Some Bedding Basics.
There are several different kinds of accuracy. A hunter wants long term consistency for his
rifles. The first shot has to go where
he’s expecting it every time. The
ability to shoot long strings of fire rapidly is important to the varmint or
target shooter. Everybody wants the
bullets to land as close to point of aim as possible but standards may
differ. Someone hunting whitetails at
close range may be happy with a rifle that puts all it’s shots into five inches
at 100 yards. A serious high-power
competitor needs all his shots to go under an inch. A bench rest shooter will demand less than a
tenth of that. Target shooters will
usually get sighter shots and can adjust for any changes in impact point from
one day to the next. Hunters don’t
usually get sighters. Hunters usually
aren’t concerned about firing more than three shots at a time. They can get by with a lighter barrel
contour. Everyone will have different
accuracy standards.
The reason the bedding is important lies in how the rifle’s
barrel acts when you fire a bullet through it.
The barrel vibrates like a tuning fork.
As the bullet travels down the bore the muzzle will move through an
elliptical pattern and may also move up and down or side to side. As long as the bullets leave the barrel at
the same point in the vibrations every time, they will land in the same
place. These vibrations will also cause
the bullet to shoot away from any hard objects in contact with the barrel (or
stock). That’s why you must not rest
your rifle directly against a hard object when you shoot. Put your hand or a hat, glove, jacket, etc.
between the hard object and the stock before you shoot.
A change in bullet weight or velocity may change the
vibration nodes or the point at which the bullet exits the muzzle. By trying different factory rounds or
handloading it is possible to determine which bullets will shoot best in a
given barrel. Sometimes a velocity sweet-spot can be found at which the barrel will show
maximum accuracy for a given bullet.
Twist rate determines what range of bullet weights your barrel will
stabilize at a given velocity.
There are factors that go into minimizing the effects of
vibration on a barrel. These include
cross section area, resistance to torque, inertia, ability to break up the
vibration nodes, etc. I’ll refer to
these attributes together as “stiffness”.
(That’s so I don’t get a bunch of letters from mechanical engineers
correcting my terminology. Bootes
Obermeyer, Dan Lilja, Ed Shilen, Robert Hart or any of the other premium barrel
makers are more than welcome to correct me.)
For maximum accuracy you want to either make the vibrations
as consistent as possible or (better yet) minimize them. Barrel contour determines how relatively
stiff a barrel is and how much it will tend to vibrate. Generally the longer the barrel the bigger
the vibration nodes and the slimmer the barrel the larger the nodes. By increasing the diameter of the barrel you
can increase its stiffness and decrease the nodes. fluting helps break up the nodes and a fluted
barrel will be stiffer than an unfluted barrel of the same weight. Fluting a heavy barrel will actually
decrease the stiffness but it will lighten the barrel considerably. Fluting must be done correctly or it will
induce stress in the barrel and decrease the accuracy. Certain patterns of fluting will break up the
elliptical vibrations better than others.
The short heavy barrels are used in target competition to get the
stiffest barrels for a given weight as the weight of the rifle is limited by
the rules. A straight taper or octagonal
barrel will be stiffer than a standard stepped barrel contour. Support from the forearm or top ribs will
also lend stiffness and help break up the vibration nodes.
As a barrel heats up it will expand. If there are any stresses on the barrel–
either internal or due to outside pressure– the barrel may walk it’s shots as
it heats. As the bore expands, group
size will also increase. Heavy barrels
take longer to heat up. Octagonal and
fluted barrels have more surface area and tend to cool off a tiny bit more
quickly. Cryogenic treatment of barrels
relieves internal stress that my have been induced by machining and improves
accuracy. Another point of stress can be
where the barrel socks up against the receiver.
The face and threads of both must be straight and square with each other
to reduce stress.
Wood stocks are traditional.
A well seasoned, dense, straight-grained, properly sealed wood stock is
as stable as any. A wood stock that is
lacking in any of these points may warp with changes in humidity or
temperature. As the stock warps it can put
pressure on the rifle barrel. This
pressure will change the vibration nodes and/or push the barrel causing changes
in accuracy or point of impact. Quality
wood is virtually impossible to find today.
Most stocks now are very soft and frankly not suitable for use in
firearms. When installing sling swivel
studs on new stocks I could very nearly turn the bit in with my bare
hands. Once in a while I’d get in an old
rifle and had to be very careful as the wood was usually dense enough to burn
up a drill bit run too quickly.
Wood laminates aren’t as subject to warpage but they can
warp. Laminates tend to be heavy as
well.
Synthetics are great.
They are dimensionally stable and not affected by changes in temperature
or humidity. Synthetics tend to be
stronger than the wood that’s available now.
Synthetics can be lighter than wood but will usually end up being the
same or a bit heavier than a comparable wood stock when they are constructed
properly. The most popular synthetics
include fiberglass, graphite, and Kevlar fibers in an epoxy matrix.
Some custom rifles are being built with stocks made out of
aluminum alloys. H.S. Precision stocks
make use of an aluminum bedding block in a synthetic stock. The AWP sniper rifle also makes use of a
metal bedding block that provides the structural integrity to the stock. The stock itself is two plastic halves that
just bolt on. I have found that bedding
H.S. Precision stocks with a thin layer of epoxy bedding compound can improve
their consistency by supporting the receiver more fully.
