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Concentricity:How much ?

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Unread 02-09-2010, 04:27 PM
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Re: Concentricity:How much ?

Interesting thread.

This is what I do and I'm sure some of you more expereinced will have some ideas one way or the other:

1-pick up 300RUM cases where I can inspect each case before purchase-can't be completely sure they are from the same lot, but most likely are--each case is inspected for centered primer/flashhole, flashole inspected from both sides and from inside checked to make sure no obstructions and properly factory deburred, neck reasonably round, any other visual anomolies noted and dealt with

2-anneal case mouth/shoulder--I know I've heard that they are supposed to be annealed from the factory, but I don't see the look of the 338 Lap, for instance and i'm not sure how much I would trust the annealing job of most American made brass anyway--just trying to treat every case as consistently as possible

3-trim to length I've decided on based on measured chamber length--chamfer and debur

4-I don't sort these cases by weight and don't sort them at all other than that mentioned in step one--did a bunch of testing a few years ago prompted by a member that used to post here alot years ago from AK (Brent, I think) that he'd found that the case volume had little to do with the case weight on these 300RUM cases and that the case volumes were remarkably consistent regardless of case weight differences--I found the same so I don't weigh these anymore

5-run 'em through the matched Sinclair mandrel for the Sinclair neck turning tool and then neck turn to take off approx. 75-80%

6-run through the Redding FL die with .008 competition shellholder (to fit the chamber more closely and reduce the amount of brass being worked)

Edit: just trying to get the case as straigt as possible so that the first firing might potentially be used to start to get an accurate load development process going. Some of these cases do look straighter/better to me after doing this and it seems to have provided more consistent results with fireforming loads. Could be wrong--lots of variables.


This usually leave me with less than .001 runout on the Sinclair concentricity guage. Edit: ...after reloading again, that is. Bullet and case neck runout is typically held to less than .001 AND I have gone back to used the regular FL dies with button expander. Big no no, I know, but I get straighter cases than when I've tried 2 different bushing dies. There's several keys here, I think, two of which are to let your shell holder float (nothing retains mine) and let your button expander float but soft seating the adj. nut against and o-ring. This lets the expander find it's own center--the center the die just made on the case. Use Imperial Sizing wax as well--have yet to find anything that does as well as this stuff. Just whst seems to work well for me. I seat .010 from the lands with 210 Berger and have been getting very consistent precision from this factory rifle, but there's alot of work that has gone into improving the entire system (including my shooting form), so it's difficult to pinpoint any one area. Lots of little things might add up.
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Last edited by jmden; 02-11-2010 at 12:31 PM.
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Unread 02-09-2010, 04:34 PM
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Re: Concentricity:How much ?

Originally Posted by Mikecr View Post
Why on earth would you 'restrain' what you're trying to measure?
You're masking variance. This is why your numbers are so low.
Sounds like you're using a concentricity gage rather than a Runout gage.

As a note, the Sinclair 'Concentricity' gage is actually a 'Runout' gage.

Why are you guys FL sizing NEW brass & culling by runout?
New brass could measure any runout, even if turned, until it has been fire-formed into it's final product.

#1 Stop FL sizing new brass. It is wasted time.
#2 To see your brass at it's lowest runout(it's best potential), measure it after fireforming, & before doing anything to it.
#3 Measure after each sizing stage to see the specific runout contributor
#4 Measure loaded runout off the ogive at a point near land contact, with the bullet unrestrained. Don't assume anything about the neck, or even think about it at this point.
Think TIR off the ogives.
That's why I posted this question : to get some different approaches to the problem.

And it brings up some interesting questions=

When I think of Concentricity I think of every thing on the same center line from one to
the other.

When I check a barrel for run out I place it between centers and gauge from end to end
(This tells me if the barrel is strait)

When aligning a set of scope rings I use a bar or a lap. (The pointer type can match at
the points and still not be aligned strait for the scope.

So when I check the finished round for concentricity I want it to be absolutely strait from
the tip of the bullet to the base of the case (On the same center line). I have checked
the cases and they were near perfect but when I check the bullet ogive it would be off
quite a bit(Pointing away from the center line) .

This made me improve the bullet seating process ,and Like you after the case has been
fired in a good chamber it is about as good as it gets. so the problems seamed to be in
the sizing and bullet seating.

Now if I do both processes (Concentricity and run out) I find that I can detect a problem
and fix it easier and don,t have to guess where the problem is.

The dimensions I am getting are the worst reading I can get no mater where I gauge them
and the method I use.

And your question about case prep befor fire forming is also a good one. I simply do it to
minimize fore forming (If it is good why fire form it just use it to hunt with and fire form at
the same time.

If this is the wrong way to achieve the minimum run out and the best concentricity please
guide me in the right directions.

