Headspace

Headspace is simple.  It’s the distance from the gun’s bolt face to the surface inside the chamber that stops the forward motion of the cartridge as it’s inserted.  In a bottleneck cartridge, the case headspaces on the shoulder.


Many shooters, and all reloaders, know that.  But I think there’s a misunderstanding of case length (maybe it’s just my misunderstanding).  We’re told in all the manuals to carefully check the length of our cases before reloading, and to trim them if they’re beyond a certain specified length.


Actually there are two important case lengths to a bottlenecked case.  The distance from the head to the shoulder, and the distance from the shoulder to the case mouth.


My Winchester has what I regard to be excessive headspace, which means that if I fire a factory load, the case will stretch backward, to fill the extra space.  I suspect most of the stretching is between the shoulder and the bolt face.  If I neck size the case, or size it so the shoulder is pushed back only a thousandth or two, the case is now “too long” and I am told, in all the loading manuals, to trim it.  That would be shortening the neck in response to stretching behind the shoulder, and it would accomplish nothing.


Sure; the cases being loaded should all be the same length so they’re crimped equally, but I won’t know how far the case’s neck extends into the chamber by just measuring the overall length of the case.  Once I have a case that’s fire-formed to my rifle’s chamber, the neck may or may not need trimming.


Are there case gauges that make it easy to take a measurement of the mouth-to-shoulder length as well as the headspace?  Should I just shut up and full-length size all my cases, trim them to spec, and wait for the cases to deteriorate from excessive stretching and sizing? (the rifle was checked by a “gunsmith” and declared to be within spec, BTW)

16 thoughts on “Headspace

  1. I don’t know if there are case gauges like that, but a good micrometer should be able to do the job. Most cases are tapered–that begs the question, should one measure along the taper (easy), or on a straight line making a right triangle with the taper (more difficult)? Also it might not be bad to check the taper from the shoulder to the neck…or maybe that’s a little OCD. But OCD is good when reloading.

  2. My Arisaka has been rechambered for 30-06. THis makes the case bulge just a tiny bit around the case just ahead of the inner belt.

    I never use the brass for anything else, and I only neck resize it, and then I use 303 bullets because they fit the 7.7 barrel better.

  3. The main goal of trimming cases is to ensure that the chamber leade does not wedge down on the case mouth into the bullet. This would cause a lot of neck tension and excessive peak pressure. I’ve seen guns destroyed from firing cases where the case length went to excess in this way. If you have ensured that does not happen, then you can live with a longer case trim measurement.

  4. Most precision long range shooters have gone to full length resizing on all cases. I full length resize everything that goes through my AR’s, because gas guns are very picky about that. I have upwards of 20 reloads on some of my Lapua brass without having to toss it yet, and I haven’t annealed anything. I tend to lose about 2-3 cases per hundred somewhere after the 15th reload or so, mostly due to neck splits, but that is more than sufficient for me to think I got my money out of my cases!

    In my experience, full length resizing is no harder on cases than neck resizing, anyways.

    There are case gauges, like this one, http://www.midwayusa.com/viewproduct/?productnumber=610800.

    Is that what you are asking?

  5. I’m guessing this is for your 300 win mag?

    There is headspace and then there is headspace. Non-belted bottle necked cartridges head space from the bolt face to a datum point on the shoulder. Since no one uses the exact same point, this is why you should always use go & no-go chamber gauges from the same manufacturer and not mix. Straight wall cases measure headspace from the bolt face to the case mouth, and rimmed cases headspace is determined by rim thickness. 38 supers, a semi-rimmed case originally headspaced on the rim. Accuracy was terrible in most guns. Sometime in the 70’s, someone figured out that reaming barrels to headspace on the case mouth was way more accurate and repeatable and this led to the fantastic 38 super race guns of the 80’s and 90’s.
    So what does this have to do with 300 wins?? Turns out most 300 win mags (and lots of other belted cartridges)are reamed to headspace on the belt and tend to be a little long from the belt to the shoulder. Since the belt varies quite a bit you get a lot of accuracy complaints from 300 win mag shooters in general, especially if they aren’t consistent in their ammo purchases. Most owners don’t reload and it used to be that reloaded 300 win mag cases had a terrible reputation for lots of problems with case head separations. Reloading a magnum case 3x was considered the maximum. This is still true if you full length resize. Every time you fire a round that case stretches to fill the chamber then you resize it back to new ammo specs. This really puts a lot of stress on the case and in 2-3 reloadings one will normally start seeing a bright ring on the base on the case just above the belt. This where is the case stretching and failure takes place. So what to do about it?? First, after every firing, take a paperclip, straighten it out and put a 1/8″ or so right angle bend on 1 end. Run the bent end into the case to the back and gently drag it forward on the wall of the case. If you feel a bump/ridge (and it is distinctive)as you drag it out, it indicates an incipient case separation and that case is trash. Second, don’t full length size you cases. Get a chamber headspace measuring tool like this:
    http://www.hornady.com/store/Headspace-Gauge-Kits/
    and resize to your actual chamber dimension. A good instructional is here:
    http://blog.sinclairintl.com/2011/01/31/setting-up-your-full-length-sizing-die.
    Third, start using a neck bushing die like the RCBS X die.. Once you start doing this you will be headspacing on the cartridge shoulder and not off the belt. You will also get more firings out of your cases. I started doing this several years ago and my cases now last at least 10 reloadings. My custom 300 win mag will easily hold the 9 ring on a f-class target at 1000 yds all day long and I’m only a mediocre long range shooter.

