Modifications to Hornady Concentricity Tool

Article 4

Modifications to Hornady Concentricity Tool

How to turn this:

Modifications to Hornady Concentricity Tool 3

Into this, and to be able to return to the original configuration.

Modifications to Hornady Concentricity Tool 5

I am sure there are many people that use the Hornady Concentricity Tool often.  It is a good basic tool and has its strengths and weaknesses.

First the strengths.

  1. Not expensive.
  2. Easy to use.
  3. Accurate as a “bullet in case” runout checker. (AKA Cartridge Checker)
  4. Allows measurements to be taken at almost any place along the case sides, as well as the bullet.
  5. Assuming the case rim is not damaged, more closely replicates measuring runout based on the actual cartridge axial centerline, instead of measuring from the case wall which has expanded in a non-uniform manner. (The user must provide some pressure on the bullet tip holder to compensate for the loose fit of the center in the frame.)

Now the negatives.

  1. The track that the dial indicator runs on is not parallel to the case axial center. (Mine was off .0010” when measuring a straight shaft)
  2. The dial indicator cannot get to the case base just above the extractor groove on rimless cases.
  3. One cannot measure concentricity on an empty case. (AKA Case Checker)
  4. The “so called” run out corrector is useless as it breaks the bullet to case neck tension which is more important than run out for accuracy.

Modifications: Make the Negatives into Positives.

By the negative numbers above.

    1. First check your tool to see if it is parallel or not. Find a straight rod that can fit between the two reversed centers.  Measure with the dial indicator at the extreme ends of the rod and see if the dial needle rest at the same mark on the dial.  If the needle does not rest at the same mark, loosen the right screw and adjust the track until the dial indicated the same at both ends.  Then tighten the left screw to lock the track.  Remove the right screw and coat the screws tapered head (not the threads) with blue or green Loctite.  Replace the screw, but tighten only enough to keep the track from moving vertically.  The threaded holes in my base were not drilled in alignment.  This adjustment is not really needed as the concentricity measurements are relative, + or – readings, not absolute diameter measurements.  (I only did it because I am a nerd).
    2. Referring to the picture below.  This part involves modifying the case base end reverse center.  (The big round heavy shaft to the left of the tool, held by an eccentric clamp.  Remove this from the tool, and with a good file, or grinder and file, remove about .100” from the center cone end.  Use the largest cartridge base you will ever use as a gauge so you do not take too much material off.  You cannot replace it later.  Remove enough material so you can just see the case extractor groove looking from the perpendicular.  Radius the edges.  Then with a file, make a flat about .300” on the outside diameter of the shaft on the center cone end.  This allows the dial slider to overlap the shaft so the dial can get closer to the case base (the part just above the rim groove).  Finally, file a bevel on the edge of the center cone to allow the dial plunger to approach the case groove.  That finishes this part.  Refer to the following picture.                              Modifications to Hornady Concentricity Tool 7
    3.  Now comes the case mouth end. We need to make a Dead Center to replace the reverse Dead Center Hornady supplies.  Your tool should have come with three bullet centers.  I only ever use the middle size one, so I chose the largest one as the candidate for modification.  If I had a lathe, I would have just turned a dead center that would substitute, but I only have hand tools, so here goes.  I took my spare LEE 7.62 x 36 expander pin and used a file, hacksaw, and drill to make it fit in the large bullet center hole.  This hole is tapered, so it took a bit of time to grind and file the shaft of the expander pin to fit.  First, I cut off the expansion pin so the long end would fit my drill chuck.  Then I ground the decapping pin end to fit the Hornady bullet center.  This hole is slightly tapered so it took some time to get the pin tapered correctly.  Once my part fit, I Loctited it in place, and then cut off the end that had been in the drill chuck.  This left me with the external centering taper shown in the below picture.Modifications to Hornady Concentricity Tool 9
    4. Take the Run Out Corrector screw that came with the Hornady tool and throw in into a spare parts bin. It will allow you to correct some runout, and at the same time destroy all your good work making reloads.  The reason it does not work is flexing the case bullet junction weakens the grip the case has on the bullet.  I find neck tension to be 5 times more responsible to good SD and ES development than concentricity.  Concentricity starts at the case.  If the case is uniform from the factory, and has the proper base diameter (many are under size), and the headspace is kept to a minimum, you have a very good chance of having uniform camber fit, and thus good accuracy.

Notes on Use:

  1. Make sure you load the bullet/mouth center with about 1/8” spring tension before running your test. The round knob should be about 1/8” from the red frame when in use.
  2. Make sure you put some pressure on the shank of the bullet/mouth center in order to eliminate the centers fitting slop in its hole.
  3. Do not allow the black knob to rotate as this will introduce artificial runout.
  4. Use care in mouth chamfering your cases, hand chamfer is preferable to machine chamfer. A uniform chamfer cut is critical to consistent measurements.

Modifications to Hornady Concentricity Tool 11


Is there anyone reading this article that can make a simple machined mouth end dead center?  Hand working the expansion pins is difficult.  It is very hard to get the two parts to fit together without run out.  Ideally the part would be a one-piece replacement male center that just screws onto the black knob.

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