Tuning neck tension - should it be done and if so, how?

[Cassidy]

I've been following threads about tuning for powder load and tuning for bullet seating depth, but I havn't heard much about whether neck tension should be tuned. I've read that for 6PPC thin case necks can provide better precision than thicker case necks (I've got a 6.5x47, not a 6mm).

Should case neck tension be tuned to obtain optimal shooting precision? My understanding is that neck tension (the neck's grip on the bullet) is something we cannot directly measure but is a function of (1) case neck thickness (how much if any you turn), (2) the neck bushing size used in case sizing (to obtain 0.001, 0.002, 0.003" etc difference between case neck diameter with and without a seated bullet), and (3) bullet seating depth.

If we keep bullet seating depth constant (and the powder load constant), what would be a good study (tuning) paradigm to determine the optimal case neck thickness and optimal case neck bushing size as two unknown variables. I don't know if chamber neck clearance is a significant factor in this question; I've read that chamber neck clearance does not influence POI precision much. This may be fun to think over.

 

[Chuckhunter]

Cassidy, My 6.5X47 is strictly a hunting rifle. It has a .291 Nk and my loaded round with Lapua unturned brass and Berger 130Gr. Hunting VLD's mikes .288 across the neck. That gives me .003 clearance and I stay .020 off the rifeling so I never get into a situation where I have to eject a loaded round and have a bullet pulled out in the chamber and powder all over my action. For strictly accuracy shooting I would assume guys would have approx. .001 to .002 grip on the bullet and clearance my be slightly less. Personally I have found, I'd rather have to much clearance than not enough and because myrifle is for hunting only my grip is probably heavier than perhaps a competative shooter may use.

 

[Cassidy]

Chuckhunter - my chamber neck is 0.2944, my mic'd case neck (unturned, fired 3-4X) with bullet seated (Berger 130gr VLD Hunting, 6.5mm, same as yours) is an average of 0.2908 (range 0.2907-0.2909), and unturned Lapua case neck thickness (cases fired 3-4x) is a range of 0.0126 to 0.0141. This means my chamber neck clearance is 0.0036, so I don't have much room to turn necks, maybe just true them (unless I get another reamer and barrel with a tighter chamber neck clearance, maybe 0.2924.

But what I'm asking in my question is, since both the neck bushing size (when FL resizing the case neck) and the case neck thickness both impact on neck tension (the grip of the case neck on the bullet and how it releases), is there a way (a tuning test method) to determine the best neck thickness and also the best neck bushing size to use (to determine these two variables in one test) - is there a shooting tuning test procedure (like the powder load tuning method Erik has posted) that can help determine these two presently unknown parameters (variables) for any given rifle, not just mine, to optimize neck tension to get better groups.

 

[jlow]

If you want to do this, and it would be very interesting, you will have to be able to keep a lot of other variables constant.

For example, apart from all the other “stuff” that can affect your final result that is not neck tension associated, there is consistent neck thickness (neck turning), consistent sizing (bushing size is easy her but neck hardness/degree of annealing is harder but will affect the sizing due to spring back), you also have to deal with how smooth and lubricated the neck is as this also comes into play when dealing with “neck tension”. Not impossible but not easy to keep consistent to the degree that you can see the slight changes in one of the variable you are changing.

 

[BoydAllen]

If you have a rifle that has the potential, you may indeed see improvement in accuracy by doing things that make your bullet pull, as estimated by seating force and difference between sized and loaded neck diameter more consistent, and by experimenting with sized neck diameter....but as the previous poster wrote, you need to control all of the other variables tightly if you want to really be able to see what is going on. Many shooters say that they want to shoot better, but when you outline the things that are normal for shooters that attain more accuracy, it becomes obvious that they want some magic answer that takes very little effort, that basically does not require that they do anything different. How do you currently prepare and size your brass?

 

[Chuckhunter]

With the Nk dimension you have you are limited. You could test various tensions by changing bushings either up for less tension or down for more tension and you could see an improvement. Your clearance is determined by the wall thickness of your brass and your reamers Nk dimension. Bench guys generally shoot tighter Nk's so they can completely clean up the entire Nk and keep them a consistant thickness all the way around, resulting in straighter ammo. As Boyd pointed out, you must have a rifle capable of shooting tight enough to actually see the difference. I have shot competatively at short range and at 1000yds. and trust me, I would not go to the lengths with my hunting loads that I go to with my match ammo.

 

[Cassidy]

Jlow, Boyd and Chuckhunter - I am not at the point in my brass prep that I can tightly control the multiple other variables involved - I am still working to control these other parameters. So right now, I'm not the person to do this test. What I would like to see in this thread is discussion focused on a little creative thinking (since this type of test apparently hasn't been done before) to describe how such a neck tension tuning test would be performed. For example, at 100 yds, with all the other parameters dialed in and tightly controlled for case prep, load, seating depth, etc, maybe the first 5 groups, all with a 0.287 neck bushing, would be 3-5 shots each, changing the neck thickness by increments of 0.0003 per group across the five targets, and the next 5 groups might be with a 0.286 neck bushing, again changing the neck thickness by increments of 0.0003 across the five targets. I'm trying to get some input on how, not just for myself (eventually I will) but others can test tune to see how much neck bushing size and how much neck thickness influences POI precision so others can similarly tune neck tension for their rifles if these tests turn out to be useful for improving precision. Trying to get ideas about how to design this neck tension tuning test.

Separately, just had a surprise this evening. I'm retired AF. Just had 7 SF guys - some active some recently separated - come visit for a beer. I love it. Will spend more time with them this Labor Day weekend shooting.

 

[jlow]

Unfortunately, it can be complicated because even if you have everything dialed in there are yet other parameters that can work as a monkey wrench against you.

For example, you cannot do this using any combination of powder, primer, case, and bullet because if you are not in an accuracy node to begin with then both the precision/accuracy of your rounds would be such that it would be difficult to see the effect of slight changes. In addition, the other parameter you would be interested in i.e. MV consistency will also be affected.

Even if you were on an accuracy node, significant changes in temperature and wind can also affect the above between days. This is why it can be so difficult to do these studies because simple things such as temperature and wind which is completely out of your control and which can be different between days can make comparison between studies done between days difficult.

Not trying to talk you out of it but just pointing out what you might be up against.

 

[Lapua40X]

I believe that some of the published emphasis on neck tension and its affect on accuracy approaches myth. It certainly makes sense that a thinner and thereby less dense (and therefore weaker) neck is likely to produce less restriction on the bullet as it leaves the case. But let's face the fact that the bullet diameter remains constant and the case neck must offer an inside diameter that is at least less than the bullet's outside diameter when the bullet is seated so that some surface tension remains to hold the bullet in place. The major difference then is in the distance between the outside diameter of the case neck and the chamber wall where a turned thinner neck has more space to expand and theoretically may expand more quickly.
Without carefully controlled scientific laboratory testing I doubt that any of us can scientifically prove that there is a measurable difference between a .284 case neck that's turned to remove .002 and sized for an inside diameter of .282 has any advantage over the standard commercial case sized to the same inside diameter or one that is turned to remove .003 under similar sizing conditions. If we can't measure it scientifically I believe we should be careful about relying too heavily on claims that someone dramatically improved accuracy with a certain neck turning formula that reduced neck tension.

 

[BoydAllen]

Many of us have already done the test that you seem curious about, but not yet prepared to do. When you have gotten to the point where you are ready, let us know. It is relatively easy, but explaining it to someone who is not going to do it anytime soon seems like a bit of a waste of bandwidth.

 

[Cassidy]

Thanks for the warnings.