Why We Blueprint Actions

Chris Taylor

Random Factor of the K Power
Moderator
#1
Without getting into a long discourse on mechanical precision in rifles, I'll give the Reader's Digest version. Mechanical precision in rifles comes from the the repeatable, rigid, alignment of parts.

Blueprinting/truing an action is remachining all the important bits so they are parallel, concentric, or perpendicular (depending on the part) to the axis of the bolt in the receiver, and that the bolt is in line with the bore of the barrel. It can get more complicated, but that's the general idea.

Since I was doing an action for a T&E gun that was particularly bad, figured I'd take some pics, and pull back the curtain for a minute. :D

The following pics are from a mid/late 80s push feed Winchester M70.

In this pic, I'm machining the face of the receiver so the barrel has a solid bearing surface. As you can see, the face ain't square atall atall. This would cause uneven torque on the bbl, which causes unequal stress in a pretty important juncture of the whole machine. This can also cause uneven harmonics, which could be a cause of erratic grouping.

Going back to our short definition, this isn't a particularly rigid, or aligned, situation, which would detract from getting the maximum precision from this rig.

Rcvr ring m70.jpg

In this second pic, I'm remachining the bolt lug locking recess, where the lugs on the bolt fit and sit when the action is closed.

These are bearing surfaces, and are the places that actually take the recoil when a cartridge is fired. This is a pretty high stress area, since most of our modern cartridges can generate 60k-65kPSI transducer. Pretty critical area.

If the lug recess isn't square, it can actually cause the bolt and lugs to bend, ever so slightly, til both lugs are in contact with their mating surface in the receiver. This can cause the cartidge to be misaligned with the axis of the bore. Match chambers are cut to minimum tolerances, but there is still clearance enough for a slight misalignment.

Neck sizing and turning can help mitigate this, since the case is fit to the chamber, but the possibility still exists.

This pic can be a little confusing if you've never seen down the action with the barrel off. And it's a crappy cell phone pic, so it ain't that great. I'm a gunsmith, not a photog. ;)

What you are seeing is the front end of the action in a truing fixture. The recoil lug is up to the left at 10 o'clock. There is a mandrel in the action where the bolt usually goes, and my cutting tool is still set up to cut. The lug recess to the right in the pic (top of the gun, left locking lug of the bolt) has already been cut and is bare silver steel. The one to the left (bottom of the gun, right lug of the bolt) hasn't been touched yet, as you can see by the fact that it still has marking fluid on it, and is dark.

Lugs m70.jpg

Keep in mind here, when we're talking about truing an action, we're talking thousandths of an inch (.001") worth of material. Not much granted, but it can be enough to be the difference of a gun shooting MOA or one-third that. This particular gun was .002-.003" out of square. That level of tolerance is actually not bad, and is well within most factories specs.

The same basic process goes on with the bolt, recutting the locking lugs and truing the bolt face. Then lapping the lugs, which is using a mild abrasive to polish the lugs and lug recess mating surface so they have as tight, and smooth, a fit as possible.

I didn't show re-cutting and truing the action threads in the gun, because they were surprisingly good. And single pointing internal threads always makes me break out in a cold sweat, since one little goof can scrap a receiver. Didn't even think of taking a pic 'til it was all said and done. :D

Hopefully, this was at least moderately educational, and made even a little bit of sense. There is a lot of time and effort that goes into building a good precise rifle.
 
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#5
Thanks for this post, it was really informative. I have at various times in my life lusted after a precision bolt gun, and no doubt if I ever go through with it I'll be having this work done.

Does the barrel ever get a similar treatment to make sure that it's square to the bore?