Headspacing Belted cartridges

bewsher500

Well-Known Member
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-mag-headspace_topic37640.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!
 

paul o'

Well-Known Member
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.[SUP][1][/SUP] 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.
 
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8x57

Distinguished Member
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. :stir:

I'll stick to rimless or rimmed cartridges, or to use the British term flanged cartridges as I have enough to worry about already. :lol:
 

bewsher500

Well-Known Member
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.
 

Brithunter

Well-Known Member
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.
 

Muir

Well-Known Member
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
 

Whitebeard

Well-Known Member
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.
 

bewsher500

Well-Known Member
Case choice wasn't coincidental

null_zpse56182c2.jpg




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)
 
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Whitebeard

Well-Known Member
Case choice wasn't coincidental

null_zpse56182c2.jpg




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.
 

bewsher500

Well-Known Member
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.


You can't have it both ways!

Its either important that it is a marked, referenced or "toleranced" dimension at a specific point or its not.

Apart from that it is not a secant shoulder like a Weatherby,
It matters not where you take the comparative measurements from on an planar angular surface so long as the opposing chamber wall is at the same angle (which it is as it fire-formed the case).
For the purpose of determining where cases should be sized to it serves function over form.

The standard approach to measuring a rifle's chamber is with a fire-formed case and a head space gauge.
In rimless this is done off the shoulder datum point (a "toleranced" dimension), in belted this is done off the belt.
What is this if it is not a headspace gauge?

The purpose of the post was not to get into the fineries of headspace and chamber construction but to highlight the issues that lie in FL sizing belted cases without taking into account the same headspace you would in a rimless resize.
Despite the standard advice that "Belted cases headspace off the belt".....I believe this is wrong, poor practice and will result in shortened case life, possible case failure and maybe even accuracy anomalies

Without going to a machine shop or casting a chamber core I have my chamber headspace dimensions from the largest possible fired case.
Same as I would from any other chamber
 

Whitebeard

Well-Known Member
Whatever mate you carry on but without determining the headspace of your chamber with the correct headspace gauges you are wasting your time and so am i trying to advise you.

Ian.
 

Sinistral

Well-Known Member
This seems such a waste of time and effort when correct headspacing should be established mechanically without fretting over the process ... belted magnum or not.

In 1500 rounds in a 7mmRM I've never had a case separate in this way. I'm guessing you're using second hand cases which have been 'set back' multiple times possibly .... although this is quite difficult to manage with decent dies.

Whether you start with factory rounds or new cases the shoulder will be correctly positioned on 1st firing. From that point on use a Lee Collet Neck Sizing Die which leaves the body & shoulder untouched. When the belted area swells to a very 'tight' chamber fit the belt won't enter the recessed base of the collet. It's then time to do a full length resize before returning to the CNSZ regime again.
 

sinbad

Well-Known Member
I have a 7mm rem mag, And i also have a headache after reading all that and trying to digest it.LOL
Paul
 

JabaliHunter

Well-Known Member
View attachment 34327
A Collet for Belted Magnum Cases - Craig Boddington, Guns & Ammo September 2002
Belts on the belted magnum case were put there for headspace control and not for strength, as is sometimes claimed. Once the case has been fired, the life of the case and perhaps accuracy, can be improved by headspacing off the shoulder rather than the belt. This is easily done by backing out the sizing die so that it doesn't set back the shoulder any more than just enough to insure that the case will chamber freely in the gun.

Because of the taper in ordinary resizing dies, the back end of the case (at the pressure ring), is usually left slightly larger in diameter than desired. Innovative Technologies has come up with a clever solution: a collet-type resizing die that sizes the body of the case right up to the belt.

In use, the die body is screwed into the press from the bottom. A fired case is dropped into the top of the die. If it drops in freely to the belt, there's no reason for any further sizing. If it fails to drop in freely, the case is removed from the die body and the collet is slipped over the case until it bottoms on the belt. The case and collet are then pressed up into the die body in the usual way. The collet is squeezed against the case by the die body and resizes it. When the case is removed from the die body the collet comes out with the case and is then slipped off.

​Neck sizing, and any required reforming of the shoulder, is then completed using the regular dies for the caliber. All the basic belted magnum cases can be sized using the same collet. This collet die works very well and is a very useful accessory to the reloader of the belted magnum cases.
 

bewsher500

Well-Known Member
I inherited the brass with the rifle so a certain amount of history there

It's then time to do a full length resize before returning to the CNSZ regime again.

neck sizing has always been the easy option to avoid die setting and headspace measurement and is on the whole what I do with pretty much all my other cartridges

but you still have to FL size at some point
 

Whitebeard

Well-Known Member
I think your understanding of headspace is at fault here bewsher, headspace is a chamber measurement and IMO you have an excessive headspace condition, 99% of the time case head seperation is down to this and no amount of die adjustment can cater for this.
With excess head space the belt does not stop forward movement of the case upon firing, the firing pin then drives the case forward untill the case shoulder contacts the shoulder angle of the chamber, it then butts up against this and expands the case backwards towards the bolt face causing stretching in the web area.
FL sizing pushes the shoulder back and the whole cycle begins again but each time the web area gets thinner until the case head seperates.
On a correctly headspaced chamber for belted cases the belt prohibits excessive forward movement of the case upon firing allowing the case to expand forwards to contact the chamber shoulder, giving you basically two headspace points, the belt and the shoulder.
You need to get your headspace (belt) checked to eliminate this, if it is a problem you need to get it rectified before you explore any means of sizing your cases.

Ian.
 

icedog

Well-Known Member
One of the cartridges I shoot and reload is the 7mm Rem Mag. Though I have not experienced a case head separation in my own rifle, one of my sons, who also shoots a 7mm, did have such a separation with a reload at an "awkward" moment during a grizzly bear hunt. Following that incident, I adjusted my die so that it resized the neck, but did not bump the shoulder back at each resizing. I expect that eventually those cases will be difficult to chamber, and I will need to touch the shoulder back a tiny bit. My 7mm is not a rifle I shoot frequently any more, but so far, so good, and as a bonus I suppose, I am now extremely thorough with my examination of spent 7mm mag cases before I go ahead with reloading them.
Not sure whether that was a "headspace" issue, or simply a die adjustment problem, but it seems to be resolved for the time being.
 

Brithunter

Well-Known Member
May I suggest that you measure the thickness oft eh belts and compare it with what they should actually measure!

I have noticed that many cartridge manufacturers don't seem to be able to make cases to the design specs. Winchester seem particularly bad with rim diameters and thicness being down of what it should really be.
 
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