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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:

  1. The receiver face is cut square.

  2. 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.)

  3. 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

 

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