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From: (Bart Bobbitt)
Subject: Re: Precision Brass ?
Organization: Hewlett-Packard Fort Collins Site

I don't think non-flashhole cases would be worthwhile.  They're pretty
good on all commercial cases; even those used for competition.  Most
folks accept the flash holes as they are.  Those wanting the uniformity
of primer pocket and flash hole dimensions modify what the factory makes.
Flash holes vary a few thousandths in diameter and using a small drill
bit to open them up to the same diameter is easy.  The hole will be more
uniform if an existing one is enlarged rather than drilling one in a case
that doesn't have one.  For example, my .308 Win. case's holes range from
079 to about .081-in.  I use a number drill (44, I think) that's .082-in.
in diameter running in a low-speed drill press to uniform them.  As they
are quite well centered in the first place, I don't find any slight off-
center positioning detrimental to accuracy.  I then chamfer the inside
of the flash hole to remove any burrs on the inside.  Primer pockets are
then uniformed for depth to about .131-in. below the case head.

In machine rest tests at long range, primer pocket depth uniformity makes
a very small improvement in accuracy, but very difficult to detect.  Flash
hole diameter uniformity also makes a very small difference; again, very
difficult to detect.  Some of the top highpower shooters only uniform the
pocket depth and not concern themselves with flash hole diameter uniformity.
But they do deburr the inside of the flash hole; that tends to make a more
noticable difference.

Some years ago, I did some velocity uniformity tests between cases with their
primer pockets/flashholes uniformed and other cases that weren't so uniformed.
The only thing that made any repeatable difference was pocket depth being
uniform.  Flash holes varied about .002-in. in diameter; pocket depth .004-in.
All the pocket and hole uniformity accuracy improvement tests I've done
show little improvement.

I'm convinced more noticable accuracy improvement from things related to
primers is the selection of the primer itself.  The primer's ignition
uniformity is crucial to accuracy.  All brands vary in uniformity; within
a lot and from lot to lot.  And the milder ones seem to produce better
accuracy than the stronger (hotter?) ones.  Once in a while, each company
will make a very uniform lot of primers.  But finding that lot is where
the time and trouble begin.  I've found the most uniform large rifle primer
made on this planet is the RWS-5341.  Some lots are better than others, but
I've found the odds of getting a uniform lot is much higher with RWS than
any other.  In a recent test of several lots of Federal 210M primers, the
muzzle velocity standard deviation (best test of primer uniformity I know
of) ranged from 5 to 40; that's about a 15 to 120 fps velocity spread.  With
RWS primers, the lots varied from 4 to 15 fps Sd; a 12 to 45 fps velocity
spread.  That's about 3X the difference I've seen compared to uniforming the
primer pocket depth and flash hole diameter.


From: (Bart Bobbitt)
Subject: Re: [RIFLE, RELOADING] quality of new cases
Organization: Hewlett-Packard Fort Collins Site

J. Spencer ( wrote:

: So I took a 2mm (?) drill bit and my slow-speed drill
: and cleaned the flash holes.

A 2mm drill bit diameter is .0788-in. in diameter.  That's OK to use
to clean up flash holes.  I think your `(?)' implied you werent' sure
of the bit size.  If it's actually greater than 2.1mm (.083-in.), you
might have to reduce powder charges a tenth of a grain or more to be
safe.  Larger diameter flash holes let more heat into the case which
starts more powder burning faster.  Some folks have enlarged flash
holes too much; result's have been much higher than normal pressures.
But if you did use a 2mm bit, you're in good shape.


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