Photos #11 and #12 show our GTR receiver blueprinting jig. This is the workhorse of the entire operation that makes everything else possible. Because it is very difficult to see the entire receiver truing set-up in a photo, I will briefly explain how all these tools fit together: First, the correct size raceway bushings are inserted fore and aft into the receiver; the fit should be tight, but not so tight that they must be “driven” into place. Second, the small end of the reamer/mandrel is inserted into the center of the raceway bushings and the mandrel is pushed back toward the tang until the reamer flutes come up against the internal lug seats; a squirt of WD-40 goes a long way here. Third, the receiver and mandrel are placed into the blueprinting jig and everything is roughly centered by adjusting the eight bolts on the jig. Finally, the entire apparatus is placed in the independent four jaw chuck on the lathe. Then, the real fun begins.
Now, although it is not mandatory to “zero” the jig itself, I would strongly suggest doing so; simply because anything with “run-out” spinning 600 RPM right off the end of your cutting tool messes with your depth perception (at least it does for me) and there is no room for error here.
Photos #13 and #14 show the jig, receiver and mandrel in the lathe. You will note the large dial 0.0005" Mitutoyo indicator out on the extreme right-hand end of the mandrel. Although it is hard to visualize, in this right-hand position you are indicating the tang end of the receiver; and when you move the lathe carriage and indicator to the left (toward the receiver face) you are indicating the breach end of the receiver. Some guys use two dial indicators simultaneously for this operation, but because our lathe has an accurate DRO that allows us to return to the EXACT same indicating position, we only use one – sliding it back and forth the length of the rod.
Next, it is just a matter of indicating both ends of the rod to absolute zero/zero. Which is a lot easier said than done. The trick is just to have both patience and fortitude; whether it takes one hour or three, just keep adjusting the jig bolts until the needle on your 0.0005" indicator doesn’t even wiggle. Then, when you are satisfied that you have indicated both ends of the rod as close to perfection as humanly possible, stand back, turn on your lathe and let it run for five minutes. Now, go back and start all over! (And you wondered why some guy’s guns always seem to shoot better than others.)
So now that we have the receiver spinning on its true center axis, which is what this entire process has been about, we are finally ready to make the “face cut.”
Photos #15, #16 and #17 vividly show the progressive “clean-up” of the receiver face. Obviously, the cutting tool removes the blue Dykem from the “high spots” first, advances to the “low spots,” and eventual overall clean-up. Total “run-out” on this receiver from first tool touch to complete clean-up was 0.008"; better than some, but not as bad as others.
Photos #18 and #19 depict the truing of the internal receiver lug abutments. Because this is a totally blind machining operation, and very difficult to execute and photograph, I failed to capture a picture of one “high” lug in a state of “total clean-up” and the opposing “low” lug in an “untouched” condition; which happens often, and if left unchecked, creates the common factory rifle condition of having one bolt lug bearing 100% and the other lug floating in space like a drunken cosmonaut. Which is one of the many reasons why 1/4 MOA factory guns are few and far between.
Finally, we come to photo # 20 and the finished product. But wait, you exclaim, what about the internal threads!!! Aaahhh yes, the threads and other loose ends; well that, my friend, will just have to wait for another day.
Quarter Minute Magnums
PO box 18
Winchester, ID 83555
Precision Shooting Magazine is one of the oldest continuously published firearms magazines in the US; its first issue was in May of 1956. While it if often thought of as "home" by Benchrest shooters as well as Highpower and Smallbore shooters, its subscription base is considerably broader. Other areas that are given frequent coverage include wildcat cartridges, military sniping, long range varmint shooting, accuracy gunsmithing and reloading, equipment reviews and firearms-related history. Its cadre of writers is unique and most of them write only for this publication.
Join the discussion of this article HERE at the Article Discussion Forum.