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Case “Crush up”

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britrifles View Drop Down
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    Posted: January 13 2023 at 9:02am
Weather sucks today with very high lingering winds from numerous tornadoes in the area yesterday.  So, I needed something to do today…not going to the range in these conditions. 

I noticed something yesterday I hadn’t previously in a January 1961 Technical Report on the No. 4 Conversions to 7.62 written by DCRA Armorer and shooter Dave Reynolds.  The second paragraph of the report was on the status of the “NRA Bisley Progress”.  I believe this is in relation to the UK NRA status on the 7.62 conversion project.  

Here is what he wrote:



The bit in the red box surprised me.  This suggests the headspace is set to be shorter, not longer, than military 7.62 cases.  Anyone speculate on why this would be done?  I know precision long range shooters will typically neck size, or partial length resize cases to get around .001 to .002 crush up of cases so that they tend to center themselves in the chamber improving accuracy.  But .006 inches is a fair bit. 

I went thru my 7.62 brass that I fired over the last month and measured the base to shoulder lengths to establish the case headspace length at which the slightest resistance was felt when locking the bolt.  To do this, I used very light finger pressure on the bolt handle.  What I found in my DCRA was a case headspace length of 1.6250 had no resistance and 1.6260 had the slightest resistance with about a 1/32 inch gap between the bolt handle bottom and the action body.  About another 1/32 inch gap per .001 inch increase in case headspace length. 

Don’t take these measurements as absolute, I’m using a Hornady case headspace comparator and digital calipers, consider these as relative measurements.  

My Manson Precision .308 Win GO Gage measures 1.6225 and NOGO measures 1.6280.  The bolt closes on the GO, but about 0.35 inch gap under bolt handle on NOGO.  These gage lengths are not what is identified on the gage, just what they measure with my Hornady case headspace comparator. 

The fact that both my DCRA rifles have headspace below the .308 SAAMI NOGO and were built by Long Branch suggest that these rifles were indeed headspaced tight relative to 7.62 NATO specifications.  They do not have “long chambers” such as found on 7.62 MGs. 

About the longest fired case length I measured was 1.629, slightly longer than the NOGO gage, but brass is much more elastic than the steel headspace gages, so I’m not surprised by this. 

Where I’m going with this is:

1) Why would the 7.62 converted rifles be set up for a .003 to .006 case crush up?
2) At what point do you decide to do a partial length resize to set the shoulder back?
3) How much should the shoulder be set back for this rifle? 

It’s well known that neck sizing extends case life considerably, but what affect does “crush up” have on bolt thrust loads?  Might be negligible.  And, it might improve accuracy to have crush up. I don’t think they were worried about case life back then, competitive shooters did not reload, ammunition was supplied for matches, and in some cases, for practice too!

No doubt the Lee action elastically strains more than a Mauser action from firing a cartridge as the locking lugs are 4+ inches behind the case head where as the Mauser is about 0.5 inches. Theoretically, the Lee will displace the bolt aft 8 times the distance a Mauser will assuming similar stress levels in the bolt and receiver. 

I wonder if as part of the initial study of converting and testing the No. 4, RSAF Enfield decided that because of the additional strain on the action and how much the case will stretch on firing, it should be somewhat “pre-loaded” in the chamber.  This would reduce the amount the case will stretch in firing. 









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Post Options Post Options   Thanks (0) Thanks(0)   Quote Doco Overboard Quote  Post ReplyReply Direct Link To This Post Posted: January 13 2023 at 4:59pm
Maybe its becuase NAto spec ammo could not be reliably counted on to meet US/Uk standards particularly for auto loading rifles. (I'm thinking reliable ignition of the cartridge in a bolt gun with all kinds of ammo for this  this instance?)
Anyway,Gus Fisher/Jim Adell provided an update on using 7.62 Nato and 308 Win ammo in the latest GCA journal which may or may not shed some light on the question.
The article is dedicated to the realm of USGI standards for the m14 service rifle and Navy match rifles if I recall correctly and specific Nato match ammo/pressure safety characteristics.
It also provides #'s for the differences in 7.62/308 Hs specifications with USGI equipment-small arms.
I think field reject for the 308 is nearly the same as no go for a usgi m14 chamber or very close based on what information has been provided in Kunhausens service rifle book too.
I dont know if this helps your query very much, or if at all but I remembered this topic touched on recently in the GCA publication.

