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.
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:
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
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!