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:

: Some other variables that I have wondered about are:

: bullet to barrel alignment. Its common for cast bullet shooters to start a
: cast bullet in the rifling to prevent misalignment. While jacketed bullets
: are less sensitive to this problem, are they immune to it?

In my own tests at 600 yards with a .308 Win., groups tend to open up
about one eighth MOA for eacy .001-in. runout the bullet's point has
that is greater than about .0025-in.  Cartridges with less than .0025-in.
runout as I measure them don't seem to have any impact on group size.
With a well-centered throat, `round' throats and chambers (a lot aren't
round because of lousy reamer quality and machine shop practices), they'll
shoot great.  Ammo with greater than about .007-in. runout all shoots the
same size groups pretty much; they are all large and horrible.  I've checked
LC match ammo for straightness, both M118 and M852.  Some lots are better
than others.  I've seen the runout go from .000 to .030-in. in the same
box.  When the M118 stuff started being labeled `Special Ball,' I found out
why it was so special.  Those folks at Lake City Arsenal had to run this
stuff through special machines to make it more crooked than the M118 match
ammo was.  Some of that M118 Special Ball had so much bullet runout, you
could not close the bolt on a round in your chamber.  I've measured some
that had over .060-in. runout.  M118 LC64 Lot 12040 had about .000-.005
runout for all of it that I shot; the most accurate M118 ammo I've shot.
Runout on the '92 Palma Match ammo averaged about .0025 and ranged from .000
to about .005-in. of the 1000 or so rounds of 4 different lots I measured.

But one needs to know what I mean by .001-in. runout; depending on the
method used to measure it, the same cartridge can have different amounts of
runout numbers.  Here's my method:
       _____________________           V
      |                     \___ ________
      |                         |        \
      |                      ___|________/
      |_____________________/    ^
        ^

       `^' is where the round rests in precision bearings; one at the back
           on the pressure ring, the other about .010-in. in front of the
           case mouth.  Two bearings are used at each position.  Each is
           about 5/8ths of an inch OD on 1/4th-inch rods.  About 1/4th of
           an inch between them lets the cartridge spin freely.

       `V' is where the plunger of the dial indicator sets.  The plunger is
           opposite one of the bearings.   The plunger is two bullet
           diameters away from the bullet's bearing point.  This gives
           the same reading for a given amount of runout regardless of
           how far out the bullet point is from the case mouth.

: case registration. Cast bullet shooters commonly mark their cases and chamber
: them with the same registration. Do jacketed bullet target shooters do this?

Some highpower shooters index their ammo after spinning it and marking a
point on the case relative to where the dial indicator was to show some
amount of bullet runout.  I've done this with ammo with .007-in. runout and
shot great scores at 1000 yards; the same ammo not indexed would shoot
about twice as big of groups.  With a good lot of cases that don't vary
more than .003-in. in body wall thickness and have necks that are uniformed
to less than .0005-in. in thickness, there's no reason to index cases except
for the ones with greater than .0025-in. bullet runout.  Also, the more
the bullet has to jump to the rifling, the straigher (less bullet runout)
it has to have to maintain accuracy.  And soft-seating the bullets so the
jam into the rifling tends to help the ones with about .003-in. runout do
better.  Tests have shown that when this is done, you can get the same
accuracy as with perfectly straight rounds with several thousandths of
bullet jump to the lands.

Indexing cases helps black powder arms because they don't lock up the same
as a bolt action rifle.  Orienting the same case the same way for these
rifles seems to work best when the same case is used over and over for each
shot.  Otherwise, the preload on the action would be different for each
case regardless of whether it was indexed or not.  All this as told to me
by a .32-40 black powder shooter who drives tacks so to speak.  Others may
have different opinions/facts/theories/wild-guesses/no-idea-at-all........

Benchresters also check for runout, but to a much lesser amount than
highpower shooters do.  That's because they use smaller and shorter bullets.
22 and 24 caliber rounds require less bullet runout than a 30 caliber ones
do to get best accuracy.

: bullet resistance. Some bullets are harder than others and have more bearing
: surface. Since time pressure curves of smokeless powders depend on resistance
: fron the bullet( weight and frictional ) does frictional resistance provide
: a finer adjustment than bullet weight does to effect a consistant resonance?
: In particular, does this variable serve to fine tune barrel time?

Remember that bullet resistance is greatest when the bullet is being
engraved by the rifling's land.  After the bullet's base has moved into the
rifling, bullet resistance has dropped quite a ways down.  Some resistance
is there as the intense pressure on the bullet's base causes the bullet's
sides to want to swell out, but the groove diameter is as far as it goes.
As the bullet rides the groove diameter for the most part, there isn't much
resistance by contact with the barrel.  Well, there may be some if the
bullet encounters a tighter or rougher place as it moves toward the muzzle,
but these things tend to distort the bullet's shape and balance more than
cause enough resistance to slow down the bullet.  And if the bullet does
meet with a tighter or rougher place, I would think some of the bullet's
energy would be transferred to the barrel at this place; especially if the
roughness scraped off some of the jacket.

In chronographing ammo, one of the most interesting things I've noted is
that the harder the case neck holds the bullet, the greater the velocity
spread is.  Perhaps this is because there is a variable between the loosest
and tightest held bullet in a lot, if the average neck tension is greater,
there will be a greater spread in pounds of force needed to move the bullet
out of the case neck.  This translates to a greater variance in the front
end of the powder's pressure curve which causes a greater variation in
velocity.  The tighter the neck tension, the higher the velocity and its
spread.  That's why I keep neck tension as low as possible; just tight
enough to hold the bullet in place for slow fire; a tad tighter for
rapid fire.  A 20% variance about 5 pounds of seating force is better
than a 20% variance about 50 pounds of seating force needed.


BB