From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Newsgroups: rec.guns
Subject: Re: Bedding and Bart
Date: 22 Apr 1994 13:45:57 -0400

Accuracy is the reduction of all variables to zero.  Or at least as
close as us mortals can get them.  One of these variables is where the
muzzle points as the bullet exits relative to the line of sight.  That
pesky barreled action is like rubber; hard rubber laying on a chunk of
wood.  Smack that hard rubbery thing with something and it's gonna be
bounced off of that piece of wood; but it ain't gonna come back down
to exactly the same place.

Examining the difference between a stock without epoxy bedding and one
with epoxy bedding for the same barreled action, we note that when the
epoxy bedded stock is used groups (scores?) are excellent.  But when 
the plain stock is used, scores (groups?) are horrible.  Even when the
same ammo is used.  So, there must be a reason.  There is.  Bedding.
 
When a rifle fires, its barreled action whips and vibrates all over
the place in every direction and various magnitudes.  Such physical
trauma results in the receiver finally settling down in a microscopically
different place after each shot.  After which it now gets to start
the vibrating and whipping all over again when the next shot is fired.

But that microscopically different starting point causes the barreled
action to take off in a different direction and magnitude than before
when the next shot is fired.  This just repeats for each and every shot.

As the muzzle points in random places for each shot due to these whips
and vibrations, it will point at a different place relative to the line
of sight for each shot.  That is what causes groups (accuracy) to be
less than what makes smiley faces.  Barrel weight doesn't reduce this
situation.  Neither does handloads with extremely low velocity standard
deviations.  It is further aggravated by out-of-square bolt faces and
locking lugs not making full contact.  If the barrel touches part of
the forend, that adds another accuracy-degrading element to an already
bad situation.  And the best cases, primers, powder and bullets so
darned perfectly assembled won't help either.  If the barreled action
doesn't start from the same place for each shot, the bullets won't end
up in the same place later. 

So, if the barreled action can be somehow returned to exactly the same
place in the stock for each and every shot, the magnitude of those
barrel whips and vibrations will be greatly reduced, if not practically
eliminated.  Then the only thing left is normal barreled action vibrations
at their resonant frequency, but this can't be eliminated although it
has virtually no effect on accuracy.  Epoxy bedding was and is the
solution.

With the proper epoxy material being a near zero-tolerance fit to the
receiver, there's no room for the receiver to move around in from shot
to shot.  Clearance between the receiver and the epoxy is .0001-in. or
less.  That tolerance is at all places around the receiver.  With
the correct torque on the stock screws, that receiver will go back to
the same position with the same tension so darned repeatable from shot
to shot that the accuracy is the equal of a barrel clamped in a machine
rest with just the action hanging on the back end.  

Benchresters moved one step further some years ago.  After expoxy bedding
their receivers, they removed the barreled action, roughed up the bedding
surface and the receiver, then glued the stock to the receiver.  That
made sure the barreled action started its high-on-the-Richter-Scale moves
from exactly the same place, plus it eliminated the need to check the
stock screw torque a few times during the shooting day.

If one does not reduce the physical variables their own body has as part
of the complete shooting system, they may be large enough to mask any
improvements that have been made to the rifle and/or ammo to make the
mechanical parts of the system a flawless performer.  Sometimes, that
does happen.

BB

------
     
From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Newsgroups: rec.guns
Subject: Re: Bedding and Bart
Date: 22 Apr 1994 13:46:09 -0400

Gary Coffman (gary@ke4zv.atl.ga.us) wrote:

: .....it would seem that
: the inaccuracy caused by the necessarily loose fit of gun to shooter 
: would overwhelm any looseness of fit in the rifle itself. Comments?
: Explanations?

I went through the receiver bedding stuff earlier.  Now here's the
rest of the story.

`Bedding' a rifle to the shooter is equally as important.  The rifle
must be held with the same pressure at all its person-contact points
just like the receiver in its stock-contact points.  Here's why.....

