Concentricity Observations

[pbatesaz]

Projectile Runout: A summary of measurements obtained from a small sample of newly loaded rounds of 6.5CM ammunition

Materials: 24 new, unfired Lapua cases, (16) Berger 130gr hybrid projectiles, (8) 122 gr Warner Flatline projectiles

Methods: Cases were full length sized in a Redding standard die with the expander and depriming stem removed. Case necks were then expanded using a 21st Century expander mandrel. Flash holes were deburred with a 21st Century tool. Case necks were turned to .013 with a 21st Century lathe, washed and dried. Cases were primed, charged and projectiles were seated in a single motion using a Sinclair micrometer die in a K&M arbor press. Using a 21st Century runout gauge, runout measurements were taken at the case neck, at the bearing surface of the projectiles near the case mouth and near the projectile tip.

Results: Bullet runout near the projectile tip ranged from .002 to .008 inches, averaging .0042. Bullet runout near the case mouth ranged from .0005 to .004 inches, averaging .0016. Case neck runout ranged from .0005 to .002, averaging .0012 inches. The tip runout to case neck runout correlation was somewhat weak at R=0.27, Bullet bearing surface to case runout correlation was slightly stronger, R=.032. The bullet tip to bullet bearing surface correlation was stronger, as expected at R=0.77.

Interpretation & Discussion: This is a small sample of cartridges and subject to the shortcomings associated with potential statistical variation. That issue aside, projectile runout in these cases is mostly due to factors other than case preparation, almost certainly due to the bullet seating procedure. The source of case neck runout was not examined, possibly related to the case sizing process, neck expansion or due to manufacturing case body/neck eccentricity or tipping. Further evaluation of these possibilities is planned. Changes in the bullet seating procedure will need to be evaluated. Previous observations by others suggest that partial bullet seating followed by 180 degree rotation of the cartridge and completion of seating may help reduce runout. Lubrication of the projectile may also be of some benefit. Perhaps honing the interior of the neck, by some as yet undetermined means might be helpful. Anything that would axially align the case neck and projectile would seem to be desirable. This might include a custom seating die and/or free floating the case for self alignment

 

[spife7980]

I like where this is going. Good conscience summaries of what you see. I’d like to add fired and sized to the list, I suspect they will be better then.

 

[ken4570tc in WY]

My observation would be that your procedure doesn't include running the neck turned cases through a FL die to re-align the necks, could account for some run-out variances. However, as Spife stated, firing should iron out much of it.

 

[pbatesaz]

I plan to look at the neck runout again after firing and following FL resizing. Then I will look at runout when they are next loaded. More data to follow! Ideas and thoughts are encouraged.

 

[Kraze]

I feel like I'm reading the abstract to a scientific paper (that's a compliment). I look forward to following along as you gather more data!

 

[LA260]

Awesome project! Funny I've been thinking about measuring concentricity for few months and actually ordered the 21st century concentricity gauge just couple of hourse ago. I will be keeping an eye on this tread to get some ideas and also to compare with my results.

 

[_Raining]

I am sure I am wrong, so feel free to correct me.

What makes sense to me is that when you measure case runout, you are measuring the irregularities of the case neck thickness as well as how well it is centered up. Turning should reduce the irregularities, the degree of which depends on how tight of a fit the case is to the turning mandrel and how much material was removed.

The problem with the most common concentricity gauges is that you are running on 2 spots on the case body, which could also have runout and irregularities in thickness. So you could essentially be canceling out the irregularities and runout of those points on the case or stacking them. (A low spot on the contact with the concentricity bearings cancels out a high spot on the point that the dial indicator sits, same for high/low. Vice versa, a low spot on the bearing + a low spot on case neck = a larger runout value on the dial, same for high/high).

When you measure on the bullet you are adding any runout that exists in the bullet jacket with that runout from the case neck (inside runout from not being centered up. Not sure if thickness irregularities on the inside will show up as runout on the bullet or if it is just a factor of inconsistent release friction upon firing). The same stacking or canceling out can occur in this situation also.

For measuring the runout on the tip of the bullet, the best I can come up with to describe it is:
The purple line is longer then the blue line. Plus you also have jacket irregularities there as well.

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I assume you would need a different tool to measure just the bullet but that info might be relevant as well as putting the dial on the case body and putting the dial on the inside of the case neck. Or I could just be an idiot babbling about nothing ¯\_(ツ)_/¯

 

[spife7980]

@_Raining I think this also helps demonstrate how those differences in material thickness can influence your results.
First is perfect all through such as with a turned neck, the next the bullet/inside neck is perfect but the outer case neck shows the differences, the last is how those differences can push the bullet out (https://journalofmountainhunting.com/lock-and-reload-part-4/)

And then you also have this to go into your readings even more.

So ultimately what it comes down to is what kind of concentricity do you actually have and where does it come from? Thats the answer I cant reason and why I havent gotten a conc gauge. I dont know what I would do with the info once I had it without purchasing even more measuring equipment to figure out what the first was actually measuring.

 

[LA260]

@_Raining So ultimately what it comes down to is what kind of concentricity do you actually have and where does it come from? Thats the answer I cant reason and why I havent gotten a conc gauge. I dont know what I would do with the info once I had it without purchasing even more measuring equipment to figure out what the first was actually measuring.

This is why it took me two months to order the gauge. I still dont know what I'm going to do with the measurement info once I get it but figured I'll start with a fired case and just measure every step of the reloading process to try to come up with some kind of solution to a problem that might not even be there or does not matter much. Plus I like nice tools lol

 

[pbatesaz]

There certainly remains a lot to study. FWIW I also have a Hornady concentricity gauge (which supprts the cartridge at the bullet and case base) and compared a few random cartridges at the bearing surface of the bullet. The readings were as nearly identical to those obtained with the 21st Century gauge as I could determine. I plan to look at these cases again after firing and will look at inside neck runout as well as outside runout and outside case body runout. With the next batch I want to look at the virgin case body and neck, virgin resized case body and neck and turned case neck inside and outside runout. With the current batch, post turning neck thickness was uniform within .0005 as measured with a tubing micrometer. Gratified by the interest. Thanks for your contributions guys!