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Thread: Headspacing Belted cartridges

  1. #1

    Headspacing Belted cartridges

    Too much time on my hands lately so I have been researching madly

    I stumbled across the SAMMI text on headspacing dimensions and case drawings for pretty much anything you like.
    here:
    http://www.saami.org/specifications_and_information/publications/download/206.pdf


    I am newcomer to belted cases and various authorities discuss the merits of headspacing off the shoulder, ignoring the belt and headspacing off the belt ignoring the shoulder..... and everything in-between!

    What I found when running some increasing loads in a load test of 208gr AMax and 3 different powders was that despite having checked the FL die setting to get a "suitable" FL size as I would with rimless cases , I was getting some case head separation marks ("rings"), some case breaches and an extreme example of separation at the mid to high end of some the loads.


    front to back showing 2 case "rings" indicated in green, one breach and one complete separation




    During the test firing no noticeable difference in recoil, blast, POI or extraction was observed, to all intents and purposes every one of these rounds was behaving normally
    with the exception of the separated one which showed normal bolt lift but very hard extraction, I suspect it was the actual extraction that completely separated the case rather than the stretch

    As a result of this I am paying much more attention to the belted cases and am trying to reduce case stretch and match my FL sized 300WM cases (belted) to the rifle chamber as much as possible without relying on Neck sizing alone.

    This relies on the chamber length data and case length measurements
    As headspace data and measuring tools for belted cartridges are based on case head to belt measurements I came up with an off the cuff idea

    I have a bag of rifle and pistol cases of a variety of shapes and sizes
    If I found one that gives me a consistent datum point on the shoulder I can use it as a headspace gauge!

    cue the .41 Rem Mag pistol case
    fits the 300WM shoulder perfectly and sits about bang on the middle of the shoulder.



    (case shown is not one of mine, but shows a very short fired length compared to mine below)


    in my travels I also stumbled across this thread:
    http://www.opticstalk.com/300-win-ma...opic37640.html

    A post down the page the chap measures a new case vs a once fired case and gets a 0.040" difference!

    "That is a gap of .040" which is huge. But remember that the case will not move forward in the chamber that much because forward movement is stopped by the belt. But the case WILL expand to fill the chamber.
    HOWEVER it will not expand all the way to the shoulder on the first firing. The case may take 3 or 4 firings to fully expand until it fills the chamber and there is contact between the case shoulder and the chamber shoulder.

    For example on my 300 win mag the measurements are as follows taken with the Hornady tool

    new case - 2.253" (4.253" minus the 2" for the Hornady insert) (57.22mm!)
    once fired - 2.270" neck sized only (shoulder not pushed back)
    twice fired - 2.272" neck sized only
    3 times fired - 2.2725" (57.72mm) when chambered the case is tight (crush fit) so time to push the shoulder back for easy chambering"


    the bit that intrigued me was the observation that cases to not fireform to the shoulder every time. I am not convinced by this part and suspect what he is seeing is the less and less case retraction under cooling that a newly annealed case shows vs a 4x fired one.
    the pressures are just too great to hold the shape and not form to the chamber.
    (as an aside compare his fired case sizes to mine! almost 10 thou shorter)

    Anyway. to test my new device I set about measuring batches
    all measurements in MM

    I measured
    a) N160 fired cases going up through the charge levels
    b) N165 fired cases going up through the charge levels, (only a few as a I stopped firing after the first noticeable seperation mark)
    c) H4831 fired cases going up through the charge levels
    d) fired cases from some IMR4831 185gr Lapua mega loads
    e) FL sized cases that had the N160/N165/4831 loads in them
    f) FL sized cases sized in a different die (Lapua Mega loads in this brass)

    the numbers were very interesting (at least to me!)
    a) started at 57.93, rising up to 57.95, 57.97 (most accurate by some margin), 57.99 with the last but one load at 58.01 (case ring shown and next charge up split completely)
    b) started 58.01, rising to 58.02, 58.03 (all showing rings with one breach)
    c) 58.01, 58.03, 58.04 (all showing rings to lesser extent)
    d) all between 57.97 and 57.98, (very consistent lengths within 0.1mm or 4 thou, no case rings)
    e) 57.67mm (short by 0.2-0.25mm or 7-9 thou based on fired cases)
    f) 57.80 (short by 0.15mm or 5 thou)


    I have drawn some conclusions from this:
    1) Optimum case length after FL sizing needs to be at least 57.80mm
    2) the most accurate loads all have fired case lengths of 57.97
    3) loads with fired cases longer than 57.99mm are not accurate and have case separation rings and are scrap!

    I intend to neck size the fired brass in the 57.97-57.98mm range and see what I get in terms of accuracy improvements and compare them to FL sized cases in the 57.80-57.90 range

    I suspect that there is a sweet spot to not just charge level, seating depth but sized case headspace length.

    more to come!