Its the fall 2022  GCA volume- I just double checked


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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: January 13 2023 at 5:48pm
Augh, I’ve let my GCA membership expire. That would be an interesting article, although, not likely addresses this question on the No. 4 7.62 conversions.  It would be be very unwise to short chamber a self loading rifle given the potential for a slam fire. 

This may remain a mystery and lost to time as the few who fully understood this project are likely no longer with us. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 5:17am
Thanks to Doco for sending me the article by Gus Fisher on headspace for the 7.62 M1/M1A and M14 rifles.  They indeed found accuracy improved in the USMC rifle team M14’s when setting headspace to just .001 over the SAMMI .308 NO Go (1.6300). This was needed for reliable cartridge feeding.  This is about the same as my DCRA No. 4 rifles, and well short of the NATO 7.62 Minimum.  

The RSAF Enfield likely used something different for the L8/L39/L42, I suspect Peter Laidler would know, perhaps the Armourer knows?  The L39 being a target rifle, I’ll bet headspace was less than NATO minimum. 





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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 6:45am
It may be of interest to shooters why headspace is more than a safety consideration.  

Rifle cartridges that have a tapered case body will not center themselves in the chamber unless they are held fully forward in the chamber.  The further back they sit in the chamber due to increasing headspace, the lower they sit relative to the bore axis due to the effects of gravity.  This is the reason for the concept of a slight “crush” fit to get the case center itself to the chamber used by Benchrest and other precision long range shooters. 

The concepts of neck sizing only is to get the case to perfectly match the chamber dimensions and therefore align the cartridge to the bore axis.  A bullet that does not enter the rifling well centered will not give tight groups on the target. 

The other affect is case life.  The larger the headspace, the shorter the case life.  We can minimize this by neck sizing only and when full length sizing becomes necessary, push the shoulder back by .001 or .002 inches and no more to retain that very slight “crush up” of the cartridge case.  I adjust my .303 and 7.62 full length sizing dies so that there is just the slightest resistance to fully lock the bolt. I used to screw the die down until it stopped on the she!! holder, but this practice will quickly lead to case web cracks and head separations.  With neck sizing only, and “partial length” resizing, I’ve been able to get 40 to 50 reloads of my .303 and 7.62/.308 brass.  

While 0.074 inch headspace in your .303 is the maximum for safety reasons, it is not desirable for accuracy or case life.  If you’re fortunate to have some spare bolt heads, see if you can get the headspace down to just over 0.064.  That 0.01 inches will make a big difference to case life and may improve accuracy.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote m38swede Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 7:26am
wow, good info, thx!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Goosic Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 8:41am
I totally understand the "Crush Up" when using a rimless cartridge like the the 7.62mm especially if you are a Bench Rest or Long Range shooter but, and Laidler and other professional Armourers have made it a point to inform everyone within earshot that, to try to apply a, "Crush Up" on a rimmed cartridge like the 303 British cartridge, provides no benefits other than to ensure that you have flattened the case head. Even the O-Ring trick is a hit or miss solution to a otherwise arbitrary problem.
The 303 and the 7.62mm were born of wartime conditions and given larger chamber areas just for the purpose of a "FailSafe" to assist if the issue arises. This is where custom fitting a LE bolthead to suit the needs of its owner comes in handy but, only to certain conditions. Whereas if, you want "Crush Up" on your 30-06, 308W, 243W or whatever rimless cartridge your Mauser styled rifle is using, you best have alot of money to play with because you will need to have a custom bolt assembly created because "Factory" made bolt assemblies won't get you what you desire. You mentioned BR and LR shooters and "Crush Up" plays a vital role within their game plan.  For your average "Joe Shoot Em' Up" at the range, as long as it made a hole in the paper, they're good to go. 
Thank you for your take on the topic of Crush Up though Geoff...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 11:41am
After a few firings with neck sizing only, the .303 case will no longer headspace on the rim.  It is the shoulder that stops the case going further forward in the chamber, not the rim.  You can feel this by the slightest resistance on fully locking the bolt because the case comes up against the shoulder of the chamber.  This bolt resistance can be felt after only one firing of my recent 7.62 long range loads, all headspace slack has been taken up by case stretch on hat first firing.