    Best
    Joe

  6. Jeez I need to read things closer. I just realised this was Lyle’s post and not Joe’s and I don’t have a clue which cartridge he’s using. The last part regarding case sizing and chamber measurement is still relevant, though……some days!!!

  7. Ahhhhh…..as Joe in Reno says, belted cases are a different ball game. I don’t reload for belted cases, but what he says makes great sense!

    I’m surprised to hear about the WinMag accuracy issues, because the military just stepped up and is rebuilding a large number of their sniper rifles into 300 WinMags. They tend to want accuracy in those rifles, for obvious reasons. Of course, their ammo will be new and to a specific spec, but I wonder….. They are ordering an initial order reported as something like 38.5 MILLION rounds of ammo, though, so they should be able to figure it out!!

    I was shocked at that figure, 38.5 MILLION rounds in a sniper rifle caliber is a BUNCH. Figure 2000-3000 rounds per barrel to wear them out, thats a bunch of barrels! Remember, 1 Shot….1 Kill. That’s a heap o’ kills, son! Figure they shoot 100 rounds in practice for every round at a real target, that’s still a heap o’ kills, 385,000!

    Or…figure they shoot 1000 rounds of practice for every one on a target, thats still 1000 x 38.5 sniper kills, or 38,500 sniper kills! Do we even have that many sniper kills in all the wars of the 20 and 21st Centuries?

    If they shoot 1000 rounds of practice for every kill, that means the average rifle will get 2-3 kills before the barrel is gone….That means the per kill cost is 1000 x whatever the cost per round is, and I’m guessing something like $1/round for this grade of ammo, plus the cost of the rifle system….That’s figured at somewhere around $6-10,000 including the scope, mags, etc. Figure low end…so each rifle gets 3 kills before the barrel is gone. That’s $3000 in ammo and $6000 in rifle, or roughly $9000/3, or $3000/kill.

    WOW!!!

    And I took this thread WAY off topic!!

  8. Bill
    As I understand it, they aren’t actually “rebuilding” the M40’s as they are gaming the procurement system. With what we know of building accurate rifles now, I’m sure they will headspace off the shoulder, not the belt, and have a high quality chamber held to very tight tolerances as well as match grade ammo. If this is done, my above post is largely irrelevant. Remington’s custom shop has the contract and I’m sure the rifles will be right.

    Joe

  9. Go to Midway and search for “L.E. Wilson Case Length Gage” or just “case gage”. I got one for .45ACP, drop the round in, if it goes smooth and sits flush then it is smaller than SAAMI maximums.

  10. Joe in Reno,

    I agree on the M24/M40 rebuild. The only thing left appears to be the action, (and most of those will be replaced, I bet!) with new barrels, furniture, etc. But you play the game the way you have to play the game.

    I had forgotten that the 300 WinMag was belted and headspaced on the belt when reading about the conversion. It’s interesting that they chose that cartridge, but it was already in the system, and from what I read, nothing was considered that wasn’t there already.

    I had wondered how and why so many guys shot the 300 WinMag and how they got decent accuracy from that belted cartridge, which aren’t renowned for their long range prowess. (I know, there are exceptions!) You cleared that up and I appreciate it!

