From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel Vibrations vs. Accuracy
Organization: Hewlett-Packard Fort Collins Site

K. Karcich (kkrh@troi.cc.rochester.edu) wrote about using the same case
for a string of shots in a modern rifle:

: Do you know anyone who has actually tried this with a modern rifle, that is
: using the same case over and over again? I guess I don't fully understand
: the quanatitative difference between the lockup on a rolling block and the
: lockup on a bolt action. Maybe the reason this works for a black powder arm
: is that the cases are not as precisely assembled to control runout?

Shooting the same case for a number of shots can be interesting indeed.
I've done it.  In a test at 600 yards, I fired 40 shots.  20 were with
the same case; the other 20 were twenty cases already loaded.  Someone
else loaded the one case while I shot one from the box.  Every other fired
round was with the same case, indexed the same in the chamber and all on
the same test target.  The 20 from the box were fired on another target
and were not indexed.  Bullet runout of all rounds was less than .002-in.

Both 20-shot groups were about 3.5 inches in diameter.  That told me that
indexing a case the same in the chamber doesn't do anything significant.

One interesting thing about falling/rolling block rifles is their breech
face is typically not perpendicular with the chamber axis.  Because of
this, it makes sense to always orient the case in one of these rifle's
chambers the same for each shot.  The case head will be flattened against
the breech face the same for each shot.  Otherwise, the barrel/receiver
will vibrate differently if the pressure point from the case head is
at a different location on the breech face for each shot.  I'm not aware
of any `block' action breech face that is squared up with the chamber
axis; it wasn't done a hundred years ago and I don't think it's done
anywhere today, even on the Ruger No. 1 rifles, by anybody.  So, with
one of these rifles, you almost have to index the case the same way for
each and every shot.

One highpower rifle action, the Paramount (originally the Swing), was
designed to shoot factory ammo.  Knowing that factory ammo has its head
not squared up very will with the case axis, this action was designed
with 4 medium-sized locking lugs.  With this arrangement, it doesn't make
much difference where the case is indexed in the chamber.  As the head of
the bolt is well supported all around, the initial impact of the case
head against the bolt face doesn't affect accuracy.  These actions are
the best thing on this planet for .308 Win. factory ammo; the round they
were designed for.  It's a credit to the Englishman who designed it.
And when handloads are used, there isn't anything better.

Some actions have 3 lugs; better than 2 if factory ammo is used.  But
neither a 3- or 4-lug action has an advantage over a 2-lug action when
handloads are used and the 2-lug action is trued up correctly.

In a standard 2-lug bolt action rifle, if a new factory round is fired,
chances are that the first contact point could be to the right or left
side of the bolt face.  When this happens, the barreled action tends to
vibrate side to side more than normal and the shot group is strung out
horizontally.  Creighton Audette had an article in the American Rifleman
some years ago that covered this quite well.  If the first contact point
is at the top or bottom where the lugs are, vertical stringing will occur
but not nearly as much as when side contact is made.

In a decent bolt action target rifle (free, highpower, benchrest), the
bolt face is squared with the chamber axis.  Note that squaring it up
with the bolt axis, front of receiver, etc. is not doing it correctly.
The bolt lugs first have to be lapped to full contact on all of them.
A threaded guide screws into the receiver thread and is held in a lathe.
The front of the receiver is then faced off perpendicular with the guide's
axis.  Then a pilot is threaded into the reciever threads and a carbide-
tipped cutter is slip-fit in it with a stop and is hand turned against
the bolt face squaring it up with the receiver thread axis, hence the
barrel/chamber axis.  Once this is done and a cartridge case fired 2 or 3
times, the head of the case will be in full contact with the bolt face
for each shot regardless of how it's indexed in the chamber.

: I'm not convinced that jacket hardness is not a factor effecting velocity
: spread. If case neck tension increases velocity and spread, then engraving
: resistance should do the same since both impact the front of the pressure
: curve. One thing is that with case neck tension, we are talking about variances
: in a single target loading. With engraving resistance, we are changing
: the load entirely, as a tuning mechanism. ( I'm assuming that engraving
: resistance remains the same withing bullet lots, in practical terms )

If there were a difference in jacket hardness within a lot of bullets,
there would probably be a greater velocity spread than if jacket hardness
was uniform within a lot.  Anything that's different between two rounds
of ammo can cause a velocity difference between them.  But I'm not aware
of any extensive tests regarding engraving resistance.  But I'm sure the
velocity would be higher if all things were equal and only the jacket
were harder creating more resistance as it entered the rifling.

BB