Headspace Control Shims^TM Test

Shell Holder Bump vs. Case Head Runout

by James Nelson

Summary

While shooting High-Power competition with M1 Garands and M1-A’s I had to full length size for reliable feeding but needed greater than 0.010” under sizing of the cases to minimize any headspace condition and accordingly optimize any more accuracy I could squeeze from the combining factors.  I found I could dial in the exact headspace sizing solution I was after using Headspace Control Shims^TM, but I was concerned about giving up accuracy to case head runout created or aggravated by reloading press deflection (we back issue types can read “Adjustable Headspace Full-Length Sizing” by M.L. McPherson – Precision Shooting 11/96, and “Considerations of Reloading Press Deflections” by M.L. McPherson – Precision Shooting 1/98).  Because of the promoted notion that shell holder bump sizing was a best practice for full-length case sizing to relieve potential case head runout propagation, I was surprised that my test showed negligible difference in case head runout between shell holder bump sized and non-shell holder bump sized cases. 

The Case Sizing Set-up

Two headspace sizing adjustment methods; left Competition
Shell Holders (CSH), right Headspace Control Shims^TM (HCS)

To conduct my test I specifically compared a sizing method of Eclipse Engineering & Design's Headspace Control ShimsTM (HCS), which elevates the die in the press creating a non-shell holder bump sizing condition, to the shell holder bump sizing condition of Redding's Competition Shell Holders (CSH).  The cases were sized through an RCBS full-length size die in a Hornady Pacific-007 cast aluminum reloading press.  My 007 reloading press has a 0.010” drop of the ram in the cam-over (full stroke) position.  In order to maintain constant contact between the shell holder and sizing die during the “bump sizing” sequence the die was set such that the reloading press system absorbed or deflected to absorb a 0.010” interference between the shell holder and sizing die when the ram is at it’s highest point.  The CSH used for my test was the +0.010” shell holder.  In order to achieve the same linear sizing condition (+0.010”) on the HCS (non-bump) sized cases it was necessary to use a 0.020” shim stack between the bottom of sizing die’s lock ring and the top of the press. A standard RCBS #3 shell holder was used for the HCS sizing.  Dillon Case Lube was used for all sizing.

The Test Batch

The Test Batch - 600 Lake City 88 Match 308 Cases.

I sorted 600 Lake City 88 MATCH 308 cases into ascending order of case weight, then I recorded the fired cases' weight and head runout respectively. Case weights ranged from 172.9 to 178.1 grains, case head runout ranged 0.2 to 5.2 mils (0.0002-0.0052”).  The fired case head runout seemed reasonable from a M1A rifle, I wouldn’t expect that wide range from a bolt gun...,

I used a Dillon D-Terminator electronic scale to weigh the cases, and the NECO case measuring tool to measure the case head runout...,

     NECO gage with Eclipse Case-Mouth Cone^TM
I found using the Eclipse Case-Mouth Cone^TM in the Neco case gage gave a more constant and stable feel when measuring case head runout than did the graduated case mouth guides that came with the gage.  I then assembled lots of 100 cases per lot while maintaining the ranked case weight order (lightest to heaviest), and each case’s respective fired case head runout.  The first lot (1-100 lightest) cases were sized with HCS, the next lot (101-200) sized with CSH, 201-300 HCS, 301-400 CSH, 401-500 HCS, and finally 501-600 CSH.  The sizing methods were mixed and staggered that way with regard to case weight so that one sizing method was not offered a favorable condition over the other.  After sizing I re-measured and recorded each case's "Sized” case head runout.  Sized case head runout ranged 0.2-4.2 mils.  I entered all data into MS Excel and cranked out more data and a couple of fancy charts to interpret the data and form conclusions.

 Here is my view on what the charts reveal about the data that was collected over the test.   I wanted to consider the change (delta- “D”) in the case head runout for both methods of case sizing.  To obtain delta I subtracted the fired (unsized) case head runout from the sized case head runout.  With this in mind realize that a negative delta represents the condition of case head runout being reduced through the sizing process (sized runout < fired runout).  The reason we are interested in this change is because it will give us a comparison between the two resizing methods and of their tendency to introduce or stifle case head runout.  The first chart “Histogram – HCS vs CSH” shows the number of occurrences that each sizing method resulted in a change of a particular value.  These occurrences are evaluated to give and “average” change, or what we could predict to be typical behavior of the particular sizing method. 

(The black & white chart:  if it’s printed in black and white and we loose the distinguishable colors of the chart it’s a struggle to make sense of it, here is some help reading it.  Delta values are the uppermost row of unmarked values ranging from –1.6 to 1.2.  Four columns centered on each delta value.  Columns left of center of each delta value are for HCS and those to the right of center are CSH.  The bold wide columns represent frequency, which is scaled on the left hand axis as well as noted in the frequency block below each delta value.  The skinnier columns superimposed over the wide bold columns represent the percent of lot, which is scaled on the right hand axis as well as noted in the % block below each delta value.) 

The lot of HCS sized brass dialed out and average of 15/100,000 of an inch (0.00015") greater case head runout change than the CSH cases. In other words CSH corrected the case head runout an average of 0.00015” more than did HCS.  For all intensive purposes I believe 0.00015" additional case head runout represents no discernable adverse effect to accuracy.

One of the interesting characteristics about the sizing methods were that both sizing methods showed a tendency to correct the case head runout more in heavier cases.  This behavior can be seen in the following chart “Change in Case Head Runout vs Case Weight”.  The two linear equations and representative “average” lines through their respective data show the inclination for the change in case head runout to become more negative (smaller “sized case” head runout) as case weight increases.

Conclusions

I have read a couple of articles showing the deflection of reloading presses during simulated sizing operations.  It has been suggested that such reloading press deflections could aggravate case head runout if shell holder bump is not adhered to during the sizing operation, promoting shell holder bump as the safety mechanism to squaring-up the case heads under the press deflections.  Regrettably I’ve not been able to find any studies that compare the topic of case head runout compared across shell holder bump and non-bump conditions.  My first hand analysis and testing of the two methods have reassured my confidence in using non-bump headspace solutions (such as Headspace Control Shims^TM) for precise under sizing of the cases to minimize a headspace condition, or when I need to interchange die sets between presses or rifles.  Hopefully this information will promote a paradigm shift to achieve more exact case sizing methods resulting in greater accuracy and satisfaction.

Dillon Precision Products, Inc.           Eclipse Engineering & Design                RCBS

Hornady Manufacturing Co.               NECO                         Redding Reloading Equipment