Precision Barrel Fitting, Part Three

Suppose you weren’t working on a barrel fitting job. Suppose you were making a wooden picture frame. If any one of your four frame sides isn’t squared and true on its ends there will be gaps instead of tight joints where the frame angles meet. In such a situation, these gaps can be filled and blended smooth before the frame is painted. The last time I looked, I found no rifle barrels or receivers made of wood so we won’t be able to cover such mistakes as easily.

The first guns were made prior to metallurgy being advanced enough to cast tubes. Cannon barrels then were slat constructed similar to casks and kegs. The result was then heavily banded with metal. These bands were applied primarily at the breech but I have seen etchings of them spaced out along the tube during one of my visits to the Tower of London. Sometimes the banding It must be accurately turned with all diameters and center holes concentric. Any uncorrected misalignment will cause a loss in accuracy.

The process takes minimal time but can be the most important machining step taken inn precision barrel fitting. worked. Sometimes it didn’t. When it didn’t, it wasn’t enemy arrows that knocked down defenders. It was splinters. The point being made here so far has been an alert. I’ll make it a warning: Never begin any barrel fitting steps before the receiver has been squared. The process is simple and can be done in about ten minutes.

Leave it undone and any imperfection of the receiver will be amplified when the barrel is attached. As for a few examples, consider that if the receiver tilts down, up, to the right or to the left, any barrel joined to it will tend to point low, high, right or left, only more so. Sometimes the sight system installed might be able to compensate, but it is safe to say the receiver will never be center bore with the barrel. Or, if you prefer, visa versa. Actually, the procedure for squaring the face is simple.

It can take as few as ten minutes. Drawing 18 shows you the setup for turning. You may have noticed the receiver wears no external parts. They have all been removed including the trigger assembly. Whassup? Having stuff attached to and sticking out from the receiver during turning would

The table it refers to was reproduced in an earlier article in the barrel fitting series. be dangerous for the operator and ruin the receiver if, for example, the trigger group whacked some part of the lathe. I add, the potential damage is frequently reciprocal. manufacturing operation always keep top of mind the requirement just made regarding center holes and diameters. Make step-by-step notes The receiver facing mandrel illustration is your road map for making a mandrel.

Avoid trying to square a receiver face without one. All center holes and diameters made in the fabrication of a mandrel are required to be absolutely concentric. Receiver threads must be precision turned. Throughout this mandrel As indicated in this tool application chart, all metal removal excepting the left and right finishing shapes is accomplished with the tool point. This includes boring and internal threading. to support your memory from here on in.

The blank round stock to be transformed into your mandrel can be held in a three-jaw chuck, but a four-jaw self-centering one is much preferred. You’ll discover why as this article proceeds. Center drill one end of the round. Reposition the round leaving about half its thread relief end secured in the chuck. Turn the blank to the proper thread diameter. Mark the thread area. Turn the remaining length of the round to be a very close fit with the receiver bolt way. Turn the receiver threads.

Reverse the workpiece end to end. The diameter of the round may have to be reduced to be secured in the jaws. Then, indicate the piece to be on center, plus or minus 0.0005. Center drill the remaining end of the round and turn the result to match the bottom of the receiver threads, minus 0.025 to 0.030. The mandrel is now complete. Of course a self-centering threejaw chuck could be used.

The problem is most of them return to center with a plus or minus of 0.001 to 0.002, while their four-jaw counterparts return to 0.0005. Therefore, the three-jaw is not recommended. Squaring The Receiver Face Setting up will require more time than the work. Either a general pur-

Actual work on the barrel such as crowning and chambering will be addressed soon in future articles. Breech diameter, contour, length and taper are usually specified by caliber and cosmetic specifications provided to the barrel maker when the order for it is placed. pose cutting tool or a dedicated facing tool can be used. Whichever you select must have its left side face set at a 5 to 10 degree angle to the receiver face. The point of the tool, not its side, will do all the cutting.

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