To turn or not (necks)

[nheninge]

Yes, I think fire forming followed by FL resizing should reset this difference because your dies body/shoulder/neck are drilled/honed with concentric bits and should have a single axis if they are quality dies.

However, thick/thin sides run the length of the case. This is only noticed if ID measurements of variance are taken.

I find a lot of value in culling cases with increased neck variance. If I get 200 good cases with no variance (followed by neck turning), those 200 cases may last the life of the barrel if the brass is treated properly. A lot of concentricity/runout problems are avoided since I never start with a problem.

 

[woods]

Good stuff!

How do you sort for neck thickness variations? Do you use a ball micrometer?

With brass as hard to come by lately it would hurt to have to throw some away.

 

[nheninge]

I sort neck thickness variation with the Sinclair tool. It has a pilot stop for each caliber that fits perfectly after using an expander mandrel (to round the necks on new brass. Then you spin the cases by hand and read the variation on a dial mic.

Thick and thin sides can be marked easily for indexing
A ball mic w/ stand and feeler gauges are used to verify thickness

I know brass is expensive, but so is a miss...

 

[tlk]

I am using an RCBS case master guaging tool, and I dont think I am too happy with it as it is. It works similiar to the Sinclair tool, but there is a bushing that slips on top of the case neck. You measure off of this bushing, but I found it to vary up to .003" in places if you roll it instead of just take specific data points. I think that if I took the bushing to my local machinist and had it smoothed out it would be fine. Probably will do that. Or I will get a good ball micrometer as I suspect the cost will be the same.

 

[tlk]

OK, I think I found a problem: If the brass is f/l sized as stated before and everything is centered, neck is turned, etc., what happens to the brass upon firing?

Will the thin portions of the case body expand faster than the thicker and cause the case to swell into the chamber at an uneven rate around the case body? If so, will this cause the bullet to no longer be aligned by the time pressure has built up enough to get it out of the case and into the barrel? Is the case thickness variation causing innacuracy in this case? Somewhat difficult to explain what I am talking about, but I am thinking that the uneven expansion might have an effect by placing leveraging force on the shoulder/case mouth.

But would it? Would it matter if you had the case body smaller than the chamber and the whole case pinned between a trued boltface and the datum? I wonder with this being the situation if the shoulder being a slight crush fit would help prevent/minimize this issue.

 

[tlk]

Still working on this one, let me know if these thoughts are correct:

If your brass is slightly crush fit (as per above with the case body smaller than the chamber), then the only time that the thickness variation of a case body would matter would be IF the case body wasn't finished expanding by the time the bullet began leaving the neck, which may negate any advantages to neck sizing because it would cause the bullet to be off-center again as it moved into the rifling.

The question is if the pressure required to launch the bullet out of the case mouth is greater than the pressure to expand the case itself. If it is less, then the bullet could leave before any case expansion occurs, giving you the full advatage of the trued neck. Same thing would happen for the bullet leaving after the case had fully expanded, since it would be leaving after the case would be at an O.D. equal to the chamber and all of the inaccuracies in thickness variation are back to the inside of the case (during expansion it is moving both to the inside and outside as the case expands from the inside). But pressure would be higher than a lower neck tension because you need to have the bullet hold until ALL of the brass is expanded to the chamber.

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The real issue is when the bullet leaves the imperfect (but f/l sized) case that has started expanding but has not finished. Any off-center expansion could be transferred to the bullet. Also, inconsistency for the same case at different reloadings could be thrown into the mix because the case is not in the exact same position each time in the chamber, causing the "off-center" of the bullet to not the the same shot to shot (could be off-center at the 4 o'clock position, next reloading at the 8 o'clock, etc - rotation of the case in the chamber).
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If I look at it like this, it means to me that neck turning serves two purposes on the imperfect case: alignment and neck tension, but the bullet must leave at a certain time for it to be advantageous. For nheninge's perfect cases, it serves only for neck tension, and bullet launch can be at any time.

I think I see now why neck turning is not a "nit". Please correct any wrong assumptions in all of this. I want to get this right so we all know.

Woods, I still would like to know how the relation of case neck thickness to neck tension is derived. Is there research or numbers somewhere?


TLK

 

[woods]

I think we are well past the simple basics of internal ballistics and reloading here and are operating on theory, sorta like how they discovered black holes with lack of direct evidence but with the combination of theories and implied influences. Almost all this is not verifiable with direct measurement and at best is an educated theory.

Also we are down to minutiae (sp?) when it comes to what the sequence of events is in the milliseconds of the firing sequence. The variance in neck thickness is one of the larger and more verifiable things that we are talking about here and thus may be one of the last controllable things that you can do that will have any significant and identifiable effects on consistancy of the bullet release and firing sequence.

"Woods, I still would like to know how the relation of case neck thickness to neck tension is derived. Is there research or numbers somewhere?"

I'm not sure where I have said that the brass thickness at the neck has a direct correlation to neck tension. What I'm sure I have said somewhere is that you can vary the neck tension (inside diameter) by sizing the entire neck to a smaller dimension. Also I do believe that if you have thicker brass on one side of the neck then that brass will resist stetching from the bullet seating more than the thin side and cant the bullet (runout) and that the thicker brass will expand to the chamber walls slower than the thinner side during the firing sequence. But I do not believe that these effects (other than the runout) will have a large enough significance to make an identifiable significance on paper.

