What Amount of Neck Runout Drives Turning?

[JDSwim3]

A few questions on neck runout and turning:

1) Should I neck turn for tension consistency regardless of even very minor neck runout?

2) Is there a generally accepted maximum neck runout number (eg. 2 thou) that is "good enough" to skip turning?

3) If I need to turn even one neck in my lot, should I then turn all of them to that wall thickness for consistent tension regardless of runout of some of the cases?

FWIW - my once-fired, fire-formed Lapua 308 looks to be averaging about 2 thou in neck runout, with some close to zero and some upwards of 4 thou.

Thank you!

 

[David Christian]

@JDSwim3, see if you can play with you sizing die to ensure the least amount of runout (float the die, install an o-ring, try a different die).

I turn all of my necks for consistent interference fit and grip between the bullet and case neck. I mostly don’t measure and don’t care about runout. I’ve never seen a difference on target.

Inconsistent neck wall thickness = turning.
Improper die alignment = runout.

Yes, if you are going to turn, turn the entire box or you will end up with more mass in some case necks which will provide more tension on the bullet. If you have a bit of runout, you will end up turning into the shoulder at a bit of an uneven angle, which as long as you don’t go to deep into the shoulder will get straightened out after the next firing.

Run a mini blind test on your various runout cases and see if there is a difference on paper. I can’t see any all the way up to 6-7 thousands.

 

[Scott Harris]

A few questions on neck runout and turning:

1) Should I neck turn for tension consistency regardless of even very minor neck runout?

2) Is there a generally accepted maximum neck runout number (eg. 2 thou) that is "good enough" to skip turning?

3) If I need to turn even one neck in my lot, should I then turn all of them to that wall thickness for consistent tension regardless of runout of some of the cases?

FWIW - my once-fired, fire-formed Lapua 308 looks to be averaging about 2 thou in neck runout, with some close to zero and some upwards of 4 thou.

Thank you!

What discipline are you shooting in? Since you mention 308 WIN, I'll guess f-class? IMHO, neck turning is not needed. We did extensive testing of carefully turned necks versus no-turn: no difference in score or x-count. This was a year of testing across several barrels. Runout seems a little worse than zero with no-turn necks, but it did not impact the results on paper. It's free advice, take it for what it's worth.

 

[mikecr]

Should I neck turn for tension consistency regardless of even very minor neck runout?

When you open a new lot of brass, it's valuable to measure thickness around each neck. You might find they're close together and cull away only a relative few. Problem solved. You want necks very near same thickness, and if runout is a concern, then you want low thickness variance as seen around necks(as it actually runs FL of cases).

Is there a generally accepted maximum neck runout number (eg. 2 thou) that is "good enough" to skip turning?

Runout is only a problem when it exceeds your chamber clearances. This producing chambered pressure points, which affect barrel vibrations.

Turned necks aid in reducing neck-born runout(only), and improves tension consistency.
But there are other ways to help with these. Most people, even most competitors, do not bother with neck turning. They focus on better bang for their efforts.
I neck turn only where I had planned to do so (my reamer/chamber). This was based on up front measurements of all brass that would be used for a particular barrel, and because I just want to put in every effort for best I can achieve.

 

[chromatica]

I turned my first batch of ADG brass in 260 Rem and I am pleased with the results. I wanted to start paying attention to runout and some say that you shouldn't even begin to test for runout until necks are turned. (Factory brass does not have a smooth outer surface on the neck.)

David Christian pointed out that most turning lathes will not make an even cut if the neck axis is off center. I use the 21st Century lathe where the cutter is mounted with a shock mount that allows the axis of the mandrel to "wallow" with any neck alignment problem. When fire forming trues the neck, I don't wind up with thin spots on the neck.

 

[mikecr]

When things wobble you need free floating. The 21st setup is not free floating, in that both ends of axis are pinned. Yes, it wobbles with your case, but about a pinned center, causing angular errors with each wobble.
Terrible design IMO.
Better to just hold the cutter body in one hand. That's free floating, and is proven for many decades.

You can test for LOADED runout with factory cases like any other. Just set the indicator on exposed bullet bearing. While using expander mandrels in a pre-seating operation, unturned necks, even those with reasonable thickness variance, can point seated bullets dead true.
There are sound reloading methods to solve most anything.

 

[PopCharlie]

Mikecr is right about neck turning and runout.
I've done my own runout testing using 3 different rifles in 3 different calibers. A Cooper 6mmbr, a custom 6.5x47 Lapua and a custom .223 bolt gun. After 2 years and about 3500 rounds I have concluded the following.
1. In my Cooper factory rifle, runout of up to .006 on the ogive had no effect on target at 100 yards. (Mikecr's point)
2. In my 2 custom rifles, runout of more than .003 on the ogive would cause bullet impact to change on target at 100 yards. In an unpredictable way.
3. Runout can be avoided, and corrected by using a good, properly set up full length non bushing sizing die.
Neck turning thoughts:
There are only 2 real reasons to turn necks; to fit a tight necked chamber. To fix excessive neck wall thickness variations (for me, more than .0015).
1. Turning necks will not fix runout. If the csae is bent before it's turned, it will be bent after its turned. Necks gotta be straight before turning.
2. Holding the turning tool in my hand, with the case locked in to a neck turning shell holder (Like the ones Sinclair makes) and chucked in a drill, produce the best results for me.
3. The cutter in the tool is the most important thing. If the cutter is ground wrong, your cases will be a mess after turning. Next is mandrel, too much play in the mandrel to case neck fit will cause poor results.

Just my thoughts...your results may vary.

PopCharlie