  2. #2
    Innovative Technologies - Reloading Equipment
    have a look at this site. Seems to sort out some of the issues. My mate and I use one for 300 win mag.Might be of interest.

  3. #3
    found you this you prob already have this but its a nice in sight as to why the belt was put in place:

    Some large rimless magnum or military cartridges have a belt formed above the extractor groove. This belt is of slightly larger diameter than the adjacent case, so the cartridge may headspace on the edge of the belt closest to the bullet.[1] The original purpose of the belt was to give accurate headspacing for cartridges with shallow shoulder angles, where longitudinal precision of seating using such a shoulder presents difficulties. In effect this was similar to the headspacing function of a rim, but gave a long enough surface for cartridges to lie side-by-side in a magazine without risk of interference during the feed-stroke. The adoption of the belt as headspacing feature on rounds such as the .300 Holland & Holland Magnum and its derivatives ultimately started a fashion that resulted in most later Magnum rifle cartridges featuring belts.
    Some more or less straight cartridges have no bottleneck to headspace on. Such cylindrical shaped cartridge cases use the case mouth (the forward end of a cartridge case) as a forward positioned flange used for headspacing.
    Last edited by paul o'; 02-11-2013 at 21:40.

  4. #4
    Nah too much bother with belted magnums - I think I'll give them a miss as they don't really offer any benefits in a rifle, machine gun maybe rifle no.

    I'll stick to rimless or rimmed cartridges, or to use the British term flanged cartridges as I have enough to worry about already.
    It's the calibre of the shooter that counts not the calibre of the rifle.

  5. #5
    Quote Originally Posted by Jagdmatch View Post
    Innovative Technologies - Reloading Equipment
    have a look at this site. Seems to sort out some of the issues. My mate and I use one for 300 win mag. Might be of interest.
    One of what?
    The headspace gauge? Looks very nice but am not sure it does anything more than my Heath Robinson case/gauge.

    Interesting point in the thread link above, if the guy is to be believed then using once fired cases to measure your chamber is not enough as they don't all expand right to the shoulder of the chamber.
    my longest fired case was 58.04mm my shortest fired case was 57.92mm
    a difference of 0.12mm or almost 5 thousands of an inch. SO HOW BIG IS MY CHAMBER?

    It occurred to me that a lot of reloaders spend hours measuring seating depths
    how many measure headspace of there lots of FL sized cases?

    if your case size is not absolutely consistent on a rimless case then the distance to the lands that you have so carefully worked out is immaterial as the firing pin will drive the case forward into the chamber shoulder reducing your distance to the lands a varying amount depending on what you have sized it.

  6. #6
    if your case size is not absolutely consistent on a rimless case then the distance to the lands that you have so carefully worked out is immaterial as the firing pin will drive the case forward into the chamber shoulder reducing your distance to the lands a varying amount depending on what you have sized it.

    Hey bewsher ............................ are you trying to upset them?

    Small factual details like this do not belong in their world nor does the fact that the throat moves forward with wear and erosion.

  7. #7
    When at gunsmithing school, the head machine shop instructor/gunsmith was the man who built the Army's long range target rifles said that they never headspaced the belted 300WM guns on the belt, always the shoulder. They were more accurate. Bewsher: They same trick works for bullet seating depth.
    Good diagnostic work.~Muir

  8. #8
    bewsher, all you have is stated is all very well but without a set datum to work to its all a waste of time all you are effectively doing is comparisons.
    Firstly you need to establish that your headspace is within the given SAAMI spec of .220" to .227".
    There is also a datum measurement from the base of the cartridge to a point on the shoulder where it is .420" diametre, this measurement is 2.270" +.007" for the cartridge or 2.279" +.010" for the chamber.
    Using a 41 mag case on the shouilder will put close but not close enough to the .420" dimemsion to get a proper reading, you need an insert for you comparotor that is exactly .420", do you know a machinist who can knock you one up?
    With this insert you then have a set datum to work to and your length measurements will give you more accurate idea of what is happening with your cases.
    If the length from the base to the .420" datum diametre exceeds 2.289" you have a problem with headspace which will need to be rectified.

    Ian.

  9. #9
    Case choice wasn't coincidental





    The point was that the measurement at the .420" shoulder point is not a reference point used by SAMMI for headspace purposes it's a basic dimension. (Even marked "B" in their drawings)
    Last edited by bewsher500; 05-11-2013 at 21:23.

  10. #10
    Quote Originally Posted by bewsher500 View Post
    Case choice wasn't coincidental





    The point was that the measurement at the .420" shoulder point is not a reference point used by SAMMI for headspace purposes it's a basic dimension. (Even marked "B" in their drawings)
    It does not have to be a reference dimension, even if it is marked basic, because it it is a tolerenced dimension it can be used as a reference point when other dimension are not suitable, however without ascertianing your headspace dimension all that you are doing is futile.

    Ian.

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