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"Barreling Up" a Rifle "Barreling Up" a Rifle From "Trade School" episode DTRS-106

Figure A

In this episode of Trade School, gunsmithing student Tim Mullin learns that becoming a quality gunsmith means having a mechanical mind. Today's gunsmiths are capable of not only repairing guns, but manufacturing them as well. Here we join in Tim's education as he crafts a custom target rifle, a gift for an administrator at the school. Tim aims to balance both beauty and precision in every inch of his project (figure A).

Here we join Tim as he focuses his attention on the metal portion of the rifle by "barreling up" the gun. He starts with a commercial bolt-action receiver and a commercial barrel blank, but must refine both so that they fit together precisely. The receiver basically contains all the mechanisms that transfer a cartridge from a magazine into the chamber. The barrel is essentially the steel tube that a bullet travels through when fired. A metal "recoil lug" fits between the receiver and the barrel and serves to keep all the metal components from separating from the wood stock ....

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Basic Tools:

Machining Lathe
Cutting Oil
Calipers
Micrometers
Cobalt Steel Lathe Cutters
Lapping Compound
Reamer
No-Go & Go Gauges
Pull Gauge/ Tape Measure
Vertical Mill
Combination Square
Plastic Mallet
Chisels
Hand Plane
Barrel Fitting tool
F1 checkering tools
Inletting compound

• Tim starts by "truing" the front face, or torque surface of the receiver on a machining lathe (figure B). The lathe spins the receiver at a high rate of speed and a custom cutting tool mills the ring surface until it is flat (figure C). Later the barrel secures through the recoil lug and screws into the receiver and this meeting point must align perfectly.

  Figure B

  Figure C

  
  

• Next, using various micrometers, Tim measures components of the receiver and the diameter of the recoil lug (figure D). He then transfers these dimensions to a hand-drawn blueprint (figure E). All cuts based on these dimensions must be within a tolerance of +/- .002" -- or less than 1/3 the width of a human hair! (Note that he also draws the dimensions he intends to cut on the breech end of the barrel).

  Figure D

  Figure E

  
  

• Tim "laps the lugs" of the bolt into the lug seats of the receiver (figure F). This properly aligns the bolt with the bore of the barrel. After applying a lapping compound comprised of 1200 grit powder mixed with heavy oil to the back sides of the bolt lugs, he manually works the bolt back and forth in the receiver until the lugs seat precisely. This ensures the lugs on the end of the bolt lock tight within the receiver (figure G).

  Figure F

  Figure G

  
  

  Figure H

• Next, Tim "zeroes," or centers the barrel in the adjustable four-jaw chuck of the machining lathe. He uses a dial indicator to check for any variance in the spinning motion (figure H). Once truly centered, the barrel remains secured in this position until all the refinements are made.
  

• Then, Tim cuts down the diameter of the breech end of the barrel to fit into the receiver and recoil lug. The lathe spins the barrel and a customized piece of cobalt steel makes the cuts. A marking agent called dykem blue indicates when the cutter engages the barrel. Tim's first cut "trues up" the face of the barrel to match the refined torque surface of the receiver (figure I). Next, he repositions and guides the cutter, removing a little bit of metal on each pass. (figure J). He measures frequently with calipers until achieving the desired diameter (figure K). He double-checks by fitting the recoil lug over the barrel end.

  Figure I

  Figure J

  Figure K

  
  

• Next, Tim cuts threads on the breech end of the barrel to match the threads existing in the receiver. This time, he uses a custom cutting tool ground to a sixty-degree angle. He employs a referencing tool to ensure a ninety-degree engagement (figure L). He makes many passes to cut the threads to the appropriate depth. While doing so, he applies cutting fluid to cool the surfaces (figure M).

  Figure L

  Figure M

  
  

  Once satisfied the threads are cut properly, he tests-fits by screwing the receiver onto the barrel (figure N), but it doesn't work! The alignment of the cutting tool slipped sometime in the process. To save the barrel, Tim improvises by manually working the threads together with an action wrench and more lapping compound (figure O). The technique works, and the receiver, recoil lug and barrel all fit together properly.

  Figure N

  Figure O

  
  

• Next, Tim cuts the 2 1/2" cartridge chamber into the breech end of the barrel. Still using the lathe, Tim carefully guides a reamer and hand cuts directly into the bore of the barrel, opening it up. The reamer mimics the size of the cartridge used for this gun. He remains focused to make sure the reamer remains centered (figure P). Between cuts, he frequently cleans the chamber and reamer with kerosene to remove metal chips and also measures for the proper diameter and depth of his cut (figure Q).

  Figure P

  Figure Q

  
  

  Figure R

• Next, Tim proceeds with the defining stage of this process: The go vs. no-go test. This test determines whether or not Tim’s work is valid by verifying that the chamber is appropriate for the cartridge used. He uses two gauges that mimic the size of the cartridge used for this gun, only the no-go gauge is just slightly larger (seen here next to a dummy cartridge) (figure R). The bolt locks in place when the go-gauge is inserted, but will not lock when the no-go gauge is inserted. Tim's work passes the test!
  

  Figure S

• Before removing the barrel from the lathe, Tim polishes the chamber of the barrel with 600 grit sand paper and kerosene. This keeps cartridges from sticking inside the gun barrel. Then he assembles the metal components and torques them tight with an action wrench. He double-checks his work by inserting a .223 caliber dummy round into the chamber and locks the bolt (figure S). It works properly.
  

• With the rifle barreled up, the gun's owner test-fires a live round in a special booth at the school (figure T). Tim inspects the spent bullet casing and determines that the lack of markings indicate that the chamber is properly polished. He also inspects the primer on the spent cartridge and determines that the firing pin hit it flat and soundly (figure U).

  Figure T

  Figure U

  
  

  Figure V

• With the metal components working properly, Tim fits the inletting pins into the receiver. In addition to securing the metal components to the wood stock, the pins also help by serving to guide the inletting the metal into the stock. (figure V).

In the next segment, learn how Tim crafts the wood stock of the target rifle ....