7mm SAUM: Sorting cases by neck thickness

[ajvanwyk]

I recently went through a process to ensure that I have well prepared brass for this year's upcoming shoots. I won't bore you with everything I was doing or think that I should be doing but during the process I had the bright idea to batch my cases according to neck wall thickness. So with ball mic in hand I went through and measured every case at least on three points in the neck and for many, over four points. Very interestingly the cases broke out into 3 distinct groups,- 1. >0.013 <0.0135 // 2. >.0135 <0.0145 // 3. >0.0145 >0.015

My target neck was 0.014 and the majority of cases sorted into this bracket. My question though is whether I have taken too broad a view on neck wall thickness of 0.001 and wheter I should be batching these into smaller groups....

It there anyone out there with a view of when tedious overshadow precision

Albert

[saum2]

Just neck turn if it rocks your boat. If you changed your target by a half thou smaller and neck turned the rest to your new target, you would end up with a lot more cases and the necks would be all the same.

[DenisA]

The variation in neck wall thickness with unturned brass is the reason that people neck turn for uniformity. It's also the reason for expander balls on dies. To be used with factory less-uniform cases.

Using a bushing die without the expander ball fitted to size neck turned brass is the process that results in the least amount of neck run out or best concentricity. In this situation, if the O.D of the neck is measured and concentric then so is the I.D and inturn bullet alignment.

If you were to neck size unturned brass without the expander ball, meaning that the neck sizing was only set by the bush on the O.D, then you have the possibility of allowing the most I.D neck run out. The variation in neck thickness in this situation can have a poor effect bullet alignment. By only neck sizing the O.D of case necks with varying thickness, the O.D may be concentric, but the I.D won't.

If you neck size unturned brass using an expander ball on your die, the O.D is sized first by the bush and then the ball is dragged back through opening the neck a little via the I.D. In this situation the I.D should now be the closest to concentric dimension and the outer diameter of the neck will show the variation as run out due to non uniform neck wall thickness'.

In my experience with turned and unturned brass, expander balls have only ever increased run out measured on the neck O.D.

All that said, I think the result that you'll achieve through sorting cases by neck thickness entirely depends on how you size your brass.

[ajvanwyk]

Just for clarity. The cases I am sorting has all been through a process using 21st Century neck turning gear. All cases has been fired 3 times and before measurement,- cleaned and expanded with expander ball without a bush in the FL sizing die. I didn't think that either approach will result in a different measurement with a ball mic but you certainly have me thinking.

In any regard I think that even though the perception is that with neck turning you have complete uniformity of neck thickness, I think the realty is that there is some variation. This is caused even more so by an inexperienced "case prepper" like myself.

The question remains... is 0.001 variation of neck wall thickness too much in a batch ?

Albert

[Tim L]

"Using a bushing die without the expander ball fitted to size neck turned brass is the process that results in the least amount of neck run out or best concentricity."

Have you tried a Lee Collet die?
You may have to get it made for the SAUM but I think you'll be pleasantly surprised.

[DenisA]

"Using a bushing die without the expander ball fitted to size neck turned brass is the process that results in the least amount of neck run out or best concentricity."

Have you tried a Lee Collet die?
You may have to get it made for the SAUM but I think you'll be pleasantly surprised.

Oh no I haven't. I only had conventional dies in mind as that's what I have experience with. I have heard lots of high performing shooters swear by collett dies now that you say it.

I didn't think that either approach will result in a different measurement with a ball mic but you certainly have me thinking.

No the different process' won't result in a different measurement, but it will affect how the variations in those measurement effects bullet alignment and potentially accuracy.

The question remains... is 0.001 variation of neck wall thickness too much in a batch ?

Albert

I don't think so personally, but the point I was making is that .003" of neck wall variation can still achieve .001" or less bullet run out depending on the sizing process. Especially the case since you do use the expander ball.
Have you measured run out of loaded rounds at the bullets bearing surfaces approx. 1mm back from the ogive?

[ajvanwyk]

I don't think so personally, but the point I was making is that .003" of neck wall variation can still achieve .001" or less bullet run out depending on the sizing process. Especially the case since you do use the expander ball.
Have you measured run out of loaded rounds at the bullets bearing surfaces approx. 1mm back from the ogive?

On the odd occasion that I have used the 21st century concentricity gauge I was able to measure some at 0.0005 but the majority was less than 0.001 run out.

