.011). I thought that I might have the wrong bolt or something. I arbitrarily decided on .005 for shoulder bump
                      and it has been functioning just fine through my build. I did have to do some extractor work to keep unfired factory ammo (PF) from sticking in the chamber
                      while my handloads with the .005 bump did not. It is running well with the cases landing in a neat little pile.

                      Without the comparator tool, this could have been a lot more "fun". ANYONE that handloads needs a way to measure shoulder length, and the Hornady
                      tool is as good as any, but it needs to be noted that it is a comparator only. I often set mine up with a headspace guage, but I don't have one for the Grendel.

                      Comment

                      • VASCAR2
                        Chieftain
                        • Mar 2011
                        • 6219

                        #12
                        Will the Hornady Head Space Comparator Bushing fit in the Hornady Lock N Load C.O.A.L length gauge body. If so all I'd need to order is the B350 bushing. If the Bushing will only fit in the Head Space comparator body I'd probably order the kit.

                        Comment

                        • Drillboss
                          Warrior
                          • Jan 2015
                          • 894

                          #13
                          The headspace and COAL bushings both fit into the same carrier body. You can actually use a .35 bullet comparator bushing to check your headspace on empty brass. The measured length won't be correct, but the amount of shoulder bump between fired case and sized case will be correct.

                          Comment

                          • VASCAR2
                            Chieftain
                            • Mar 2011
                            • 6219

                            #14
                            Thanks Drillboss!

                            Comment

                            • lrgrendel
                              Warrior
                              • Jul 2013
                              • 662

                              #15
                              This is a great thread! 52 and KLEM great explanation.

                              So I am going to a bolt only now. I have a Forster FL sizing die and I always FL sized because I was only shooting a gas gun.

                              Please explain the technique I now need to do i.e. Half FL. Ideally I would get a neck sizing die as I have for my other bolt action rifles but I would like to try this out.

                              Thought I was the only one with that Lyman calibers! Mine must be 20 years old!

                              Comment

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