Blueprinting a rifle action

by Jim Green, Gunsmith on January 24, 2014

To get the most accurate shot possible from your rifle, you have to ask yourself a few questions. Is the face of the rifle’s receiver square with the centerline of the rifle’s bore ? Are the barrel threads cut on the exact centerline with the bolt raceway ? Is the bolt face at a right angle to the center of the rifle’s bore ? Are the locking lugs perpendicular to the centerline and do they mate squarely in their recesses ? Considering all the human errors that influence a group on target, it’s sometimes a good idea to make sure you have no sloppy tolerances in your rifle’s action to start with. Does everyone NEED a precision target rifle ? No, for most hunters and casual target shooters… minute of angle surgical precision is not a necessity. But some of us are shooters that really demand a lot from our guns. So the rifle’s action gets a “blueprint” job done to it. If your rifle’s action is not perfectly aligned, the pressure from firing the cartridge will cause adverse harmonic vibrations that can cause the projectile’s flight path to disperse.

Blueprinting a rifle action takes a LOT of machine work. Why would anyone want to blueprint an action ? To squeeze as much accuracy as humanly possible from their firearm. By taking out all the sloppy tolerances from a factory action, you can get serious precision from your target rifle. Were you aware that factory production rifles can be as much as 10 or 12 thousandths out of tolerance and still be considered “within spec” ?

 

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Here I’m using my barrel vise to strip the old Remington 700 action down to the bare receiver.

 

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The old method of blueprinting meant chucking the receiver between centers on a lathe and making a “sacrificial collar” to be turned on the forward receiver ring, then transferring the entire thing to your four jaw chuck. Well, as soon as you move it to the chuck, you just changed the way your dial indicators interface with the collar. What I prefer doing is placing the receiver inside a GTR fixture and dial indicating off a mandrel placed inside the receiver. On an average Remington 700 action, the inside diameter of the front part of the action can be as much as .705″, while the rear is slightly smaller at .703″. I use a reamer to cut them both to the same size. I place the mandrel in the bolt raceway after reaming the action. Then I use TWO dial indicators calibrated in ten thousandths. One up close to the ring, one further out away from the receiver ring. This eliminates run out and prevents me from having to fight against a cone shaped wobbling of the receiver that’s slightly off center. By doing this, the receiver is running true to the concentric of the bore.

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Here you can see that both dial indicators are reading exactly .0001″ on an inch while the mandrel is turned by hand to take the readings. That’s very little run out.

 

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After the set up comes the lathe work. The indicators are removed, then the mandrel and machining can start. The first thing I do is go deep inside the receiver to clean up the surfaces of the locking lugs.

 

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You can see the locking lugs inside have been re machined as clean as they can get without taking off too much metal and thinning them. The reason for this is so you can clean up the locking lugs on the bolt later on and have pretty much 100% lockup when you close the bolt. The idea here is to get everything concentric to the bore of the rifle for top accuracy.

 

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Next, I turn the surface of the receiver face to clean it up and remove any run out here too. This will ensure a good mating surface for the barrel and shoulder once it gets screwed back on.

 

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Once this is done, the next step is tricky. It involves a bit of mathematical contortion to figure out where you’re going to finish up your cuts… but you’ll basically use an internal threading tool to clean up and single point recut the threads of the barrel to screw into. WHY ? well, if you have a certain amount of factory slop that’s within tolerance, the threads may not be as perfectly aligned with the centerline of the action as they could be. In the accuracy game, closer tolerances mean a lot when it comes to precision shooting.

The next few steps will be the topic for another blog at a later time when I have photos to post. You chuck the bolt into the lathe next. Square up the bolt face with a cutter, the re face the rear portion of the locking lugs to get as perfect a match as possible when the bolt is closed. Then, I make sleeves to go on the bolt to make it run as true as possible in the bolt raceway. This prevents loss of accuracy by preventing the bolt from having any slack that could rob more potential accuracy from your rifle as the tension of the firing pin spring is lost when you squeeze the trigger. More will be explained in detail later once I have pictures to post.

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