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Neck Tension For Hand Loaders

In Blog by Rich21 Comments

This one’s for Don and the other folks out there scratching their heads about neck tension and how it applies to reloading. To put it simply, neck tension is how tight the neck is holding the bullet from slipping forward into the rifling as pressure builds in the case when fired. Instead of being measured with physics terms like friction and force, it’s typically referred to in thousandths of an inch. How does all that apply to your hand loading practices, though? Does it matter at all? Should you be annealing? We’re going to cover all of it in this article on the merits of neck tension!

Neck Tension

I’m going to say this from the get go, do what works for you!┬áIn my experience, neck tension is one of the things that you should be paying attention to when you hand load your own ammunition. The reason is, I’ve seen it’s negative effects. The biggest example or story I can think of has to do with sizing 6.5x47L brass down to 6x47L. The first few batches I necked down I used a bushing die to do it. Bushing reloading dies are useful because you can select different size bushings to really control the neck tension. My problem is, it wasn’t the best tool for this task. The bushing doesn’t size the entire neck, so in sizing down the necks, I was getting inconsistent neck tension. Part of the neck would be at the 6.5mm size and part of it was at the 6mm size after I ran the neck through the die.

How did I know neck tension was an issue? The easiest things you’ll see to determine inconsistent neck tension are how much force it takes to seat your bullet as you lower the handle on your press and the consistency of seating depth. If you seat ten bullets into cases and they all have different seating depths, that’s probably inconsistent neck tension. The bullet will bind and grab the bullet on tighter cases so the bullets will seat longer. The reverse is also true, on a loose case the bullet will push into the case with less effort, though it likely won’t affect the depth the way a tighter neck will.

Neck Tension Nuance

I think neck tension is probably one of the keys to producing single digit SD ammunition at home. I didn’t throw those bushing dies away for my 6x47L, I just use a full length die for necking down now. On all subsequent firings of the brass I will typically size the brass with a Redding Type-S full length sizing die with a bushing halfway between the fired diameter, and the resized diameter. After tumbling, cleaning, etc. I will run the brass through a Redding Type-S neck die with the final size bushing that I want the neck sized too. This is the second to last operation in my reloading process. The only thing that comes after this, before powder and bullets, is trimming.

When discussing neck tension, it’s typically described in thousandths of an inch. So say you measure the outside neck diameter of fired 308 brass and you get a measurement of 0.340.” A common neck tension settings is two thousandths, so 0.002″ worth of tension. Say after sizing the brass it drops down to 0.330″ for the outer diameter. When you seat a bullet, the neck will swell and expand around the bullet. Say the diameter of the neck then measures 0.332,” well then you’ve arrived at two thousandths of neck tension. The easiest way to do all this is load a round with your components, measure the neck diameter, and then subtract those two thousandths. The outer diameter, minus the desired tension in thousandths, is the size of the bushing you want for sizing the neck.

neck tension bushing die

Here I’ve disassembled the bushing die, the die is on the left which is either cut to size the entire case, or just the neck, the interchangeable bushings can be selected for precise amounts of varied neck tension

Why size the neck in two steps? One of the things that can affect neck tension, or is thought to, is the work hardening of brass. As it’s used it becomes more brittle and springs back farther than when it’s new. So picture squeezing the neck down inside the bushing to set the tension, but now it springs back out more than it did initially. That would lead to a looser neck over time. The theory is if you make a big jump in neck diameter in one shot, it’s more work hardening than if you do it in two steps. Honestly, I’m just a bit anal about neck tension and I think it really matters so that’s why my second to last operation for brass preparation is running the necks through a bushing neck die. I want to make absolutely sure the neck is sized the way I want and consistent before I start seating bullets.