On factory ammunition, and full length sized cases, the case will always headspace on the .303 rim, the relatively long .303 chambers (head to shoulder length) compared to the case dimensions assure this.

While it is debatable how much accuracy improvement can be seen on a No. 4 rifle by having a slight crush fit I describe above, this does in my experience greatly increase case life, something that military armourers are not interested in nor ever needed to consider.  

For the .303 in particular, in my experience, it severely reduces case life to full length size the case such that it headspaces on the rim, particularly when fired in a chamber with headspace on the long side.  Even worse with thin rims.  The stack of tolerances here are such that the rear 1/2 inch or so of case body will stretch by more than 0.01 inches on firing and it is that strain that causes case head cracks.  

I’ve yet to determine how much the bolt body compresses between the locking lugs and the bolt face, but I’d bet it’s at least a few thousands of an inch under that 18 ton load. There is also some action body stretching.  This combined displacement might be perhaps as much as another .010 inches in total, and that is added to the 0.010 inch stretch outlined above.  It’s fairly easy to see why the Lee actioned rifles give short case life. 

I’ve not yet had to use my No. 4 rifles in battle situations and environments as it was designed for, and probably never will.  But, I shoot and reload a lot for target shooting, matches and practice.  Having headspace at the short end and neck sizing only has paid dividends for me and will for others too.  

I’m an advocate of the Lee collet sizer.  This puts the absolute least amount of plastic strain (work hardening) on the case and will not pull the neck out of alignment with the case body as standard neck sizers can.  That’s a good thing.






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Post Options Post Options   Thanks (0) Thanks(0)   Quote Shamu Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 12:27pm
All the reasons & explanations above are why I still do the old time "Partial Full Length Resizing". To my mind it gives the best of both neck sizing (minimal brass working closed fit to chamber dimensions) & full length resizing (utterly consistent case dimensions every time no gradual expansion then a full resize).
I set it up so the full length die just leaves a complete "ring" around the case shoulder. This centers like crush up, & keeps brass stretching to a minimum.
Of course the disadvantage is I have not just loaded ammo, but fired cases as well, sorted & labelled by which rifle its sized for! Luckily I have come by several resizing dies over the years so I actually have a die set up for each rifle now as well! Being a bit obsessive each die is marked a different color (just regular paint) on the die body top & they all live in a single die storage box with the 3 "compartments" which are labelled in the same color!Confused


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Post Options Post Options   Thanks (0) Thanks(0)   Quote britrifles Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 1:59pm
Yes, that’s exactly what I’m suggesting.  Although there are two other ways to get there: 

1. Use a case headspace comparator gage, like the Hornady gage, measure your fired case, set up your die to move the shoulder back by no more than .002 inches.  

2. Place a neck sized case in your press she!! holder, run the ram up all the way, screw in your FL die with the neck expander removed until it bottoms out on the case.  Lower the ram, screw in the die 1/16 of a turn more, and size the case. Chamber the sized case. If excessive pressure is needed to lock the bolt, add another 1/16 turn to the die and size again.  Repeat until you’re satisfied with the amount of sizing of the case.  Install the neck expander and proceed with “partial length” sizing your cases.




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