Blueprinting
for Improved Accuracy
Blueprinting your rifle's action will not guarantee tiny groups, but if
you are to build a super-accurate rifle, you must start with the action.
Once you know you have a good, true action, you can build on it with barrel,
stock, bedding, and sighting equipment.
By
RICK JAMISON, RELOADING/RIFLES EDITOR, SHOOTING
TIMES/NOVEMBER 2003
Of
primary interest to any rifleman is accuracy. A lot of factors go into
getting a rifle to shoot accurately, including the load, the action bedding,
the quality of the barrel, the precision of the scope, and the ability of
the shooter.
But
perhaps nothing is more important than the very heart of the rifle, the
action itself. The action holds the trigger, which must be crisp and
relatively
light. The action supplies the fast locktime, which is a major contributor
to tight groups. What few shooters know, however, is the trueness of their
action and the precision with which it was made.
Important
questions about the action should be asked when building up a super-accurate
bolt rifle. For instance, are the barrel threads cut on the same centerline
as the bolt through-hole? Is the boltface concentric and at exactly a right
angle with the centerline of the bore? Are the locking lug recesses perfectly
perpendicular to the centerline? Do the locking lugs mate completely and
squarely with their recesses? Is the face of the receiver square with the
centerline? Does the washer-type recoil lug have parallel sides so that when
the action and barrel are tightened the centerline of the bore is maintained
with the centerline of the action?
When
a rifle is fired, thousands of pounds of chamber pressure are generated,
placing the entire barrel and action under tremendous stress. If any of the
parts are not perfectly aligned and abutted, disproportionate stress creates
adverse harmonics throughout the firearm, tending to throw shots awry. Most
rifles are made on production machinery, and advances in CNC machines in
recent years have increased the ability of machines to produce parts to close
tolerances. Still, there are tolerances and the parts are produced very
fast. It is not unusual to find critical receiver and bolt parts that have
more manufacturing tolerance, or runout, than critical machinists and shooters
would appreciate. For example, it is not unusual to find a receiver face
that is .010 or .012 inch out of square. The threads can easily be out .010
inch. Add .002 for the boltface and another .002 for the locking lugs, and it
all combines to do nothing good for accuracy.
While these tolerances are better than
they have ever been, and do produce fine and sometimes exceptional hunting
accuracy, the shooter pursuing the very best accuracy possible does
everything he can to ensure that his equipment is the best that it can
possibly be made to be.
So
if we want to shoot the very best to which our ammunition and abilities are
capable, the place to start is with the action. A great many shooters replace
factory barrels with a fine barrel from a custom maker, but there are no
guarantees that the custom barrel will perform up to its potential unless the
action is true.
Trueing
and squaring an action and bolt is commonly called "blueprinting." I
recommend that you replace your rifle's barrel when you blueprint your rifle's
action. That way, the new barrel can be fitted to the trued action. Since the
action threads are recut during blueprinting, the old barrel will no longer
fit without cutting off the section with the old barrel threads and
rethreading. The best time to blueprint an action is when the barrel is
replaced.
Action
blueprinting does not guarantee tiny groups because it is only one of
several factors that go into getting tight groups. But if you are to build a
supremely accurate rifle, the place to start is with the action. Once you know
you have a good one, you can build on it with barrel, stock, bedding, and
sighting equipment.
Before
any blueprinting work is attempted, the lathe itself must be good. Ideally,
a gunsmith would pick a heavyduty lathe and one that is the best and the
most precise that he can possibly get. It must then be checked and trued. The
tailstock must be absolutely true with the headstock, meaning that the
tailstock must be adjusted or altered in all axes to align perfectly. (This is
a separate operation that will not be covered here. The purpose of this
article is not to make a gunsmith out of you but to illustrate the steps
necessary to true an action.
THE
BLUEPRINTING METHOD
A
lot of different methods of blueprinting a rifle action exist, and they
differ primarily in the machine setups used to accomplish the various tasks.
Some blueprinting methods produce closer tolerances and better results than
other methods. The method shown here, using a Remington Model 700 Action as an
example, is believed to be one of the best.
The first step in any blueprinting process
is to completely disassemble the action. Remove all parts from the basic
action, such as the trigger and safety, and any other parts.
Next,
a long mandrel is fitted with sleeves to mate with the interior of the
receiver, the bolt through-hole. Different size sleeves are available to
precisely
and tightly fit the interior of the action. Once properly inserted, the
mandrel extends about five inches forward of the face of the receiver. The
hardened mandrel is ground absolutely straight, and when properly inserted in
the action, it establishes the action centerline upon which all subsequent
cuts will be made.
With
the mandrel in place in the action, the action is then centered in a holding
fixture, a heavy aluminum sleeve with eight action-positioning bolts. The
action is tightened into the fixture so that the action is approximately
centered.
With the action disassembled, a long
mandrel is fitted with sleeves to mate with the interior of the receiver. The
hardened mandrel, when properly inserted in the action, establishes the action
centerline upon which all subsequent cuts will be made.
The
next step is to place the fixture in a lathe four-jaw chuck and true the
fixture. The chuck jaws are alternately tightened and loosened until the
aluminum fixture is turning true in the lathe.
