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Chart For Easy Computation Of Worm Blank Minimum OAL

48″ barrels require a blank of 56 1/8″

42″ barrels require a blank of 50 1/8″

40″ Barrels require a blank of 48 1/8″

36″ barrels require a blank of 44 1/8″

28″ barrels require a blank of 36 1/8″


Truing Up Your Worm Blank

     I’m often asked “How do you get your worm blanks true if you are turning them by hand?”  This is a most excellent question and one that is easily answered in many words, but more easily accomplished. Five photographs should help explain how I go about doing it in the shop.

1.  The worm needs still be mounted on the lathe to perform this task.  That being insured, obtain a strip of unmarked paper that is about one inch wide and about 10″ long or so.  Regardless of what you use, one long side of this strip must have a perfectly straight edge upon it.  A piece of typing or copy paper works just fine.  Now grab a pencil and head over to the lathe.

2.  By measuring the worm blank every couple of inches for it’s entire length, find the narrowest point on the blank and mark that spot by a line with your pencil.

3.  Wrap the paper strip around the blank at this point.  Take pains to insure the paper is wrapped tightly around the worm and the one straight edge, as the paper is wrapped around, meets itself perfectly.  No slops, no loose spots, no crooked wraps are allowed here at all.  Git’er wrapped as tightly as possible.  Where the outside end of the strip runs out, mark a pencil line on the inside portion of the strip. You are making a flexible measure to be used several times throughout this operation.

I know it’s a bit hard to make out just where the end of the strip stops, but you can see where the pencil line is placed.  You will notice the right edge of the strip meets itself perfectly.

4.  Remove the strip and go to one end of the blank or the other.  Wrap the strip tightly around the worm again and notice the distance between the end of the strip and your pencil line.  If the two align perfectly, move a couple of inches and measure again.  When they do not touch each other, make a small pencil mark at that spot on the worm.

 Keep measuring every two inches until you find a spot that matches up on the strip or you have measured about a foot in length.  Mark this location with your pencil as well.

5.  Now crank up the lathe after obtaining your sanding block that has been loaded with 100 grit paper.  Sand between the lines you indicated on the worm.  Stop sanding every little bit and take a new measurement until the strip end meets the line.  Git’er as close as possible.  I know it’s fun making all that sanding dust, but don’t get carried away and sand too much off. 

6.  Repeat this task as much as needed until you’ve gone the entire length of the blank.  When the end of the strip meets the line at the opposite end of the blank from where you started, your worm is pretty dern true.

And In My Not So Humble Opinion…

    If your worm is not perfectly true and the same circumference at each end, but are less than a 1/16″, don’t worry about it, you’re in the ballpark.  You can make it up by fudging when you lay out all your groove lines by drawing the lines a little heavy/wider.  But again, don’t worry about it because the average person will not be able to make exactly perfect grooves when chiseling them out by hand anyway.  Your rifle will never know.  Don’t believe me?  I’ve got several chunk gun barrels out there and ever dern one of them is a certified spider killer.  If that kind of accuracy ain’t good enough for you, I suggest you call up my good friend Don Getz and buy yourself a machine rifled barrel as this project is not for you.