Installing a Breechplug | The Apprentice’s Notebook Part 1

The Apprentice’s Notebook 

Installing a Breechplug

By Jay T. Close, Kentucky

Introduction

A   good   measure  of your   rifle's safety,   reliability and   durability de­ pends in how the breech plug is installed. Fitting that plug is not a job to be taken lightly but it is how I  usually begin a rifle. If I do a  good job here, I can approach the rest of the rifle with confidence that the finished gun will be a safe one. If you're intimidated by this work, some manufacturers  -- e.g. the Getz Co.  -- sell barrels already breeched. You can be assured of this work. Also, if you buy a barrel and breech plug from Track of the Wolf, they'll install the plug for a modest charge. They too guarantee the work is done right. Nevertheless,  the only way to learn to install a breech­ plug is to fit a few. Moreover, installing a breech plug is a very basic skill, certainly, one required of anyone aspiring to the appellation of  "gunsmith". So, buy a barrel and a breech plug to match the barrel threads. You'll minimize frustration if you purchase one of the so-called premium barrels. These have the exterior of the barrel-shaped con­ centric to the bore and allow you to match the tang of the breech plug to any of the eight flats of the barrel.

Getting Started

Breechplug_Fig1.jpg

Figure 1  shows a schematic of a common breech plug with its various parts named. When you buy a barrel and breech plug make sure that the threads of the two components match. Be careful here because the threads used for various caliber barrels have not been standardized. The thread sizes employed by G. R. Douglas Co. in the past are fairly common so I'll list those below:

  • .32 and .36caliber........9/16 x 18UNC

  • .40  and .45 caliber........5/8x 18 UNC

  • .50, .54 and .62 caliber.. ... 3/4 x 16 UNC

Also, make sure that the width of the breech- plug tang and height of the bolster will match the diameter of the octagon barrel you choose.

The height is most important,  the width will fall into place. For those not familiar with the thread notes like "5/8 x 18 UNC" , I' ll try to explain. The "5/8" refers to the di­ameter of the threaded shaft, or the major diameter of the thread (Figure 2).

Breechplug_Fig2.jpg

The number "18" tells the number of  threads per lineal inch. You can have a threaded shaft 12 inches long or 1/2 inch long, but a 5/8 x 18 thread will still have 18 threads per inch. Eighteen threads per  inch has a couple of conse­quences. First, to screw a 5/8  x 18 threaded shaft forward one full inch, the shaft will have to   revolve 18 complete revolutions.  Second, for each complete revolution,  the shaft will move

forward (or back) 1/18 of an inch. A machinist or engineer would say the thread has a pitch of 1/8 inch or about .056 of an inch. This is all simple geometry and you can easily figure similar consequences for any known thread size.


Finally,  "UNC" stands for  Uni­fied National   Coarse. This is a standardized set of thread sizes rec­ oganized throughout  the United States.  Unified Nation Fine (UNF) is another  common series  in this country; and then, of course, metric threads....you  can look   that up for yourself!   Seriously, there's a   lot more to know about threads  (e.g. class of fit, left hand threads, thread forms),  but if you remember that the first number in a   common thread note refers to the diameter of the threaded shaft and the second number to the   threads per inch, you'll be in good shape.

To  breech your  barrel, you need to  install the plug to fit  three sur­ faces precisely.  Most important  for the safety of  your rifle is the fit of the breechplug face. This must bot­tom  tightly in  the threaded  portion of the barrel. Any gap there will col­lect powder    fouling,    harboring sparks and corrosion. See Figure 3.

Second, at the same time the face of the plug bottoms in the barrel threads, the face of thebolster must screw tightly to  the back of the bar­ rel. Any gap here will be obvious on thefinished gun and detract from its appearance (Figure 3).

Breechplug_Fig3.jpg


Breechplug_Fig4.jpg

Third,  the breech plug tang must align with one of the flats of the octa­gon barrel (Figure 4).

If  you have a "premium" barrel, the tang can align with any convenient barrel flat, but if you have, for example, a standard Douglas barrel the outside of the bar­ rel is not parallel to the bore. Douglas measures the "runout" (the deviation from parallel) and stamps their com­pany name on the fl.at nearest the bore at the breech. If you position that flat at the bottom of your barrel inlet, you can then make accuracy adjustments easily with the height of the sights.

This means,   however,   that the tang  of your breech plug can align with only one flat of the barrel: the one opposite the flat with the name stamp (Figure 5). This is trickier than it  might  seem. The alignment will be facilitated if the bolster and tang of the breech plug you buy are slightly wider than the width of the barrel Oat. If, for example, the bar­rel is 7/ 8 inches across the flats at the breech, buy a plug for a 15/16 inch barrel.  Any slight misalignment can then be filed to fit.