Next,
the extending mandrel is indicated true with .0001-inch dial indicators
positioned at two points, one close to the action and one at the outer end of
the mandrel. The action is indicated true within the holding fixture. Any
misalignment of the action to the axis of the lathe is magnified greatly at
the outer end of the long mandrel. This prevents the action from rotating in
a cone pattern as is possible if you use only a four-jaw chuck without the
fixture. What many do not realize is that you cannot get a receiver to run
true with just a four-jaw chuck. This is the purpose of the fixture and
mandrel.
Once
the action is trued and secure in the fixture and lathe, all action cuts are
made on this same setup without changing the positioning of the action. The
action is not removed and inserted again, and this ensures that all cuts have
the same relationship. Very light cuts are taken so as to prevent any
possible disturbance to the action in the fixture, and a minimum amount of
metal is removed to accomplish the cleanup. Layout blue is applied to areas
to be cut so that when the surface is cleaned up, cutting is stopped. Again,
with light cuts and close attention, minimal metal is removed. Only the amount
required to accomplish the task is taken away.
The
face of the receiver is painted with machinist layout blue (T) and cut square.
Cutting has stopped (B) and the amount that the receiver is out-of-square can
be clearly seen in the layout blue that has been cut away and the amount that
remains untouched.
These are the basic cuts accomplished at this phase:
-
The receiver face is
cut square.
-
The action threads are recut on the same pitch and angle but using a
single-point tool to ensure that they are concentric with the
centerline of the action. (Taps are made for this, but taps do not cut threads
concentric with the centerline of the action through-hole.)
-
The lug recesses are cut square, removing only enough metal to square up the
recess.
That completes the action portion. Most of the time spent accomplishing
this portion is in the setup. It takes a long time to get the action running
true within .0001 inch. Once it is set up, the actual application of layout
blue, changing cutting tools for each of the cuts, and the actual cutting go
relatively quickly.
The
action threads are recut on the same pitch and angle using a single point
tool.
The
lug recesses are cut square, removing only enough metal to square up the
recess.
The next step of the operation involves the bolt itself, first
cutting the face of the bolt square and perpendicular with the bolt body. As
with the action, all parts of the bolt--the shroud, cocking piece, striker,
and spring--are first removed. A special threaded spud with a tapered end is
then threaded into the rear of the bolt. The bolt body is then installed in
the lathe by chucking the spud at the rear of the bolt in a four-jaw lathe
chuck. The forward end of the bolt is held by a steady rest positioned
immediately behind the bolt lugs. A live center is inserted into the bolt's
firing pin hole. When the bolt is adjusted so that it is turning true, the
live center is backed out of the way and a boring bar cutter is used to square
the face of the bolt across its entire surface. In the Remington Model 700
example used, the boltface is squared inside the projecting ring around
the rim of the boltface.
When the boltface is done, a special lathe-turned spud is inserted into
the boltface. This spud has a pin that tightly fits the firing pin hole and
the inside of the boltface. The opposite end of the spud is coned inwardly to
accept the lathe's live center and the live center is moved into position to
hold the spud and in turn the boltface. The steady rest can now be removed for
clearance to cut the rear surface of the locking lugs. Again, only very light
cuts are made until the metal surface is cleaned up.
The
receiver and bolt are now complete. What you now have are bolt lugs that
mate completely in their recesses, with full contact. If done properly, they
will scrape layout blue from the entire surface of the locking lug or recess,
even without lug lapping. The threads, boltface, and receiver face are all
square and concentric with the bolt centerline and will be concentric with the
rifle's bore when the barrel is installed.
In
the case of the Remington Model 700's sandwiched recoil lug, the lug must also
be surface ground until the two sides are perfectly parallel and square so
that the barrel shoulders up squarely against this washer and in turn the
action.
An alternative is to use an aftermarket recoil lug that is already
ground square and that is thicker than the original lug. An additional feature
that can be purchased is a pinned lug. This
way, the action is drilled with a tiny hole for pin alignment with the lug so
that the lug will not rotate around the barrel shank. It is always positioned
in the same relationship with the action and in the same position for the
bedding recess.
The
bolt is then disassembled and the bolt body is indicated so that it turns true
in the lathe.
When
the bolt is adjusted so that it is turning true, the live center is backed out
of the way and a boring bar cutter is used to square the face of the bolt
across its entire surface. In the Remington Model 700 example used, the
boltface is squared inside the projecting ring around the rim of the
boltface.
With
layout blue applied, the rear surface of the locking lugs are cut. Again,
only very light cuts are made until the metal surface is cleaned up.
On
the Remington Model 700 used in this example, the final step is surface
grinding the recoil lug so that the sides are parallel.
