Thin to Win?

[spclark]

Would a round with .012 neck thickness but the same clearance shoot the same as a .0081

No.

Reason being you’re changing system dynamics altering that single variable.

Less neck wall thickness affects neck tension and speed / uniformity with which case necks respond to increasing pressures once primer then powder ignite.

I’ve noticed this in my Palma loads (much more forgiving when X ring’s 10” dia. rather than groups measured with optical comparator, to say nothing about using a sling and match sights) when changing from ‘kiss-turned’ (~ 50% circumference cut) new Lapua Palma brass and what I get from atleast-once-fired then neck turned to 0.0120” thickness.

Admittedly this was observed using barrels with chambers cut to a single profile; neck clearance increased when necks were reduced to uniform thickness... but somehow I can’t believe the increased clearance had more impact than thinning neck thickness.

I see no reason such an effect shouldn’t also impact consistency in BR disciplines.

 

[Uthink Uknow]

There’s one other variable and that is neck tension. You can seat a bullet after sizing the neck with different neck bushings. It seems to me if you seat a bullet after having sized with a bushing that is let us say .004 smaller than the .0086 thickness neck or with a .002 smaller bushing the whole equation has changed. So, you can play with neck thickness and you can play with neck bushings. And that’s all in the same chamber diameter.

 

[BoydAllen]

I think that it depends on what the powder you are using likes. Let me give you an example. In my discussions with Tom Mosul he told me that in his experience H4895 likes quite a bit of neck tension, which I think we all assume results in more bullet pull. For this reason he does not anneal his cases, this being for the Dasher and I assume also for the 6BRA. On the other hand, the experience that friends as well as I have had with Varget is that it does well with light neck tension. The reason that I mention this is that at some point neck thickness limits the amount of bullet pull you can get, regardless of the difference in neck diameters of the loaded round and the sized case.

In short range, a parallel would be the difference between 133 which I have gotten better results using more neck tension, and say the LT powders which do not seem to have the same requirement. After I decided to go thinner for my PPC necks (to increase loaded round neck clearance), I looked at what I was getting with my .262 chamber necks in the way of seating force with the thinner necks and decided to try to get back to where I was by having a barrel rechambered with a .263 neck reamer, which has gotten me back to the old neck thickness and seating force with the new neck clearance. Thinking back to his videos, I think that Jack Neary mentioned that he shoots a .263.

Every so often I read a question about neck tension, and it takes me back to a test that I did years ago with 133 that would be very easy for anyone to do, and was particularly easy for me since I typically load at the range. I shot test groups with .001, .002, and .003 neck tension with 133 with that being the only variable. The differences were striking. If I were to do that test today, I would try to tune a bit with charge weight at each of those tensions.

In the past I have read convincing posts that indicated that once you have enough, neck clearance is not a factor for accuracy, that you can have quite a lot, without consequence. If we assume this to be true, it allows us to look at testing for the effect of neck thickness as long as we have a large enough chamber neck diameter for the thickest neck that we want to test.

Recently I have been ending some of my posts with this. Test everything, and believe your targets.

 

[dmoran]

@Terry
Knowing your primarily a 600/1000 Benchrest shooter, and most of the response has been SR orientated to neck clearance, here is the results of one of my own neck clearance test (.002" to .007") from a 6Dasher /105-VLD:



Except for the wind getting the better of the .264 increment, they all basically ended up about .3-MOA, with no POI shifting between the increments. First time I conducted this test was in 2006 and the last time (above) was 2013, and from multiple barrels, that's always yielded the same outcome; no gains or lose in accuracy from .002" to .007" neck clearance.

Myself use no less then .003" clearance, to eliminate some of the risks of running tighter.

 

[7887mm08]

I dont believe theres anything to gain with really tight clearance in the neck. I like .003 on a 6mm and more on the bigger cases. There is also a trend to larger neck diameters in an effort to remove as little brass as possible and still clean the necks up. Thicker necks will give more grip and some powders respond well to that.

