From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: RELOADING: Cleaning the primer pocket
Organization: Hewlett-Packard Fort Collins Site

I've noticed that primer pockets that have been uniformed (bottoms trued
up square and smooth and to a uniform depth) clean out almost perfectly
by either vibrating or tumbling.  I'm convinced the rough surface caused
by the pocket being punched in the case head tends to hold more primer
residue firmly in the pocket than those that are uniformed.

Some years ago, I took 20 cases with cleaned primer pockets and 20 that
were not cleaned.  All 40 cases were reloaded with the same type and amount
of components.  Each set of reloads were tested at 600 yards in a .308 Win.
Results showed the ammo with dirty primer pockets shot half-inch smaller
groups than those with cleaned ones.  Another test was done at 1000 yards
with a .30-.338 and the stuff with cleaned primer pockets shot a 20-shot
group about a quarter-inch smaller than the dirty ones did.

Seems the uncleaned batch of cases may produce better accuracy, but the
difference wasn't statistically significant to draw any high-confidence
conclusions.  So, I don't think it makes any significant difference in
30 caliber cartridges.  Some benchresters claim cleaning the primer
pockets makes a difference with 22 and 24 caliber benchrest cartridges.
But then, smaller cartridges need more care and uniformity than larger
ones for top accuracy anyway.

I think uniformed primer pockets are more condusive to accuracy than
clean ones.  Virtually all of the top long-range highpower shooters I
know don't clean their primer pockets.

BB