Recoil can also affect the receiver. The bottom of the receiver must be supported
by the bedding so that it isn’t under any stress and it won’t shift under
recoil. The recoil lug (or surfaces)
must also bear squarely and solidly on the bedding at the rear so that no torque
is induced under recoil. Pillars around
the action screws and bedding the trigger guard assembly support the wood and
keep it from compressing. When I bed a
rifle I will relieve the sides, front, and bottom of a standard recoil lug so
that the back is the only point it contacts the stock. Increasing the surface contact area between
the receiver and a wood stock will help keep the stock from splitting under
recoil.
The rear part of the forearm on two piece stocks may require
special attention. It must bear evenly
on both sides of the receiver to keep from putting side stress on the barrel.
Free-floating the barrel means that the entire length of the
barrel is free of contact with the stock.
Most free-floats leave a space the thickness of a dollar bill or
business card between the forearm and the barrel. This minimizes interference with the natural
vibration nodes. Some smiths will
free-float the barrel all the way back to the receiver. Most will bed the barrel out a couple of
inches from the receiver. Free-floating
is the only way to go if your stock is unstable. An improperly dried and/or sealed wood stock
will put varying pressures against the barrel and shift the point of impact
with temperature or humidity changes.
Free-floating helps minimize those effects. For any barrel contours heavier than a medium
sporter, free-floating will probably give you your best accuracy. Barrels other than heavy contour target
barrels will usually show a definite preference for one bullet weight over the
others when free-floated. Despite what
many people will tell you, free-floating may not give your rifle the best
accuracy, especially with a light contour barrel and a stable stock.
A full length neutral bed means that the entire length of
the forearm is bedded to the barrel with no upward pressure. This can be done with careful inletting of
the forearm or by glass-bedding the length of the forearm. This bedding method helps minimize barrel
vibrations as the stock lends support to the barrel right up to the end of the
forearm. You must have a stable stock to
use a neutral bed. Fiberglass stocks are
ideal for neutral bedding as the stock won’t shift on you. I have found very few rifles that actually
shot their best with a neutral bed but there have been a few.
A foreend pressure pad means that when the action screws are
tightened down, a small (one to one and a half inch) pad at the tip of the
forearm will exert some upward pressure against the bottom of the barrel. Usually five to twelve pounds of pressure is
all that’s needed. More pressure can
cause the rifle to walk its shots vertically as it heats up. As with neutral bedding the forearm helps
support the barrel and minimize vibration.
You must also use a stable stock with a pressure pad. I’ve usually had better results bedding light
barreled sporters with a pressure pad than a free float. I will usually start with a free float as
it’s easy to put in a pad later. If the
pad doesn’t prove out it’s easy to route it out again as well. Pressure bedded barrels tend to be less
finicky as to bullet weights or loads.
The pressure pad must be smooth and cradle the barrel so that the barrel
ends up in the same position every time you fire. If there is a hump between the forearm and
the barrel, the barrel might sometimes end up on one side and sometimes on the
other. Any time you shoot with a tight
sling your point of impact will drop slightly.
If you use a pressure pad and shoot with a tight hasty sling your point
of impact can drop considerably.
Practice with a tight sling on the range so you know what your rifle
will do.
The barrel tie-down was used in the pre-’64 Winchester model
70s. It consists of a neutral or
pressure bed and a tie-down that attaches the barrel to the forearm. The tie-down is usually a stud welded or
dovetailed to the barrel that is tapped to receive a screw. The screw passes through the forearm and into
the stud. An escutcheon or aluminum
pillar in the forearm is a good idea to keep the wood from compressing as the
screw is tightened. I like a loose
dovetail for the stud to allow the barrel to expand as it heats up. A spring between the screw and forearm is
also used on some rifles to provide a constant tension level. A stable stock is necessary for a
tie-down. I’ve seen some mediocre rifles
turn into real shooters with a tie-down.
The tie down turns the barrel and forearm into a single rigid unit and
doesn’t allow vibration nodes except in the unsupported portion of the
barrel. This makes the barrel act as if
it were much stiffer than an unsupported barrel of the same weight and
contour. A tie-down is the last bedding
method I’ll try on a rifle as they are a pain to install.
I had a customer buy a Weatherby Fibermark rifle with a
synthetic stock. Weatherby test fires
all of their rifles and guarantees three shot groups of one and a half inches
or less at one hundred yards with Weatherby factory ammo. They include the test target with each of
their rifles and most Weatherbys will do half that. At that time Weatherby used a pressure pad on
the forearm. This fellow insisted that I
free float the barrel to improve accuracy.
I explained that a free float would probably increase the group size
with this rifle and strongly suggested he test fire it first to see how it
would shoot with the pressure pad. He
was absolutely insistent so I had him sign the work receipt where I wrote that
the free float would probably increase the group size.
I fired a group and saved the target. The three shots went into less than three
quarters of an inch. Next I Acraglassed
the stock and free floated the barrel. I
took the same target out and fired a new group.
The three shots went just over 1 1/2 inches. I showed him the target and the customer
started to yell until I brought out my copy of the receipt that he had
signed. The next fall he had me
reinstall a pressure pad.
Shoot your firearms before you start messing with them. Who knows?
They might not need any work.
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