Like nheninge said the cases with identifiable neck thickness variations have those same thickness variations carried down through the case body (also something taken on faith since I have not seen proof of this). It may be possible if this is true that in severe cases you will have a dissimilar expansion and a sort of bannana effect upon firing.

If anyone has a link it would be interesting and informative to this discussion to see the actual formation process of the brass by the manufacturer.

 

[tlk]

When you take the expander out and turn the necks you can take the brass neck thickness down to a specific thickness so that you do not have too much bullet grip. For instance if you have a 30-06 full length die and take the expander out and size the case and the outside of the neck measures .329" and your neck thickness is .014" then the math would be

.329" - .014" -.014" = .301"

that is too much bullet grip of .308" - .301" = .007"

But if you neck turned your brass down to .012" the math would be

.329" - .012" -.012" = .305"

which is about right for a bullet grip of .308" - .305" = .003"

When you compared thickness to grip I took it to mean that there is a quantifiable holding force involved of some value (lbs, etc).

Part of this probably is overboard. I just put my entire thought process down for verification. Maybe all I accomplished was showing what I think on in my free time in the car .

 

[woods]

Maybe all I accomplished was showing what I think on in my free time in the car .

Man's gotta have something to think about while driving down the freeway!

Where the difference lies with respect to neck thickness and bullet grip is that the bullet grip is not derived from the neck thickness itself but rather from the inside diameter of the neck. IOW if you had 2 cases both with an inside diameter .003" less than caliber and 1 had a consistant neck thickness of .014" and the other had a consistant neck thickness of .012", there probably would be a difference in bullet grip but it would be unmeasureable, undetectable and of no significance compared to varying the inside diameter by even a small amount, like .0025".

Previously I had not sorted by neck variance but I will try it with the next batch of new brass before I neck turn. Seems to me that if the manufacturing process could make a piece of brass with a variance in neck thickness then it would be possible to make one with the neck out of center with regard to the center of the case body also.

 

[BountyHunter]

IOW if you had 2 cases both with an inside diameter .003" less than caliber and 1 had a consistant neck thickness of .014" and the other had a consistant neck thickness of .012", there probably would be a difference in bullet grip but it would be unmeasureable, undetectable and of no significance compared to varying the inside diameter by even a small amount, like .0025".

Not true at all.

Take the KM arbor press with the dial indicator on top and inline die and you can see, measure and feel the difference. No dial indicator and you can certainly feel the difference when seating. That is routinely used for neck turned brass for 1k BR shooters to sort rounds as loading for "match" or "sighter" category.

BH

 

[tlk]

Woods, I re-read your post I quoted, and see what you were saying: you are sizing off of the outside. If you size off of the outside (no expander ball)and turn the neck down you are changing the I.D. of the case neck. So instead of relying upon an expander ball, you can customize bullet grip by turning necks to various thicknesses and relying upon the die to size the outside. Didn't see that important detail before. See, this is what I mean by re-reading everything - details can get missed easily.

Can someone hand me a skillet so I can scramble this egg on my face?

 

[woods]

Not true at all.

Take the KM arbor press with the dial indicator on top and inline die and you can see, measure and feel the difference. No dial indicator and you can certainly feel the difference when seating. That is routinely used for neck turned brass for 1k BR shooters to sort rounds as loading for "match" or "sighter" category.

BH

Possibly.

IME there is a definite increase in seating pressure when going from one bushing size to another in a bushing neck sizer and also when using different size mandrels that I ordered from Lee for the Lee Collet neck sizer. IOW, by changing the bullet grip. By contrast when I have sized cases with the Lee Collet for a factory rifle on cases that I have not neck turned, I can get consistant seating force even though there are neck thickness variances. However the Lee Collet mandrel I normally use for factory rifles gives only .001" bullet grip so that may have some bearing on the seating force.

Differences in seating force IME (after standardizing the ID) has been much more dependent upon the condition of the inside of the neck. On mine I use no lube, scour the inside with scotchbrite, smooth the inside with steel wool and then use mica

in order to make for as consistant seating force as possible.

So I get consistant seating force with cases I have not neck turned and I get consistant seating force with cases that I have neck turned. IME the seating force changes noticeably with changes in bullet grip.

 

[nheninge]

The K&m arbor with force measurement is great (I also use it routinely) but it is also an indirect measurement consisting of many factors including: neck tension, neck thickness, neck uniformity/neck imperfections, lubrication, bullet shape etc. "Seating force" is the best measurement we have to measure these variations in case prep when seating bullets.

I returned my RCBS casemaster waaaaaay back. Just felt cheap, and that bushing neck thickness thing had way too much play for my liking. It just felt like it was built by the lowest bidder.

I did verify the case thickness throughout the case with the RCBS tool though. I modified a bar to fit in the tool similiar to the case head separation bar (just long enough to fit in the neck and still make contact with the inside of the brass) and found thick/thin sides consistently down the length of the cases at the same spot represented on the neck. This is fact. And the banana case theory has quite a bit of discussion/research as well. This theory's effects are mostly negated "IF" you sort by neck variation. I still index my cases squarely behind the recoil lug (which makes no sense) but I guess I do this because it is consistent.

I think in the car too. Hope we all never meet while driving or we may CRASH!!!