Other Neck Tension Stuffs

You may have heard of annealing. This is the practice of heating the necks of the brass, uniformly, and to a consistent temperature. This then softens the brass and, in theory, enhances the longevity of the brass and leads to more consistent neck tension. I’ll just toss this out there right now; the Applied Ballistics guys have done a lot of testing on annealing and the results are in Modern Advancements in Long Range Shooting Volume II. I’m not going to spoil that for you except to say it’s probably not worth doing until you’ve got in excess of ten firings on the cases. Annealing every 3rd, 6th firing, etc. is unlikely to result in any difference in neck tension or accuracy. If you do have double digit firings on brass and you plan to use it further, there are folks out there who will clean and anneal the cases for you at around $0.10/case.

Loose necks can be detrimental to accuracy. Again, that’s a conclusion I’ve seen in my own reloading over the years. I played with it a bit and found that accuracy was better with two to three thousandths of neck tension than with less. What I noticed was a velocity increase as the pressure had to build a bit more, I guess, to get the projectile moving. The end result was likely a higher, and more consistent, muzzle velocity which resulted in better accuracy when I was shooting my 308 in the first months of hand loading.

Wrapping Up

There are many myths and legends when it comes to hand loading. Some stuff matters and some stuff doesn’t. I think different things matter for different disciplines of shooting. If you shoot Benchrest and the top 10 shooters are separated by tiny fractions of an inch in group size, things like sorting your brass by weight makes sense. Sorting bullets by weight along with trimming meplats and tipping bullets can have their place. However, we cater to tactical style shooting on this site where the targets are bigger but the shooting positions and setup before the shot are way worse. As a result I try to give you guys the best advice. I don’t think weight sorting brass is worth your time for tactical shooting. I do think a bushing die and some bushings are worth the money to ensure consistent neck tension on your loaded rounds. If you have any experience you want to share on the subject, do so in the comments below!

Owner and Proprietor of AccuracyTech, LLC. Rich is a Firearms Enthusiast, Precision Rifle Competitor, and Writer. He is committed to bringing readers quality reviews and articles related to the Precision Shooting Sports. If you have any questions for him, please use the contact form on the site.

Comments

  1. I recently had an interesting conversation with a technical rep for one of the major bullet makers. I had asked him why my match grade seating die was unable to seat bullets to a consistent depth following my adjusting the die downward to get the right cartridge length from base to ogive.

    He explained to me that the seating die would seat to different depths depending on variations in casing next tension. I believe he mentioned that the bullet would shift or slip somehow if it encountered a higher neck tension. So, this suggests that each bullet needs to be carefully seated with multiple length checks required to get to the exact length and that setting the seating die once would not ensure repeatability.

    I had thought the die had a fixed grasp of the bullet and would gradually seat it deeper as you screwed the seating stem downward – and, that this dimension would then be set and repeatable. Based on the conversation I’m now instructed that if I wanted to avoid multiple seating attempts and multiple trips to the caliper/comparator, I’d need to be sure all necks were exactly the same tension. Even using the match grade bushing sizing die, I seem unable to achieve the necessary neck tension consistency.

    Any thoughts or ideas from your expertise would be hugely appreciated.
    Regards,
    Gary

    1. Author

      How full of powder is your case, Gary? Neck tension is important BUT if you’re using bushing dies with the correct bushing there isn’t a whole lot more you can do. Some folks will turn their necks for added consistency in neck tension but I’ve yet to find that necessary.

      If your case is fairly full of powder a compressed load will jack with your seating depth too. You should be using a bushing approximately 0.002″ smaller than the caliper measured diameter of the neck of a loaded cartridge.

  2. Thank you Rich!
    I’m new to reloading and I’m not really sure if the load is compressed. When I powdered the cases, the level seemed way down there but it also seems like a good idea figure out how to check into this and see if it’s compressed.

    The problem I’m experiencing is getting the die adjusted to a perfect case length, then tightening down on the seating stem to set up for the same depth on the next round(s). Sometimes I get one or two that are spot on, but then, without any changes by me, the next one seats .003-.005″ too far down. So I back out the stem and start working my way down again on the seating depth. I get to the magic number, secure the stem and it all starts over again.