More input please And thanks for your reply

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Unread 02-09-2010, 09:20 PM
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Re: Concentricity:How much ?

If your chambers are anything but damn round, cartridge runout is by far the least of your concerns.
Also, dies don't remove runout. They cycle brass to cause it.
Don't believe me? Measure runout on cases freshly pulled from a smoking chamber. Then measure it after a cycle with your dies.
I don't care how perfect your dies, even those that contribute nothing to runout, they still cannot REDUCE runout. This is because the devil's in your brass.

But you can manage this to the point of irrelevance right from the git-go.
This is how:
1. Cull plenty of brass by thickness variance in each neck, as measured with a properly set up ball mic. Settle for well under 1/2thou variance, 360deg, and rake the rest in a trash can. Don't think turning negates the need for this. It doesn't.
2. Prep brass, partial neck size, and fire-form.
3. [optional]Check H20 capacity, and what leaves your tolerance is again raked in the trash.
4. Workout each stage of sizing and seating to produce runout as low as desired.
(for some, lowest measurable).

With this & depending on your load pressures, spot checks for runout every few reloads should get it. Runout always grows, a little bit, with every cycling of the brass. But my brass will wear out many barrels before runout is a concern.
And that is why I use the process described.
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Unread 02-09-2010, 09:21 PM
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Re: Concentricity:How much ?

I shoot a lot of over bore magnums. A couple of them will only give one firing per case. Fireforming may not be an option and I do not waste barrel life on it. I will prep the unfired cases and may full length size them if I think the runout on the neck is bad.

I weigh bullets and cases and cull by weight. I have experimented with weighing cases at F-class and at 1000 yards a case that is different will shoot different. I marked every case one day that gave me a flyer and reloaded them for the next competition and they gave me flyers again. A bad case is a bad case so get rid of it.
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Unread 02-11-2010, 08:41 AM
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Re: Concentricity:How much ?

Originally Posted by Kevin Thomas View Post
Hey guys,

I have a slight problem with threads like this, in that I'm not sure we're all speaking the same language. Everyone seems to have their own methods of measuring run-out, and they're not all the same. Saw this sort of thing constantly in the QC range, with customer complaints of concentricity issues. The vast majority would turn out to be customer errors in how they were measuring what they thought was run-out.

I'd like to see some sort of standardization on how we measure, so that all our results would corelate to one another in a more meaningful manner. Don't ask me how we do that, but that's what I'd like to see.

Beyond that, I think we can all agree, less is better. But how much?
Great comments. I totally agree.

I've been facinated/measuring/testing/wondering about bullet runout for years. Even made a few tools to measure it. Depending on how the round was supported and where the dial indicator's plunger was put, different numbers would show up for the same round. And a .223 Rem. with its bullet 2/10ths of a degree crooked to the case axis will show less runout than a .300 Wby. Mag. with its bullet at the same angle.

As rimless bottleneck cases are pressed hard into the chamber shoulder when fired (firing pin impact does this with enough force to set the case shoulder back a few thousandths), that very well centers the front of the case in the chamber. And the back end's pressed against the chamber wall (typically at the pressure ring) by the extractor's force on the case head. With bolts having inline plunger ejectors, cases are pushed forward against the chamber shoulder, too. Which means every case is a tiny bit crooked in the chamber when it's fired. If a round's got absolute zero runout, its bullet will still be a tiny bit crooked to the bore axis. Doesn't matter how much case neck or bullet clearance to the barrel there is. The front of the round's centered this way in the chamber. If one wants the bullet to be aligned with the bore, the case neck needs to be well centered on the case shoulder. It doesn't matter how much clearance there is between the shoulder-body junction and the same point in the chamber, but there can be no interference at this point.

As there's not such thing as a perfectly round case, their slight out of round numbers will effect runout numbers

So I made a tool out of ball bearings on Garand cleaning rod sections held in aluminum blocks at the ends. That let the loaded round rest at its pressure ring and mid point on the shoulder. Put the dial indicator on the bullet 1/10th inch back from the tip. If I put the dial indicator just back of the case mouth, I can measure case neck runout. Whatever runout there is on this tool will pretty much match what there is when the round's chambered. And any slight out of round the case is at its body-shoulder junction won't effect the numbers.

In measuring case neck runout with several rimless bottleneck and belted cases, then using all sorts of bullet seaters (standard, inline chamber types, cheap, expensive), they all seated bullets very well inline with the case neck. And the case neck still had the same amount of runout after bullet seating as it did afterwords. Which tells me that if the case neck axis ain't in line with the case body, any bullet seating tool's not going to help. Something has to be done to make sized cases end up with straight necks. I noted on Redding's web site some time ago they've now got an article covering this very issue. It's at the "concentricity" link below:


Here's what I've found helps make sized case necks as straight as possible. And produce best accuracy, too.