Tools

In  addition  to a workbench  and sturdy vise, these are  the tools and materials I   have    on   hand   when breeching a barrel:

  • 10 inch mill file with handle. 9 inch adjustable wrench.

  • 4 inch machinists' try square. vernier caliper with depth gauge. scriber.

  • 10 oz. hammer.

  • 3/4 inch countersink fitted in a brace.

  • 1/4 inch cold chisel.

  • machinists' layout blue (Dykem blue). file card.

  • brass jaw covers for the vise.

  • brass   strip to   protect breechplug from wrench.

  • light weight oil (3-in-l oil). Q-tips.

Breechplug_Fig6.jpg

This list should be largely self ex­planatory.  The jaw covers for my vise were bent from scrap brass less than 1/16 inch thick. Use what you have available and bend them to suit your   equipment.  The pad for   the breech plug itself is a strip of 1/ 16 inch  brass 3/ 4 inch   by  2 I/2   inch bent  into a "U” as  in Figure  6. When  placed between  the breech­ plug  bolster  and the jaws  of the wrench it  protects the sides of   the plug and tang.   Make sure that  the shape you bend  allows the brass to lie close to sides of the bolster.

Filing

First,  measure the depth of the threading in your barrel -- a vernier caliper with depth gauge helps, or just use a small pocket ruler. Often­ times, barrels are threaded 5/8 or 3/ 4   inch deep: too deep for our pur­ poses.  Unless you're contemplating building some sort of cannon,  file the breech of your barrel until only slightly more than 1/2 inch of threading is left -- say, about 17/ 32. This amount of threading is plenty strong and by keeping the breech­ plug fairly short you aid the design of the lock and wrist region of the rifle. But more on that in the future. If  you have much material to re­ move, start filing with a big, coarse, double cut file until you're near the finished size. Then, switch to the finer 10 inch  mill file. Accurate fil­ ing comes with practice, but it helps to  have the   workpiece clamped se­ curely about elbow high for this type of work. Take an easy Stance with  one foot slightly forward and file with long, smooth strokes. One hand  will grasp the file handle (al­ways have handles on your files) and the other hand will lightly grasp the tip of the file.

As  you begin  a file stroke,  the front hand exerts  downward pres­ sure. Slowly,   that pressure will transfer to  the rear hand. The net result is that  the center of pressure will always be   directly over the workpiece as you file. This is what allows you to file flat (Figure 7).


On the return stroke, lift the file from the work. Don't drag it back across.  Imagine  

yourself filing just in the very center of the workpiece -­ "aim" for that spot. This will also

help you  file flat. Stop work  occa­sionally and check your progress. Rotate the barrel in  

the vise so  you are coming at  the work from a dif­ different  angle. This is another trick

that will aid  flat filing.

   

Breechplug_Fig8.jpg
Breechplug_Fig9.jpg

Check progress frequently with the depth gauge and test the breech of the barrel  with a try square.  The area you're filing  must be absolutely flat and   perpendicular to the   

threaded hole in the barrel. If the barrel is par­ allel sided, this is easy to  determine. 

Just place the try  square body on one of  the barrel flats with the blade on the breech. Stoop

down to get a close look at  your work (Figure 8). Check the  breech area for flatness from  

at least two barrel flats 90 degrees  from each other as indicated in (Figure 9). Correct  any  

deviations  from square that you find.


Breechplug_Fig10.jpg

Testing   the    breech   face of   a swamped (tapered and  flared) barrel is a little  

trickier.  Set the body  of your try square on  the breech face and slide the blade up to one

flat of the   barrel. The   blade will stand from the barrel the same amount on all flats if  

the breech  face is square and flat. (Figure 10 shows the idea.)


Sometimes, if you're having trou­ ble  keeping the breech face flat, short  reciprocating file

strokes  can help.  Grasp the middle of  the file with hands about 3 inches apart. Set the file

firmly on the breech so  you can apply even pressure  with both bands. Then work the file back

and forth   using strokes  about 1 inch long.  This technique can be  espe­ cially good if  

you've  filed a  slight crown or high spot on the barrel and need to  level it. Whatever it

takes, get that breech face flat, then countersink the ends of the threads about 1/16 inch.

Next,  measure  the length  of the threaded shaft of the breechplug. File its length to match the

threads in the barrel:  a bit more than 1/2   inch. Check your work often. Keep the face of

the  plug flat and  perpendicular to the  run of the thread (Figure 11). The try square will

Be your guide. 


When  the plug  length seems  to match the   barrel threads,   file a chamfer on the end  

of  the shaft at about 45 degrees. Hold the plug up­ right in  the vise by its bolster and

Breechplug_Fig12.jpg

cant the file at  the appropriate an­gle. Then,  walk the file around the circumference of  

the shaft end taking Jong strokes. (See Figure 12.) All you  really need is a chamfer about

1/16inch wide.

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