Concentricity Gauge


Active Member
Apr 21, 2012
South Africa
I got myself the Forster case and cartridge inspector that checks case concentricity and bullet runout. Grabbed a handfull of loads I did a while back and tested bullet runout. First 3 perfect with hardly any movement on the dial. Next 7 varied from .005 to .007. Next lot gave me .001 to .003.

The question now is how do I rectify this runout. Read somewhere of someone having drilled a hole in a block of wood slightly larger than the particular caliber, then inserting the load into the hole and manipulating same until no further runout. I need a more "scientific approach"

Any suggestions, what do you gents do?
Currently using thehole/block of wood as well. I am sometimes able to straighten out excess runout by running the loaded case back up into the seating die, then turning case in shellholder and repeating 2-3 times. Seems to work about every fourth case I try. The rest (if they are just a few), become fouling rounds. If more than a few, I pull the bullets and start over with some addl. case prep. On the problem cases, I check for runout on the neck before loading again. Using Redding competition dies with bushing. Maybe going with the next size smaller bushing would help?? Looking for answers as well.
I need a more "scientific approach"
This starts with understanding of sizing, and a better tool.

Forster claims the following functions for this tool: "Neck wall thickness, Case neck concentricity, and Bullet runout"
-Use a ball mic with a stop for thickness
-Their tool indicates only a small portion of eccentricity
-Their tool in no way indicates 'bullet' runout, nor cartridge runout

As far as bending loaded ammo to solve a problem? NOT scientific, or well advised...
Better to learn how to make & measure(to verify) straight ammo.
For me, it didn't seem to matter what I did, I still had lots of runout...then I started turning my necks.

I think the the thicker side of the neck was forcing the bullet off center (towards the thinner and weaker side) during bullet seating. This caused excessive runout on cases that had very good runout after sizing.

I picked up a Wilson Chamber Type Seating Die, hoping that it would cure my problem, and it made no difference. Once the necks were turned, all loaded rounds exhibited less than 0.003" runout.
Oh well, will use those extreme runouts as fowlers. Since building those rounds I have upgraded to Redding S bushing dies and a competition seating die. I will be testing the new rounds on the Forster tool before going to the range.

In case you are wondering... I am anal about case prep and do go the whole 9 yards. I have a RCBS case neck thickness cutter but have yet to master this tool. Having problems with cutting too deep into the shoulder area as a result of the 20 degree angle/bevel of the cutter. Might just try and grind that back to say a 40 degree angle. Any thoughts on this?
I bought a 21st Century Tool, and they make the cutters in different angles to match the case. This helps to keep that from happening. I would agree that the 20 deg angle is a bit shallow.
" didn't seem to matter what I did, I still had lots of runout...then I started turning my necks....This caused excessive runout on cases that had very good runout after sizing."

Well, case neck turning IS something, isn't it?

Given the same cases and no prep, I doubt the new bushing die will accomplish anything at all.
Well, case neck turning IS something, isn't it?

Given the same cases and no prep, I doubt the new bushing die will accomplish anything at all.

I would tend to agree - as I thought I was doing it all correct and didn't believe I needed neck turn. I proved myself wrong on that.
I use a RCBS Case Neck Turner With Auto Feed. I have found the neck turning tool remove runout to .001 or better. I have more control with a RCBS Pro Trimmer II than a Power trimmer. It is a time consuming step, but it works. When turning the outter neck, let the blade remove only half of the surface area of the neck, so only one side of the neck will be shiney. There is no adjustment when reaming the inside of the neck. Good luck.
I just turn my case necks only enough to clean them up if there not concentric in regards to thickness. One habit I got into years ago is when I'm turning necks on a new to me caliber is I will take every case I can get my hands on for that caliber and inspect them all, looking for the worst offender (being that case that has the thin side thinner than the rest) and set my neck turner to just barely clean that one up. My reasoning is that from that point all the rest will also be cleaned up and of consistent neck thickness. I will mic my turning tool and record that setting which is valid only for that caliber, so in theory with cases purchased at a later date can then be neck turned to the same dimensions.

After that, the other factors that I can see effecting runout are sizing and seating dies, and just how good your chamber was cut. If your dies induce runout, the only real fix is different dies, though I can see where turning the loaded round in the shell holder and bumping it in the seating die could help. If your rifle is of high manufacture rate you could very well have a chamber that is not quite concentric to the bore, or the neck portion isn't concentric with the case body portion, I can't imagine where a guy could do much at all that would mean anything without a visit to your GS of choice.
a rubber o ring on the seating die where it meets the press is a help.
partially seating then turning the case before final seating also helps.
the o rings can be bought in the faucet repair section at places like home depot.
I have three or four concentricity gauges, and right now the one I use the most is a NECO. I like their indicator setup far better than most of the others out there right now, and the readings do repeat consistently.

I like to shave the necks on my cases that I run thru inline dies, but that's just me. Just seems to give me loaded rounds with much less runout, and I can seem to get a better controll over the bullet tension in the neck. I use a Sinclair neck turning tool that I rebuilt (still not in love with it), but will probably buy a K&M sometime this winter. I'd kinda like to try a Wilson outside neck turner sometime in the near future as I see somethings about it the catch my eyes.
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