Case neck thickness measurements

[Brno308]

Hi All

I have Lapua cases in .308 cal that has been fired 3 times and I am preparing them for a 4th firing. I bought these cases brand new and have only neck sized thus far.

I make use of a Redding case neck gauge and have zero'd the dial calipher on the mandrel. I have my cases arranged in 7 rows of 5 in my case block.

Here are the measurements I get from the case neck gauge as per above arrangement:

Row 1

15 - 16, 16 - 17, 15 - 17, 16 - 16.5, 15 - 17

Row 2

16 - 17, 15 - 17, 15 - 16.5, 15 - 16, 15.5 - 16

Row 3

15 - 16, 15 - 17, 14.5 - 16, 16 - 17, 15.5 - 17.5

Row 4

16 - 18.5, 16.5 - 19, 15.5 - 18, 17- 17.5, 15 - 16

Row 5

15 - 16, 15 - 16.5, 15 - 16, 15 - 18, 16 - 18

Row 6

15 - 16, 15 - 16.5, 15 - 17, 15.5 - 18,15 - 16.5

Row 7

15.5 - 16, 15 - 16, 15 - 17, 15.5 - 16, 15 - 16.5

Now, I must admit I am somewhat challenged when it comes to feet and inches since we use the metric system in SA. What exactly am I measuring when the caliper shows 15 for example. Does that represent 0.0150 of an inch? Kindly explain please.

What do you see in these measurements, are the variations extreme or acceptable?

Your input would be appreciated, thanks.

 

[J E Custom]

Irregardless of what measuring system you use, the neck thickness needs to be the same all the
way around.

I prefer using a Inside Micrometer and measuring the actual wall thickness (More accurate In
My Opinion because you are measuring only the neck wall).

Measured this way, you can determine what thickness you want after neck turning to a Uniform
thickness. I like a minimum of .014 wall to maximum of .016.

Once I set the neck thickness I can determine the sizing bushing for a good consistant bullet grip.

While turning the necks you also need to maintain a minimum of .003 chamber clearance with a loaded round. .004 thousandths Is what I recomend.

You can measure a fired case (OD) of neck. Then measure a loaded round to see what clearance
you have between the neck and the neck chamber. This should be .004 to .006 thousandths.

If it is less than .003 you must turn the necks.

The math is simple=
bullet diameter-.308 + neck wall thickness .015 x 2= .030 = .338 loaded diameter.
Fired brass diameter should be at least= .338 + .004 (.342 ).

Necks wall thickness should be the same in order to load consistant loads, so the measurement's
should be accurate.

J E CUSTOM

 

[Brno308]

Thanks JEC, that clears it up nicely. I'll go ahead and apply your advise.

 

[Bart B]

What exactly am I measuring when the caliper shows 15 for example. Does that represent 0.0150 of an inch? Kindly explain please.

What do you see in these measurements, are the variations extreme or acceptable?

Yes, "15" on the dial indicator means .015 inch; or .38 mm in the metric system.

From Redding's web site for that gauge:

(Case Neck Gauge | Redding Reloading Equipment: reloading equipment for rifles, handguns, pistols, revolvers and SAECO bullet casting equipment)

– Tech Talk – Case Neck Uniformity

Uniformity of case neck wall thickness is an indication of the quality of the cartridge case itself. Any large variation (over .0015") in neck wall thickness will generally indicate a similar or greater variation in the body and shoulder of the case. This variation cannot be corrected and may lead to concentricity and bullet runout problems.

Most competition shooters that have been forced to turn necks to properly fit "tight neck" chambers will confirm the following: "Turning the necks of poor quality cases will not solve the problems associated with the poor uniformity that remains within the shoulder and body of the case.

That information's somewhat misleading. It's been my experience that 30 caliber case neck thickness can vary as much as .002 inch (.050 mm) and excellent accuracy can be had. Even with standard chamber neck diameters allowing .003 to .004 inch clearance around the case neck. Those "tight neck" chambers referred to on Redding's web site are used by folks who think best accuracy happens when necks are tight and only .001 inch or so of clearance helps center the case neck and also the bullet more precicely in the chamber. They typically don't realize nor understand that bottleneck cases headspacing on their shoulder center perfectly in the chamber neck wnen the firing pin drives the case shoulder hard into the chamber's matching shoulder centering the case perfectly there and therefore centering the bullet very well in the bore.

It's also interesting that folks in the USA getting the best accuracy from the .308 Win. case do so with full length sizing dies using a neck bushing whose diameter's 2 to 3 thousandths inch smaller than a loaded round's neck diameter. No expander ball's used as they tend to bend case necks as the come up through the case neck.