One of the biggest reasons I am so fixated on the neck wall thickness is to ensure consistent neck tension. I don't really have any way to measure this successfully. The thinking is that as the necks become more consistent, so too does the amount of pressure when using a sizing bush and therefore, should reduce ES

[ShaneG]

Hi Albert
Not familiar with your neck turner however all my hassles in neck turning and varying measurements dissipated when i went to a tungsten carbide cutter!
Prior to that i was forever adjusting the cutter for depth etc every 50 cases or so.
The tungsten cutter in a K&M body with teh alloy holder / heat sink I am using for our 308 cases has not been touched in over a 1000 necks turned.
Hope this helps?
Cheers
Shane

[ajvanwyk]

Hi Albert
Not familiar with your neck turner however all my hassles in neck turning and varying measurements dissipated when i went to a tungsten carbide cutter!

Hey Shane, thank you for the response. I fortunately received this advise early on and made sure that my cutter came in carbide. The cutter is top grade IMO and I would say that if you used it correctly there shouldn't be any/very little variation in neck thickness. Unfortunately as a new shooter and the first time turning cases, there was a bit of fiddling that has caused the imperfections.

Luckily I only had about 15 cases from 200 that was under the desired thickness and I'll be using those as fowlers. All others were either in spec (assuming still the 0.001 is okay) or over, allowing me to try and turn the others a second time

Albert

[shooter mcreid]

I have found that the pre neck turn prep can make a massive difference on the result. Hard brass that has been fired several times never seems to produce great results, whilst new brass that is neck sized down and then expanded back up using the 21st century expander produce improved results. All my measurements have shown new brass to have half a thou variation and if you are going to the effort of turning your brass you want to at most have half thou variation if not less.

[ShaneG]

I only neck turn new brass.
Lapua 308 average .0005 neck thickness variation before any turning.
After neck turning with the tungsten carbide cutter I am seeing an average of .0001
I skim turn to a 100% cut. Usually around .0015 skim.

[ajvanwyk]

I only neck turn new brass.
Lapua 308 average .0005 neck thickness variation before any turning.
After neck turning with the tungsten carbide cutter I am seeing an average of .0001
I skim turn to a 100% cut. Usually around .0015 skim.

Righto, I am way off then. I can see how this is possible though. After about the third batch of cutting, the results became much more repeatable as both man and machine learn't to work together....

Given the tolerances you've mentioned I am confident that I will have to do a few things here.... 1. Batch into much smaller groups 2. Go buy new brass much sooner than I expected 3. Really focus in on my case preparation processes.

Out of interest and just going back to an earlier comment, what could be the impact on velocity variation for a tight tolerance case like yours, to a loose as a goose case like mine could be? This could be a hard one to answer but do you think we're looking at 5fps ? 10, 20 ? more ?

I know from previous testing that I can load rounds with low ES - less than 10fps - but that has always been a rather small (5 cases) sample size. For larger combined groups I generally look at an ES of 30 - 40. Given this current fact, I am wondering (all things being equal) what quantifiable benefit the tighter necks could offer me.

Albert

[plumbs7]

In the words of a famous Qld shooter,

"You can measure something a 1000 times and still be f#*%ked by the wind !"

Anealling maybe more important! I gave neck turning away long ago except for my 7-08 Ai tight .312 neck.

[plumbs7]

In the words of a famous Qld shooter,

"You can measure something a 1000 times and still be f#*%ked by the wind !"

Anealling maybe more important! I gave neck turning away long ago except for my 7-08 Ai tight .312 neck.

Ok It's Friday Night ! .... bit of trivia ... who said that ?
Clue : he's about 6 foot 5", a good example that it's not only boat people that make it into Australia! And he could at times pass as a character from Star Wars ?

Any Guesses? ... anyone ?
Oh he is also a top bloke and one of the best gunsmiths this side of the black stump ( that is when he cuts the barrel for a Stolle and not a Barnard ! Personal joke!).
We Luv ya Mark!

[Tim N]

Hey Albert,
Our sport is the pursuit of perfection, like all sports.
Trick is knowing what really matters or what gives the greatest return for where the shooter is in their journey to "near perfection".
I'd suggest loading all the cases and see if the ones with a different neck wall give a measurable variation.
Or keep them for shorter ranges and reserve your best ones for the longer ranges.
Some of the best shooters don't neck turn at all.
I neck turn my 7saum and don't neck turn my 280AI, both shoot way better than I can.