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Discussion Starter #1
I’ve been wanting a cartridge/case neck runout gauge, but it was always one of those tools that I just didn’t want to spend the money on, so I cobbled one together from some stuff I already had. The four ball bearings are neodymium magnets along with the cartridge back stop. The Mitutoyo mag base and the dial indicator (which is a Mitutoyo also, just not labeled) I already had from my machinist days. The dials devisions are .001”, but it is easy enough to guess .0001” measurement

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Now that I have this, I’m a little dismayed at some 308 Win that has an indicated runout of .0015”-.0023” runout. Now I have re-evaluate my case resizing and bullet seat technique.
 

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I like it. If it works, run with it.
 

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Discussion Starter #4
It will make a difference down range, but that is if the rifle is capable of a high level of accuracy to begin with. (1 MOA or less) The idea of checking bullet runout in a loaded cartridge is to measure how far out the bullet is from being inline with the chamber/barrel bore axis. If the bullet engages the rifling non concentric with the bore axis, the bullet will be spun by the rifling off center and when the bullet exits the barrels muzzle the bullet will not spin along its central axis. This in turn will cause the bullet to spin in a ever increasing circle along it’s flight path, which in turn causes a deviation from the bullets intended point of aim. I should mention that aside from cartridge case/bullet runout even if reduced to an absolute minimum, there is still the rifles chamber dimensions that can affect the concentricity of the loaded cartridge in the chamber. Factory production rifles can have quite amount of dimensional variation in regards to SAAMI specs. Custom built rifles with minimal chamber dimensions along with what is referred to as a “Tight Neck” will dramatically increase the potential accuracy of the rifle allowing the full accuracy potential of a loaded cartridge with minimal bullet runout.

Bullet runout in a loaded cartridge is the result of three factors.
1- The cartridge case neck is not concentric with the cartridge body. This could be as a result of the cartridge manufacturing/forming process.

2/A- With a resizing die, this can be a result of miss alignment with the die in relation with the presses ram, shell holder, and the concentricity of the die itself and its components. (When resizing a cartridge in a conventional resizing die, allow the sizing button to float by backing off the stem retainer that holds it by 1/16-1/8 of a turn or so from the die body, this will allow the stem/sizing button to self center in the cartridge case neck when pulled through. This can be done with standard RCBS, Redding, Lyman, Bonanza, etc..) Sizing dies such as Hornady and Lee that use a collet to hold the stem/sizing button, this method of floating the stem/sizing button will not work because the collet has to hold the stem rigidly to prevent it from slipping. Neck Bushing type sizing dies are less prone to cartridge case body/neck runout if the bushing is allowed to float, but only by a few thousands of an inch. The down side to a bushing type sizing die is that it does not have a sizing button so the cartridge case neck has to be turned to a even thickness either with a hand turning tool, or a hand cranked or machine lathe. If not the cartridge case neck tension will vary and that will affect accuracy.
2/B- The conventional type bullet seating dies can cause bullet runout because the bullet is not supported fully at the first stage of it being pushed into the cartridge case neck. The best type of bullet seating die to reduce bullet runout is a floating chamber type that fully supports the cartridge case and bullet before the bullet is seated. Five examples are (listed by level of precision), L.E.Wilson, Chamber ($65), Whidden Gun Works, Micrometer ($150), Redding, Competition ($140), Forster, Bench Rest, ($60), Hornady, Custom Grade New Dimension, ($30). The L.E.Wilson will require a small arbor type press, the rest can be used in a conventional type loading press. (A side note is that in my opinion the Hornady is junk. I had a set of their C.G.N.D. for 25-06 and it produced an unacceptable amount of bullet runout.)

This is an example of a floating chamber type bullet seating die.
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3- Variation in the thickness of the cartridge case neck circumstances. If the cartridge case neck is I.E. .015” on one side and .012” directly across its diameter, when the bullet is seated into the cartridge, the thinner side will expand easier and the bullet will offset to the thin side of the neck, and will result with a bullet that is no longer concentric with body of the cartridge.

The 308 Win cartridge shown in my original post was assembled with a virgin PPU cartridge case that I ran through a RCBS standard type neck sizing die, but the stem/sizing button was held rigidly because the necks were already sized when manufactured but had dents in cartridge case mouth that needed to be rounded out to be able to inside chamfer them prior to bullet seating. The variation in the cartridge case neck thickness is what is causing most of the bullet runout. I measured some 308 Win that had their necks turned and the total runout was between .0002”-.0004”.

This is the Redding bullet Concentricity Gauge I sorta copied. Admittedly it has more adjustability then mine and is more compact. The Case Neck Gauge I don’t need because I use a .0000”-1.0000” tubing micrometer to measure the cartridge case neck thickness.
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