Those Lapua cases have a range of .0145 inch to .019 inch neck wall thickness. That's quite a bit. If best accuracy is desired, I'd get a neck turning tool and turn all the neck down to .0155 inch maximum. That'll clean up most of them and the .001 inch spread in those few that don't clean up all the way around won't matter. Case neck tension on the bullets will be a lot more uniform and that'll help keep muzzle velocity spread at reasonable levels.

I've shot fullbore matches at the General DeWet rifle range near Bloomfontein using SA arsenal 7.62 NATO ammunition loaded with Sierra 155 match bullets in cases with a .002 inch spread in neck wall thickness. That's very good for mass produced cases and that ammo shot about as good as handloaded stuff does in the USA. And folks shooting fullbore matches around the world don't use "tight neck" chambers; they pretty much all standard size. If you know anyone in SA who shoots fullbore, ask them about neck wall thickness and other stuff that makes the .308 a favorite for long range competition around the world.

 

[Bart B]

You can measure a fired case (OD) of neck. Then measure a loaded round to see what clearance you have between the neck and the neck chamber. This should be .004 to .006 thousandths.

Clearance between the case neck to the chamber neck, to me and most others I've talked with, means how far it is from the neck wall to the chamber neck. If there's .004 inch clearance then the case neck's .008 inch smaller in diameter than the chamber neck. So a .308 Win. SAAMI spec chamber neck of .344 inch with a round's .338 inch diameter neck will have .003 inch clearance around the case neck.

And I've measured a lot of fired case neck diameters and none of them are the same as the chamber neck they were fired in. All were smaller by a few thousandths. And they varied depending on how thick the neck walls and case ductility was.

 

[Brno308]

Thanks Bart B. I like the idea of uniformity of the case neck wall thickness. So I went ahead and purchased a case neck turner today. The only one I could find was the RCBS hand held case neck turner. Looks pretty straight forward, lets hope this can do the job well enough.

I sincerely appreciated all the input. This is a great site.

 

[J E Custom]

Clearance between the case neck to the chamber neck, to me and most others I've talked with, means how far it is from the neck wall to the chamber neck. If there's .004 inch clearance then the case neck's .008 inch smaller in diameter than the chamber neck. So a .308 Win. SAAMI spec chamber neck of .344 inch with a round's .338 inch diameter neck will have .003 inch clearance around the case neck.

And I've measured a lot of fired case neck diameters and none of them are the same as the chamber neck they were fired in. All were smaller by a few thousandths. And they varied depending on how thick the neck walls and case ductility was.

Maybe I did not explain it right.

The .004 to .006 is the "total" difference in the ID of the neck chamber and the OD of a loaded
round. More clearance is not a problem but it may have an effect on accuracy.If it is less than
.002 per side it needs to be turned to get the proper clearance.

Dangerous game rifles normally have .008 total clearance (.004 each side) to assure that
they will work at the worst of times.

This will allow the brass to release the round .002 to .003 per side.

I have had to open the neck chamber of some rifles with a "Total" neck clearance of .003 or less
because of pressure.

As far as fired cases being smaller, I find that they are the same size or slightly larger after
firing because they do not rebound as much as the chamber because the chamber returns
to it's original size while the brass has exceeded it's yeald strength and is the same size or
larger than the chamber and needs to be re sized to ease rechambering the round.

This is also the reason that a very hot load can cause extraction problems (The case is bigger
than the chamber because it did not return to its original size).

Also the fact that SAMMI gives a chamber dimension, does not mean that the dimension will
be exactly that because after some use the reamer will lose some diameter and also there are
a large number of out of SAMMI spec reamers that have been cut per the customers request.
So a person needs to verify the clearance he has with his loads.

J E CUSTOM

 

[Boss Hoss]

Need to get one of these--have used mine for years and LOVE it:

Mitutoyo ball style micrometer - Sinclair Intl

 

[Brno308]

Boss Hoss, now lets see... $164.99 x R8.50 per doller = R1,402.42. Plus import duties and taxes........ nope, no can do. Would you believe that all the reloading tools/equipment we have here are all made in the U.S of A. Ok we have exellent powders and our own brands of ammo and bullets that are actually exported etc, but the nitty gritty and all important tools are manufactured in America.

Ok, back to the subject... Gents I dont know too much about the SAMMI specs etc. since I am just an old hunter but keep it up, I get your point. The reason I reload is because I shoot a lot and am trying to beat the fiscal impact of buying factory ammo. In the process I find my confidence building because I shoot better with the loads I develop. The downside is the better you get the more you want to improve......

So I have cut the necks as suggested Bart B and will be at the range tomorrow to test those loads. Should my groups improve you will all know about it and I WILL bug you with more questions.