    When I spoke with the die manufacturer( also a major bullet maker) the tech support guy said that varying neck tension could cause the bullet to ‘slip’ somehow within the curvature of the seating stem. Honestly, I just don’t get that. I thought the stem firmly held the bullet and pushed it down based purely on my adjustment of the seating stem. He also pointed out the the curvature of the stem was set for a particular bullet in their line – first I knew that some seating stems were bullet-specific.

    Due to problems with my new match grade bushing die from this manufacturer, I had returned that die for their inspection and was using a neck only sizing die which was turning out pretty consistent outside neck diameters. I understand that outside diameter consistency can be undermined by inconsistent neck thickness. I’m just not ready to have to turn necks to create a decently-accurate round.

    Perhaps a bushing neck-only die would help but otherwise I’m at a loss how to consistently seat to the right depth ( relating to lands) without backing the die out every time and gradually working down on the seating stem to get the right length.

    Thanks for getting back to me Rich.
    Best,
    Gary

    1. Author

      I don’t think it’s the stem. If the neck tension is really loose it might be the problem. If the powder is compressed you will likely hear a crunch and feel resistance as you seat the bullet.

      What’s the OD of the loaded rounds measure at the neck, what bushing size are you using?

      1. Hi Rich
        The neck tension seems tight – no manual wiggle possible. Also, I don’t hear a crunch and only experience mild resistance and then only when I think I have the seating depth ‘dialed in’ and am seating down from 0 to close to the final depth.

        A batch of 21 rounds that I just measured ran from 0.289-0.292 with an average around 0.291″( suggests a range of neck tensions doesn’t it?). These were all neck sized only – since my bushing die is being diagnosed by the manufacturer. The bushings that I have are 0.288, 0.289 and 0.290 ( 0.288 & 0.290 are Hornady and the 0.289 is RCBS ).

        The problems I’m experiencing with irratic seating depths are all on casings that were neck sized without a bushing die. Interestingly, when I spot checked the sized necks before seating, they were very consistently 0.288. How is it that such consistent neck sizing results in such a wide range of finished cartridge neck OD’s? Could irregular wall thicknesses or different bullet diameters account for this? The brass is all Hornady and the bullets are Sierra & Berger140 gr hpbt. The wall thicknesses I’ve measured with a ball micrometer seem to hover between .0135-.0140″.

        Thanks for helping think this through with me!
        Gary

        1. Author

          The wall thickness probably varies as well as small variations in bullet diameter. If you are only seeing this on cases not sized with a bushing you may have found your answer.

          I full length size my brass with a bushing die and my second to last step is a pass through a bushing neck die. What’s the neck diameter of your fired cases?

          Some folks advocate using two different size bushings to step down from fired to resized in smaller increments. Instead of sizing the neck 0.010 in one pass guys will do a pass at 0.005 and then another pass 0.005 smaller to arrive at the fully sized diameter.

          I would size all the necks with a bushing die and see if you still have issues. Make sure you throw the arm on the press the same way as well. If you set up the die with a gentle throw of the arm and then start gripping and ripping you can throw your seating depth off a little just by muscling the press more.

      2. Rich
        The diameter of the fired cases is 0.2955-0.2965″.

        So, you size 0.005 with the FL bushing size and the size another .005 with the bushing neck sizer then prime, load and seat ?

        I haven’t sized and loaded any cases with the FL bushing sizer. When I tried to size with it the first time, it wouldn’t close the necks enough with the 0.288 bushing to hold a bullet. I spoke with the manufacturer’s tech rep several times and finally agreed to send them the die for diagnosing. They didn’t send me a replacement so I’ve been using the neck only sizer that I had.

        As soon as they figure out what’s wrong with the FL bushing sizer that I sent back, I’ll try the process you suggest. But, I don’t understand why my non-bushing neck sizer that’s giving me consistent neck sizes, would be causing the seating problems.
        Thanks again for your input.
        Cheers,
        Gary

        1. Author

          I’m not saying your non bushing die is causing the problem. Just that you can’t adjust the neck tension with it. If the .288 bushing doesn’t hold a bullet with one pass what I’m suggesting may work. Use say a .292 bushing in the FL die to go from .296 to .292, halfway, then use a neck bushing die somewhere in your pricess to go from .292 to .287 so you do it in two steps.