After the bullet starts down the bore, Newton's Law becomes a very big
issue.  The heavier the bullet, the more force needed to push it out
the bore.  Seems the pressure behind the bullet also pushes back on the
inside of the cartridge case with about the same amount of force.  As
a .308 Win. bullet goes down the bore and arrives at the muzzle going
about 2600 fps, the rifle has moved backwards about a tenth of an inch
as well as tilting upwards due to the center of the buttplate being 
below the bore (hence pressure) axis.  And it twists opposite the 
direction of the rifling twist.  How much it moves depends on how
firmly the rifle is being held; if tight, it won't move much at
all. 

If the amount of shooter-holding/resistance varies from shot to shot, there
is no way the rifle will move the same amount in the same direction as
the bullet goes from case mouth to muzzle.  Therefore, although the
sights were dead center on the target when you heard the shot being
fired, by the time the bullet gets out of the muzzle, its path ain't
where you'ld like it to be.

It takes about 3 milliseconds for a .308 bullet to go from case mouth
to muzzle.  During that time is when the rifle recoils and whips about.
Some examples of what causes the bullet to end up striking the target
at an undesirable place are:

  * Butt held too low in shoulder lets less mass be behind it; no shoulder
    bone behind it, just flesh.  The rifle's butt slips down a bit during
    recoil which moves the muzzle end up.  Bullet strikes high above call.

    Almost the same thing happens with different shoulder pressure; it
    causes vertical shot stringing.

  * Cheek pressed hard/soft on buttstock changes resistance laterally or
    vertically depending on pressure axis.  Bullet will strike in any
    direction away from call.

  * Forehand held at different places with different pressure on forend;
    all kinds of pressure point/axis differences.  Amount of rifle movement
    during bullet barrel time varies.  Bullet strikes typically high and
    low relative to call.

    A good example is shooting prone with a sling.  Once in position,
    do not move your front elbow; the one on the arm with the sling on.
    Use your other hand to adjust sights, pet dogs, throw rocks at your score
    keeper, but do not move that elbow.  If it moves out of place only
    half an inch, your next shot will be one-half MOA off call; move that
    elbow one inch and the next shot will be one full MOA off call.

  * Pistol grip held differently for each shot.  As the pressure applied
    to the trigger gets transferred to the stock through the hand when
    the sear releases, how that energy transfers to the stock while the
    bullet is going down the barrel adds another dimension to where the
    muzzle is when the bullet goes out.  This is the main reason why light-
    pull triggers enable the best accuracy; very little energy gets moved
    into the stock and won't change muzzle position significantly.  But
    those four and a half ton triggers (sorry, 4.5 pound) on service rifles
    used in competition.........no wonder it takes years and years for
    most folks to master them.  Three cheers for those 2 to 8 ounce wonders.

    A good rule of thumb is to hold the pistol grip with hand pressure 
    equal to the trigger pull weight; at least.  With heavy triggers, you
    need to firmly grab the grip, otherwise, when a few pounds of force
    slams back against the trigger stop, that firmer grip reduces the 
    amount of rifle movement the force causes.

All of which explains why free rifles have all those adjustable `gadgets'
all over them.  Each part of the stock is adjusted to be a perfect fit
to the shooter's body.  That way, the shooter's pressure on the stock will
be exactly the same from shot to shot.

And for those who marvel at those tiny groups benchresters shoot, even if
they are far back from winning anything.....well a great number of them
shoot free recoil; the rifle just rests on sandbags, then the thumb and
forefinger squeezes that 2-ounce trigger, the rifle goes bang and recoils
exactly the same amount in each direction.  Every single shot.  There is
no shooter contact with it except for the trigger.  But that shooting
style isn't done in other disciplines.  You gotta hold onto that magnum
you're gonna bust an elk with this fall; and that magnum moves about twice
as much before the bullet exits.
     
Rifle stocks are nothing more than an interface between the barreled action
and the shooter.  Both things on either side of the interface vibrate and
move all over the place.  If that interface is well fit to the metal at
one end and the flesh-and-bone thing at the other end, all the variables
of their fit will be reduced to zero; or pretty darn close that is.

BB