First, deprime (without sizing) fired cases then clean them before any sizing's done. I run fired case necks over a bore brush before this to clean out most of the powder fouling.

Use full length sizing dies with their necks lapped out to about 2 thousandths smaller than a loaded round's neck diameter and reduce fired case body diameters no more than 2 thousandths. This supports the case body very well when sizing the fired case neck down with minimum brass movement. There's no expander ball coming back up through the case neck that often bends it from where it was at coming out of the die's neck. Redding and RCBS make full length bushing dies that does the same thing. I've tried expensive and popular neck sizing dies (regular and bushing only types) that never produced sized case necks as straight as full length sizing dies with properly sized neck areas. I and others have got 40 to 60 reloads per .308 Win. cases using this method for maximum loads.

Every time I trim cases back to length, I debur the case mouth inside with an Easy Out screw extractor. It's got a better angle and doesn't leave as sharp an edge that scrapes off bullet jacket material when seating them. I run the case over that bore brush again to make the inside edge rounded facilitating minimum resistance to bullet seating.

It doesn't matter what type of seater I use, runout's typically 2 thousandths or less with varous 30 caliber cases. I've seen no improvement in accuracy through 1000 yards with any less runout on ammo shot in standard SAAMI dimensioned chambers.

Last edited by Bart B; 02-11-2010 at 08:56 AM.
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Unread 02-11-2010, 12:07 PM
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Re: Concentricity:How much ?

Originally Posted by Bart B View Post
a .223 Rem. with its bullet 2/10ths of a degree crooked to the case axis will show less runout than a .300 Wby. Mag. with its bullet at the same angle.
Not if the bullet angle is the ONLY source of runout.

Originally Posted by Bart B View Post
let the loaded round rest at its pressure ring and mid point on the shoulder. Put the dial indicator on the bullet 1/10th inch back from the tip.,, any slight out of round the case is at its body-shoulder junction won't effect the numbers.
Wont affect YOUR numbers, but this frees mid-body and discounts it as a problem by subtracting it from measure.
Many shooters are doing this(rationalizing/discounting runout) by jamming their bullets. And atleast several concentricity tools/case benders do the same, given that bullets are jammed into the lands, as they are in the tool.
Only the truth passes all tests.. So runout is not a problem to conceal, as straight ammo measures just that no matter what.

Originally Posted by Bart B View Post
Something has to be done to make sized cases end up with straight necks
Its called fire-forming,, the best way to straighten a case there is.
The only way to alter neck centerline after fire-forming is to FL size and/or expand necks away from centerline. If you do neither, your necks will remain centerline (although there could still be runout, that could still be an issue).

This is where partial neck sizing is a better plan. It preserves the trueness in necks that fire-forming produced. Full length sizing a neck can only alter trueness, and serves no positive purpose other than to reduce a wildcat neck to lower cal..
Expansion is required, but currently risky IMO because the operation is free from centerline (it could be off). And the worst contributor to runout ever invented is the button expander.
I currently top expand with Sinclair carbide mandrels(just before bullet seating) to drive any thickness variance outward. It is a free floating expansion, just like bullet seating, and could could alter centerline -if I had FL sized necks..
So far it has not contributed to runout off the noses, except to reduce it. Like said, and for me, seating is a very minor contributor to runout.

If you'll measure little runout from your brass fresh out of the chamber, then stop & consider the 3 ways you'll screw this up:
sizing, sizing, & sizing (in order).
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Unread 02-11-2010, 01:21 PM
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Re: Concentricity:How much ?

Mikecr, I've tried all sorts of partial necksizing. Used different full length sizing dies for both rimless and belted bottleneck cases. They all ended up pushing the fired case shoulder further forward that where it was. Bolts bound up to various degrees depending on how tight the partial neck sized case fit the chamber. Accuracy was not very good at all. Even tried expensive neck only dies using bushings of different diameters to size different amounts of the fired case neck. Again, poor accuracy.

Partial neck sizing with a full length die and neck only methods often produce better accuracy than full length sizing that sets the fired case shoulder back way too far. Most of the high power rifle matches (and records set with them) are won with full length sized cases. The smallest test groups at the longer ranges having more than 10 shots in them I know of have all been done with proper full length sized cases.

I and others get better accuracy with new cases than any partial or full neck sizing process of fired cases; especially with belted magnum cases. And in standard SAAMI dimensioned chambers, too. Sierra Bullets has been full length sizing their cases used to test their bullets for accuracy since the 1950's; I don't think anybody shoots 'em as accurate as they do. Sierra doesn't even work up loads nor weigh charges.

When partial neck sizing has a good track record in disciplines where best accuracy is attainable, I might try it again.

Last edited by Bart B; 02-11-2010 at 03:39 PM.
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