  11. My Winchester is an 1894 Carbine, caliber 30-30, bought new in the mid 1990s.

    I understand about the neck length being an issue if it starts to interfere with the tapered lead or rifling as the case comes un-crimped. I think I addressed that.

    Again; there are two case lengths of interest. One is for headspace and one is the distance the neck protrudes toward the rifling.

    Bill; Bingo! This is from the Wilson Case Gage product description;
    “Now once the initial measurement is taken, slowly dial in the full length sizer until there is a 0.001 to 0.002 drop in headspace. This will give the measurement to resize the case just below the chambers headspace in the firearm being reloaded for, giving a more accurate shot and saving brass by not causing it to be overworked.” Emphasis mine.

    Exactly. Thank you, Bill.

    Sure, you full-length size your brass in a gas gun, for optimum reliability. This is a lever gun, and if I only push the shoulder back a thou or two, I’m saving the brass from overworking, I have a case that’s custom fit to my longish chamber, and it will feed and chamber easily in the lever action. That fire-formed, custom fit case will be longer than spec, and still theoretically have a neck that’s plenty short relative to the rifling. If I trim to specified length, my necks will be extra short. They dance around the issue, but no loading manual addresses this possibility.

    I should also take a chamber casting as suggested, so I know exactly where the rifling starts relative to the shoulder. And I shall get that Wilson case gage. Thanks.
    to the shoulder.

    It would be nice if the firearms manufacturers gave us the actual chamber and throat specs of our guns. I guess that may be something you get with a custom built.

  12. I should have added this from the Wilson Case Gage description;
    Once the brass has been sized, set the brass in the gage and set the gage on a flat surface with the head side facing down. Now look across the top of the gage, this give the maximum and minimum for trim length. Emphasis mine.

    Hence the issue of measuring neck length separately from headspace, and that both are measured relative to the shoulder, which is the main point of the post. Overall case length can end up being a useless figure except with regard to crimping alone as I stated.

    Many experts (I use the term seriously and with respect here) on a forum where I described a problem with bullets wanting to seat further into the case upon chambering, talked about overall case length, but none of them hit upon this issue of there being two measurements that may have no direct relation to one another.

  13. Lyle,

    The chamber is standardized, as you know, but there is some variation. You can get the chamber drawings from multiple places, and Winchester might even give them to you. ANY chamber should be fine with the nominal dimensioned case shown here:

    http://www.reloadbench.com/popup/cart/111.html

    I reload the .30-.30 for my dad, and have been told by multiple old school reloaders that it should be full length resized because the lever doesn’t have the camming action that a bolt gun does and thus you can get stuck cases if the case dimensions get a little out. We have about 500 cases, and I can’t imagine ever shooting it enough that I’ll have to get rid of any from too much work on the brass, but I suppose its possible. I don’t plan on having to trim brass either!

    You are 100% correct about overall length vs. neck length not ever being mentioned much. I can’t find a single mention in any of my books or online! Surely this must have come up somewhere!!

    A while back I found this article by John Taffin, (and promptly printed it for my notebooks!) who has forgotten more about .30-.30’s than I will ever know, and you’ll note he always full length resizes as well.

    http://findarticles.com/p/articles/mi_m0BQY/is_1_49/ai_94765140/

  14. Joe in Reno has it right-the case stretches at the lower body area, then when it’s resized, the shoulder is pushed back, leaving the neck longer. In effect, the brass inchworms its way forward. I shim my shellholder to minimize this while still allowing free chambering, and trim at every reloading so the mouth and canalure line up reasonably well. This is for 5.56 is AR-15s. I also inside check every case at every reloading and scrap a case on slightest suspicion. So far, I have roughly 14 pounds of scrap brass. And I should probably get out more. But while I’m here, let me mention that thinning usually occurs just ahead of the base, but can also occur in the mid-body area.

  15. Ritchie,

    Really? WOW! I’ve not heard of anyone who does all that apart from the very meticulous bench resters! Certainly not anyone shooting volumes of 5.56. If I did that for my AR’s, I’d never have time to shoot, I’d be checking brass all the time!

    For AR’s I buy good brass, then I shoot it, then I tumble it, resize it, reload it and shoot it again. I check the necks for cracks after tumbling, but that’s it.

    I have had a total of 1 failure in thousands and thousands of rounds, and it was a piece of supposedly once fired Lake City brass that I bought resized and primed from a commercial reloader.

    I’m impressed by your dedication, I like shooting too much to spend that much time on brass!!

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