SLEEVING A RIFLE BOLT
One
other thing that can be done to enhance rifle accuracy is to ream the
interior of the receiver and sleeve the rifle's bolt to fit. As factory-made,
there is considerable clearance between a bolt body and the interior of a
rifle's receiver, perhaps as much as .007 to .013 inch. Proper sleeving
reduces this clearance to almost zero when the action is locked. Sleeving can
provide added precision for the shooter who wants the utmost in accuracy,
primarily the benchrest and varmint shooters. When done properly, the
procedure accomplishes a close-tolerance fit and alignment of the bolt in a
rifle receiver to prevent the bolt from cocking in any direction, a common
tendency from bolt/sear pressure. Any cocking of the bolt body, no matter how
slight, results in a boltface and locking lugs that are not perfectly square
in the receiver and are not making full contact at the instant the firing pin
falls. While there is a close-tolerance fit, the bolt will also cycle
smoothly when sleeving is properly done.
If
you want to sleeve your rifle's bolt, the decision must be made prior to
action blueprinting because it will be a part of the blueprinting process.
Here's a quick rundown of the procedure.
Before
placing the action in the lathe, two sleeves of metal must be made from solid
4140 bar stock that is 3/4 inch in diameter and long enough to make two
sleeves, one to fit under the receiver ring and one to fit under the bridge.
In the case of the Remington Model 700 used here, the sleeve for the ring was
0.4 inch long and the one for the bridge was 1.2 inches long. A precise length
is not critical. The important thing for the sake of appearance is that
they’re short enough to be hidden when the action is locked.
For
sleeving a rifle’s bolt, a bar of 4140 steel is drilled with a through-hole,
making a hollow tube of sleeving material.
Then
the bolt body is indicated true in the lathe with a four-jaw chuck.
Each
sleeve is split lengthwise and the section is laid on the bolt body for
scribing end marks in layout blue applied to the bolt body.
A
lathe is used to drill a 5/8-inchdiameter hole lengthwise through the center
of the bar stock. This results in a tube, or pipe, of sleeving material. It
is then cut to the proper lengths for each sleeve, as mentioned earlier. Next,
a bandsaw is used to cut each short section of sleeve lengthwise into equal
halves. The sleeves are now ready to be installed on the bolt body.
The
sleeves are then laid aside and attention is turned to the receiver. The
receiver is installed in the lathe and indicated true within the holding
fixture, the same as for basic action blueprinting. The interior of the rifle
receiver is reamed with a tightly sleeved cutting mandrel arrangement to
provide a straight through-hole of a precise diameter slightly larger than the
original. Reamers can vary in size for this process but must be large enough
to clean up the existing receiver through-hole.
The
reamer is specially made with a mandrel and sleeve arrangement that allows the
tight-fitting sleeves to move along the mandrel as the cutter advances,
maintaining perfect alignment while cutting. (A .703inch reamer cleans up
most standard two-lug action receivers, but not all.)
Without
removing or changing the position of the receiver in the fixture, the
remaining action cuts to the receiver face, lug recesses, and threads are
made as mentioned in the main article on blueprinting.
Next,
the rifle bolt is indicated true in the lathe and layout blue is applied to
the area where each sleeve will be attached. Each half section of bolt sleeve
is then placed on the bolt and the end marks for each sleeve are scribed onto
the layout blue on the bolt itself. The bolt remains in the lathe for the
remainder of the sleeving operation and for the bolt lug cuts and boltface
cut as mentioned in the blueprinting article.
Relief
cuts are made into the bolt body 0.05 inch deep and extend to the scribe marks
already made so that the halves of the sleeves fit into these grooves. The
sleeve halves are glued into place in the grooves using J-B Weld while the
bolt is still in the lathe. The sleeves are held in position with small glue
clamps.
Once
the J-B Weld has hardened, it can be machined and then polished to the same
diameter as the reamer used to cut the receiver through-hole. In the Remington
Model 700 just sleeved, the diameter was .703 inch. The sleeves are this
diameter at the midpoint and taper to bolt body diameter at the ends of the
sleeves to form a smooth edge that flows into the bolt body. Any excess glue
is machined off at this point for a clean and smooth surface. This makes
for a very snug fit of the bolt in the receiver, too snug for smooth and
reliable bolt operation. The next step is to remove metal along the sides of
the sleeve at opposing points that align with the lug raceways when the bolt
is locked. This way, when the bolt is rotated open, there is increased
clearance for cycling in the action with the high points of the bolt running
along the action raceways. The
bolt has a zero-tolerance fit in the receiver, at both front and rear, only
when the action is locked. There is no room whatever for bolt cocking in any
direction.
There
is an increased probability for grit to interfere with action cycling in a
big-game rifle in the hunting fields. In this situation, it is best to
sleeve only the rear of the bolt, under the bridge, to reduce the possibility
of such an event from occurring. At the same time, the rear sleeve is by
far the most important for maintaining bolt alignment.
Relief
grooves are lathe-cut into the bolt body to accept each sleeve section
If
the operation is properly done, the sleeve halves fit around the bolt body and
the edges are glued together. Glue clamps hold the sleeve sections in place
for setting.
Each
sleeve is machined to a precise diameter for a close-tolerance fit in. side
the receiver, and the surface is polished for smooth bolt operation.
When
finished, the bolt with the sleeves in place will have a zero tolerance fit in
the receiver, at both front and rear, only when the action is locked.
SHOOTING
TIMES/NOVEMBER2003