          1. Makes sense to me Rich. Would the Redding neck size bushing die be reasonable or is there a better option?
            Gary

          2. Hi Rich…. one manufacturer’s rep told me it was a big mistake for a new reloaded to start out with a set of match grade dies. Is that a prevailing belief in the reloading community?

            Thanks again for all your input!!
            Gary

          3. Author

            Not that I’ve heard. Better dies and tools just let you do a better job, not sure how that could be seen as a bad thing.

            If anything not having the right tools is detrimental.

  3. Seeing Gary’s comments on measuring the wall thickness with a ball micrometer got me thinking- what is a decent ball micrometer to pick up? I’m currently using a Frankford Arsenal-branded analog caliper with a stated +/- of .001″ and frequently see a variation of .002″ when measuring the same item (even when zeroing). Any recommendations for an upgraded caliper?

    1. Author

      I don’t mess with measuring wall thickness myself, but the big brands produce ball micrometers if you so choose. RCBS, Hornady, etc.

  4. Gary, I have had this exact same issue. There is nothing wrong with your bushing. What you are experiencing is due your case necks having an inconsistent thickness. When you bushing size your necks the OD of the necks are all the same yet the IDs are inconsistent which equates to varying neck tension. This will in turn result in various seating depths. If you take a look at the actual bullets that seated short you may see a ring and a very slight depression where the seating die contacted the bullet. This is caused by the bullets jacket giving a little as the bullet was being seated and the bullet being pressed further into the seating die instead of the case neck. The only real solution for case neck thickness variations is to turn your case necks but before I would recommend this I would say shoot the loads with the varying depths to see if this is actually degrading your accuracy.

    1. Thanks Jason. I’m still new to a lot of this but help from this and other forums has been great.

      I really am not ready, mentally anyway, to start annealing or turning necks. I’m still unclear how neck turning actually equalizes neck thickness. It seems likely to shave off high spots on the outside as the neck rotates but I haven’t yet grasped how it uniforms the thickness on both the interior and exterior.

      I’ll keep thinking on this and listening to helpful folks like yourself, Rich and others.
      Cheers
      Gary

  5. Hey Gary, you may want to think about your bullets and their tolerance ranges, as I have found out the hard way! Also, how are you measuring the loaded round? Base to ogive will give the only results worth having, as base to meplat will vary due to the swaging process in the manufacturing process

  6. Luke, forgot if I responded to you before but I am measuring to ogive – both in sorting bullets by length and in verifying my final seating depth relative to lands.

    Rich feels sorting cases by weight’s not worthwhile but, being new, I am sorting bullets by weight and length and sorting cases by weight. As I get to be a better shooter, I’ll be able to verify downrange if these activities are worth the time.
    Cheers!

    1. Author

      This is a preference thing, like so much in this sport. It doesn’t seem to be worth the effort for me, if it works for you… that’s really all that matters!

  7. Gary,

    Sinclair makes a good ball mic that measures down to .0001″ and is a good tool for evaluating neck wall thickness. If your necks are inconsistent, then you probably should be running an expander of some sort. I use the sinclair neck turning expander die with the carbide turning mandrel. K & M also sells a similar setup with custom mandrels in 1/2 thousandth increments so you can dial your neck tension in better. Also case neck finish inside can play a big part in inconsistent seating depths. If you have scratches or a rough finish, rounds will seat longer and vice-versa.

    Holland shooting supply also offers expanding mandrel sets that work with the sinclair expander die and they’re only about $30 for a set of 3.

    1. Thanks Darin
      I’ve assumed that the seating stem, being part of a mechanical mechanism that ensured a given depth of seating regardless of neck tension or the neck interior’s coefficient of friction.

      Even so, I get differing depths for a given seating depth setting. I get how neck wall thickness, interior neck surface friction, etc., can effect neck tension, but it baffles me that a fixed, mechanical seating depth system can yield different bullet seating depths. I’m pretty new at this so hopefully more experienced folks can help me figure things out. Thank you.

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