Carlsbad, you can turn to .0001". Believe it or not. Getting the mandrel fit and cutter edge is critical, then controlling the temperature of everything. Once you get there, you can turn necks well enough that you cant measure the difference with a .0001 mic. Most cutters are too sharp, you want some tool push off to force the neck against the mandrel. Now, Im not saying you can do it quick or profitable like a machinist has to

It's hard enough to find 10 machinists that can mic the same part to within .0001" Now show me the 10 reloaders that can do it. Not happening, sorry

 

[Alex Wheeler]

It does not matter if 9 other guys measurements are different than mine. All I care about is that I get the same measurement on every case. I may have misled with my wording. Holding .0001" is what I meant, not necessarily nailing the exact number. But like seating depth its ok to use your measurements as a reference even if your off a tenth or two.

Im usually hesitant to give out neck thickness numbers due to the fact that guys measure differently. What I like for guys to do is turn a case and seat a bullet. Measure the loaded round and adjust your numbers from there.

 

[BoydAllen]

A long time back I read something about turning and it clicked. I started doing it that way, and have ever since. In order to keep significant cutter force on the neck, forcing it into the mandrel, I move the cutter to the case shoulder much faster than would produce a good finish. I do this to leave a lot of material to remove on the return to the case mouth. After I make a slight print of the bevel of the cutter on the case shoulder I start back very slowly, and then I do not go back. This contributes to good uniformity of neck thickness.

In addition I believe that you should never try to force a case into alignment as you turn a case neck. Typically I use a power drive adapter, either PMA or 21st Century to drive the case with a cordless drill, and hold the turning tool as loose as I can, so that if the case wobbles I let it do whatever it want, and just keep the tool from rotating in my hand.

I find that these power adapters are quite a bit better than others that I have tried in terms of case wobble. They both have a little post that fits up in the primer pocket. which does a good job of centering the case, as well as reducing how tight the unit has to be tightened on the case, because it cannot spin out under load.

After I am finished turning, while the case is still under power, I use a trimmer that indexes off of the shoulder, and then chamfer and deburr. After that I grab the spinning the neck tightly with a wad of 0000 steel wool while the case is spinning, and force the wad against the case mouth, again while the case is turning.

Generally I check necks for thickness as I turn them. I use carbide turning mandrels, and keep my RPMs moderate to slow.

 

[MrMajestic]

top machinists with $30,000 lathes have trouble working in the .0001 in range but this guy is turning his necks to the .0001"? OK.

$330k lathes also...

 

[Terry]

The right tools help.

 

[Uthink Uknow]

And I’m awaiting the constructive answer from all the critics.

 

[jamesdmock]

And I’m awaiting the constructive answer from all the critics.

Jack was talking about a .262 neck in a 6mm PPC. At a thickness of .0086 and a .2435 bullet, he would have a total of .0013 clearance. This according to Jack and Bill was not enough. With necks turned to .0081, he would have a total of .0018 clearance. I used a "fat bullet" with a .2435 measurement on the pressure ring for demonstration purposes. Another HOF shooter, Gene Bukys, told me liked at least .0028 total clearance. Each person must test and see what works for them. Good shooting....James

 

[Uthink Uknow]

My remark, James, was at those or one who thinks one isn’t capable of measuring to the fourth decimal point. It takes time, years, to learn to do this.

 

[Dusty Stevens]

My remark, James, was at those or one who thinks one isn’t capable of measuring to the fourth decimal point. It takes time, years, to learn to do this.

And a good turning tool is a bunch more rigid than a $330,000 lathe with a boring bar hanging out 2” running at 6k rpm on a shaky floor trying to hold .0005 tolerance

 

[Northridge]

top machinists with $30,000 lathes have trouble working in the .0001 in range but this guy is turning his necks to the .0001"? OK.

Ummm.. not sure if your poking fun or not but turning necks even by hand can produce neck wall thickness that would be a +- .0001 not that hard to pull off actually.

Shawn Williams

 

[kneedtospeed]

 

[SG4247]

He aint going to win anything but a hot dog eating contest.

 

[bpmoss1]

View attachment 1081488

DAMN!!! That thing must be belt fed lol.