Case Concentricity

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forty_caliber

forty_caliber

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Did some work with the new tooling for 6.5CM and my match grade ammo project. The main goal for now is to get best possible concentricity across a batch of cases during the sizing operation. The cases that I'm using are on their 3rd firing. I'm seeing some promising results using the concentricity gauge from web to ogive in the realm of .001 - .002

I've come up with kind of a process that seems to be working fairly well but thought I would run it by the membership for any input.

Forster Co-ax press
Body only full length size
Neck and shoulder with Forster neck bushing bump die
Mandrel for neck ID

This is basically separating the processes going on in a normal full length sizing die but with more control on the diameters of various case dimensions. Haven't shot any of these yet but the numbers look good.

.40
 
You are going down the rabbit hole!!!!

If your process is good, you will see .0005-.001 runout with good brass.

Here is what i have found:

Forster Press - Check
Good brass like Peterson or Lapua that has consistent neck thickness - varying neck thickness will be pushed to the outside diameter of the neck when you expand with a mandrel or seat bullet. Movement in the dial indicator is often not an issue in the process but an offset bullet caused by variations in neck wall thickness.
Anneal every firing - This has helped with shoulder bump consistency (.0005-.001 variation) and reduced concentricity issues.
Forster FL sizing die - I have found bushings can impart run-out if you are sizing the neck more than about .004 so I now use a regular die (all one piece) honed to .004 smaller than finished loaded round. Forster charges about $24 to hone the neck. Some even have them honed to the exact diameter for the neck tension they want but I prefer to run an expander mandrel through to set final neck tension.
Graphite or Moly - on the necks before running the expander mandrel through them.
Forster Mico Seating Die - Its important to seat the bullet straight. The spring loaded sleeve aligns the case with the bullet before it is seated for the best possible alignment. I have found that once i have seated the bullet, I get lower concentricity issues if I rotate the case 90 degrees after seating the bullet, seat, another 90 degree, seat a 3rd time.

Ten guy who load precision might give you 10 different answers but short of an Arbor press, Wilson arbor dies and turning necks, that is how I have been able to get consistently low run-out.

EDIT - sorry for the poor spelling. I guess my coffee had not kicked in yet.
 
Last edited:
forty_caliber said:
Wascally wabbits. I'll look into obtaining a honed die.

.40
Click to expand...

The form can be downloaded from their website. I have found it cheaper to buy the dies on eBay and send them to forester for the honing work. Turn around is about a week.

My Redding busing and competition seating dies have all been replaced and I couldn't be happier.
 
Turning necks will gain you a 0.001" less as an average. Most of the brass I've checked varies by 0.001-0.002" in wall thickness. Evening this out helps keep everything in alignment.
 
The cases that I'm using are on their 3rd firing. I'm seeing some promising results using the concentricity gauge from web to ogive in the realm of .001 - .002
Click to expand...

I can’t visualize what you are measuring, sounds like a loaded round.?

If I am going for ideal, I always start with the fired case, then remeasure at each step along the way.

Lots of times a case can start out great then get worse.
 
Yep, a fired case in a quality chamber should start off at pretty much zero, then check after each step and see where it goes wrong if the loaded rounds are not straight.
 
Walkalong said:
Yep, a fired case in a quality chamber should start off at pretty much zero, then check after each step and see where it goes wrong if the loaded rounds are not straight.
Click to expand...

From what I have seen it’s generally lube type or application and expanders, if they are used is the #1 cause for the cases getting out of shape.
 
jmorris said:
I can’t visualize what you are measuring, sounds like a loaded round.?

If I am going for ideal, I always start with the fired case, then remeasure at each step along the way.

Lots of times a case can start out great then get worse.
Click to expand...

I made a dummy round using one of the same projectiles that I plan to use with one of the cases that I put through this particular ringer. I set the CG so that the bearing surface was just above the web of the case and the other riding the neck. Dial indicator was riding the ogive.

When the DI was riding the neck, I was seeing about .002 runout, on the ogive .001. So this tells me that the ID of neck is a little more concentric than the OD of the neck. I'm looking to see if neck turning is really necessary...or not so much.

If the center-line of the projo is inline with the center-line case what is neck turning going to do for me? When the round goes into the chamber it's going to space off the shoulder not the neck.

.40
 
forty_caliber said:
I made a dummy round using one of the same projectiles that I plan to use with one of the cases that I put through this particular ringer. I set the CG so that the bearing surface was just above the web of the case and the other riding the neck. Dial indicator was riding the ogive.

When the DI was riding the neck, I was seeing about .002 runout, on the ogive .001. So this tells me that the ID of neck is a little more concentric than the OD of the neck. I'm looking to see if neck turning is really necessary...or not so much.

If the center-line of the projo is inline with the center-line case what is neck turning going to do for me? When the round goes into the chamber it's going to space off the shoulder not the neck.

.40
Click to expand...
Neck turning is about fit in tight neck chambers and even neck tension in all others. It can impact consentricity but that is a secondary factor.
 
forty_caliber said:
I made a dummy round using one of the same projectiles that I plan to use with one of the cases that I put through this particular ringer. I set the CG so that the bearing surface was just above the web of the case and the other riding the neck. Dial indicator was riding the ogive.

When the DI was riding the neck, I was seeing about .002 runout, on the ogive .001. So this tells me that the ID of neck is a little more concentric than the OD of the neck. I'm looking to see if neck turning is really necessary...or not so much.

If the center-line of the projo is inline with the center-line case what is neck turning going to do for me? When the round goes into the chamber it's going to space off the shoulder not the neck.

.40
Click to expand...

What portion of your case is making contact with the chamber, once the bolt closes?
 
index.php
 
It’s nice to be able to dial back and forth between bullets, and less room for error. Convenience almost always costs more. :)
 
Walkalong said:
It’s nice to be able to dial back and forth between bullets, and less room for error. Convenience almost always costs more. :)
Click to expand...
To be honest I use mine on the same bullet all the time. I may have to make a small adjustment based on lot number but otherwise nothing. Being fair how a product is used changes the value of things like the micro top.
 
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