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 calipher shows 15 for example. Does that represent 0.0150 of an inch? Kindly explain please.

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

Your input would be appreciated, thanks.

 

[Joe O]

Yes,you are measuring the wall thickness in inches .015 etc.If you are shooting a factory chamber,and not neck turning,I would sort the cases, by case neck thickness, for better groups(possible).If you are using a bushing neck die,don't over size the neck,retain the expander button.You're looking for the same light resistance when extracting the case from the die.You should get consistant neck tension,as long as you use the expander button.If and when you decide to turn necks,you will be able to uniform all the necks,use a neck bushing die and size .002 less than a loaded round,such as neck thickness .013 x 2 +.308 = .334 use a .332 bushing,without the expander button.Hope this helps.

 

[brno308]

Thanks Joe O, youv'e cleared it up nicely.

 

[Martin52]

With unturned necks, variations in case neck thickness (outside) may transfer to the inside when re-szing creating bullet seating inconsistencies that affect accuracy. Retention of the expander ball in the resizing die will mitigate this but in turn this may contribute to run out due to the expander ball stretching the neck as it exits.

Proper neck turning will produce consistent case neck thickness, reduces another variable in reloading and enables the use of a bushing die. The expander ball can then be removed as there are (theoretically) no variations in case neck thickness that otherwise would transfer to the inside of the neck causing the above problems.

When sizing new cases before neck turning, use a conventional non bushing die as this sizes the neck to the neck shoulder junction and enables a cut (about 1/32) into this junction to avoid donuts later on. A bushing die stops just short of the neck/shoulder junction and should not be used for that reason. For subsequent resizings however, a bushing die can be used.

Particularly if you're interested in long range accuracy this will help. The neck thickness variations in your list are not unusual but if I were using these at 600m+ then I would certainly turn for the above reasons.

Martin

 

[brno308]

Thanks Martin, I believe you have it right technically and I will definitely follow your advise.

 

[wapiti25]

Get a tubing mic to measure case neck thickness. A caliper is not accurate enough.

 

[Martin52]

Fully agree with the tubing mic suggestion, I don't think brno308 is using a caliper to measure case neck thickness, it would be the dial indicator on the redding case neck gage. Either way the mic is the way to go.