Ok I'm off to bed now.. Cheers

 

[Bart B]

JE Custom, I've only measured two chambers for actual neck diameters; a 7.62 NATO Garand with a mil spec chamber whose neck measured .3453 +/- .0002 with my hole micrometer and a Hart barreled bolt gun whose SAAMI spec chamber neck read .3439 +/- .0002" with the same hole mic. Both had Lake City Arsenal mil-spec proof loads (42 gr. IMR4475 under a 172 gr. bullet in LC 65 lot 12638 primed cases) rated at 65,000 CUP fired in them. Loaded round neck diameters were .337" and after firing in both chambers about .341" This is from my notes taken back in 1988 when the test was run. Regular lot 12638 7.62 NATO M80 ball ammo had fired case necks a couple thousandths smaller.

All my .308 max load fired cases dropped into their bolt gun chambers from gravity; none stuck part way in. They all had neck diameters smaller than the chamber necks.

 

[J E Custom]

JE Custom, I've only measured two chambers for actual neck diameters; a 7.62 NATO Garand with a mil spec chamber whose neck measured .3453 +/- .0002 with my hole micrometer and a Hart barreled bolt gun whose SAAMI spec chamber neck read .3439 +/- .0002" with the same hole mic. Both had Lake City Arsenal mil-spec proof loads (42 gr. IMR4475 under a 172 gr. bullet in LC 65 lot 12638 primed cases) rated at 65,000 CUP fired in them. Loaded round neck diameters were .337" and after firing in both chambers about .341" This is from my notes taken back in 1988 when the test was run. Regular lot 12638 7.62 NATO M80 ball ammo had fired case necks a couple thousandths smaller.

All my .308 max load fired cases dropped into their bolt gun chambers from gravity; none stuck part way in. They all had neck diameters smaller than the chamber necks.

I believe you Bart. I just have not seen that scenario myself.

After reading you reply I even went back and compared the reamer dimension, fired case and
the chamber to see where the difference was In order to see if I had been doing the neck
measurements wrong. (Stranger things have happened).

I found nothing that lead me to change my method of assuring that the neck had enough room
to expand and release the bullet without a pressure problem.

There are allways exceptions to every rule and this just one example.

Irregardless whether we agree or not of how to measure the neck clearance, the main thing
is to prevent the necks from being to tight causing an over pressure condition.

I will admit that I am anal about such things, But I have seen the aftermath of improper loading
and other dangerous practices, so i take extra steps to prevent any surprises. This philosophy
has kept me out of trouble for over 50 years and thousands of rounds fired.

J E CUSTOM

 

[Trickymissfit]

if you goto the Sinclair webpage and look in the measuring tools you will see an "L" shaped affair that sets on a stand. I built one two or three years before Sinclair came out with theirs. All you need is a band saw, and belt sander and a drill press. I use a .218" gauge pin that's held in place with a #10-32 set screw. You can buy the micrometer heads seperately from the place you buy the gauge pins. I might have 4 hours of work in mine with a total cost of well under $80 (all the pieces came from ENCO)
gary

 

[J E Custom]

if you goto the Sinclair webpage and look in the measuring tools you will see an "L" shaped affair that sets on a stand. I built one two or three years before Sinclair came out with theirs. All you need is a band saw, and belt sander and a drill press. I use a .218" gauge pin that's held in place with a #10-32 set screw. You can buy the micrometer heads seperately from the place you buy the gauge pins. I might have 4 hours of work in mine with a total cost of well under $80 (all the pieces came from ENCO)
gary

+1
I like the micrometer stands because they leave your hands free to accuratly measure with
a good micrometer.

In my last post I left out one important method of measuring the neck/chamber.

I personally find inside micrometers difficult to use in the confines of the chamber, especally the
neck area so I use Cerrosafe and make a casting of the chamber.( it is dimensionally stable between
30 minutes to 1 hour) and it is easy to measure.

Cerrosafe also gives you a good tool to measure the rest of your chamber and a look at the
quality of it, It will show the actual free bore, lead angle and throat condition also.

I also use a ball micrometer to measure neck wall thickness and a good (Starrett or Mitutoyo)
micrometer for all other dimensions for the accuracy of my measurements.

J E CUSTOM

 

[drbill]

if you goto the Sinclair webpage and look in the measuring tools you will see an "L" shaped affair that sets on a stand. I built one two or three years before Sinclair came out with theirs. All you need is a band saw, and belt sander and a drill press. I use a .218" gauge pin that's held in place with a #10-32 set screw. You can buy the micrometer heads seperately from the place you buy the gauge pins. I might have 4 hours of work in mine with a total cost of well under $80 (all the pieces came from ENCO)
gary

Hello,

Is this stand you are talking about?

Sinclair Digital Case Neck Micrometer - Sinclair Intl

Or it this it?

Sinclair/Starret Case Neck Micrometer - Sinclair Intl

Or am I missing it all together

Thank you.