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From: (Bart Bobbitt)
Subject: Re: Precision Mic
Organization: Hewlett-Packard Fort Collins Site wrote about using the RCBS Prec. Mic:

: I found that most of my cases fired from original military factory
: loads, were at +3 thousandths relative to the  minimum chamber size.

This sounds confusing to me.  What do you mean by `relative to the
minimum chamber size?'  Your '06 chamber can have headspace as much
as .010-in. over the 1.946 minimum standard.  When you use the Mic,
your sized-case headspace needs to be about .001-in. less than what
a normal, fired, new case has after it's been fired in your chamber.

It's OK if your chamber is a few thousandths longer in headspace
than what the `0' reading is on the Mic.

: This was fine, but then I noticed some commercial reloads I had
: were sized at -8, and the shot cases also measured -8 with the
: primers protruding a measured 11 thousandths.

Sounds like those reloads were sized too much if they were -8 mils
shorter than the zero point on the Mic.  I would consider them not
usable in a normal chamber that could be almost 15 thousandths longer
in headspace than those reloads are.

: when fired gas leaks around the case and keeps it pressed forward
: until the case is expanded, then the primer backs out until it hits
: the bolt.  In this case the load was of moderate pressure so it did
: not cause the case to yield and lengthen.

Exactly what's happening.

: These loads were quite accurate.  This must be equivalent to the
: case John mentioned with the 30-30 where you didn't need a breechblock
: except for the primer backing out.

: Some other gun show cases measured +14 and were not suprizingly
: the ones that were hard to chamber.  After resizing they still
: were at +14 which told me my Lee dies were not doing any shoulder
: sizing even though they were touching the case holder.

You might grind about .020-in. off the bottom of the die.  Then it
could be set correctly in the press.

: I was thinking that using the hard to chamber cases would be sort of
: equivalent to using greased cases, since there would be little resistance
: to the backward pressure since the brass would not be strained
: much.  This sounds like a bad thing.

Right, a bad thing.  When you have to close the bolt on a hard-to-chamber
case, you're probably going to gall the bolt lugs/recesses.  The lubricant
will no longer work.  Doing this has opened up headspace on a lot of
rifles and ruined them as far as safety is concerned.

: Maybe even using cases that are right at +3 in this case, would
: not be as good as using cases at 0, allowing a little more stress on
: the brass?

You should resize your cases to where they are about .001-in. shorter
than a fired case.  That's the headspace your rifle has, not the `0'
reading on the Mic.

: Since my cases never had any yield, except when sizing the necks,
: I found no head thinning, and reloaded them at least 6 times until
: the necks started cracking.  I probably could go more if I annealed
: the necks.  I guess a good test would be to see at what pressure
: the cases start yielding and see if I can stay under that.
: Maybe this would give the best accuracy and case life, even though
: the primers back out a few thousandths?
: Wouldn't it be possible to reload a dozen times this way?

Best accuracy typically happens when the case completely fills the
chamber.  Too short of cases don't shoot too accurately.  If you use
the Mic correctly and set your dies up according to what your rifle
needs, you can get 40 to 50 loads per case neck sizing only.  That's
neck sizing with a neck sizing die, not a full-length die.  Even a
full-length die will force the shoulder forward when it's used as a
neck sizer because the shoulder diameter of the die is smaller than
that of a fired case.  Backing off the die enough to size just the
neck will still result in the shoulder being forced forward as the
body of the die shrinks the body diameter just a bit without touching
the shoulder.


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