Cartridge Case Dimensional Consistency

This is one in a series of short articles dealing with cartridge brass irregularities that can be improved.  Here are two slight improvements some precision reloaders perform in brass preparation.  Measuring the outside diameter of the rim on 10 quality .308 Win. cases at point A we see the dimensions below.  Each case dimension was taken by rotating it 90 degrees for the second reading revealing eccentricity. 

Some may ask why would I consider this area?  The only reason is to attain consistent extractor pressure when each case is chambered. 

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Below dimensions are inches taken at A in the drawing.

Case           Rim diameter               Variation at 90º

1                 .4657 ~ .4646                         .0011

2                 .4680 ~ .4667                        .0013

3                 .4672 ~ .4656                        .0016

4                 .4677 ~ .4672                        .0005

5                 .4664 ~ .4661                        .0003

6                 .4647 ~ .4641                        .0006

7                 .4661 ~ .4656                        .0005

8                 .4665 ~ .4653                        .0012

9                 .4665 ~ .4660                        .0005

10               .4661 ~ .4655                        .0006

The above average is .0008”.  You may surmise hey that’s not even .001 so what’s the problem here, well the spread from the largest .4680” to the smallest .4641” is .0039 and that’s significant.  No precision reloader would shoot cases with this much headspace difference.  Case rim eccentric will introduce extractor pressure variance.   

So, how can this be corrected.  A small metal cutting lathe is the answer, and it doesn’t have to cost an arm and a leg.  Some new import micro mini lathes start at around $200.00 at this writing, or shop around for a used lathe.  If your lathe has a Moris taper in the headstock you can make a split mandrel with/drawbar to fit and hold your cases, this being the preferred method.  Optionally L.E. Wilson, Inc has case trimmers (not related to the author) with various case holders available.  Their case holder can be held in a lathe chuck.  Now all cases can be turned to the same and concentric outside diameter.  Also, one precision reloader said he turned the bottom of the extractor groove (B in the drawing) attaining precise concentricity here.  This can be done with a cutting tool ground to fit; this is subjective as your extractor may not even reach this depth.  If that is the case then correcting eccentricity here may not affect accuracy. 

We sincerely hope this has helped you in preparing precision ammunition whether it’s for 100 – 1,000-yard target or varmint shooting.  Small groups are always welcome.

Tom Wilson                                                       Fort Smith, AR 5-17-2024©

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