This is the way I file fit breech plugs , and offer it only as that, nothing more. 

 

I would like to preface the following by saying files are nothing less than hand operated lathe and milling tools.  If anyone thinks that filing is just put it to the metal and start stroking hard, I advise you to re-think that concept carefully before starting to fit breech plugs, or any filed metal work requiring a degree of precision.  To file, and file correctly, takes practice, practice and more practice until skill approaches art.   I have not reached the latter proficiency after several decades, and have been proven wrong more than once so far as having skill.

 

If your unmarked barrel is from round stock where the flats were ground on center after drilling, reaming and rifling, you can almost always go to the nearest flat when fitting bolster.  If octagon stock was drilled, reamed and rifled, there is a good chance you will have to check carefully for any run out, that to be indexed in the vertical when fitting tang bolster.  This may require filing the rear face of barrel.  Otherwise, you may have some rather extreme looking front and rear sights drifted to the left & right to get on target. 

 

When fitting, I always hope for the  threaded length of breech plug to be more (.750") than depth barrel is threaded to the shoulder, (.625")   I know then it will be a quick and easy job.   I start by checking rear barrel face for square, check internal shoulder (sometimes a knife edge needs to be removed) and clean up milled tang bolster face edges.   Then with a depth gauge or veneer caliper check the depth of internal shoulder from rear of barrel face, and length of breech plug from bolster to face.  This tells one quickly which to reduce, the breech plug, or the rear of the barrel.  If the breech plug is longer, I run it in by hand until I feel it start to bottom out solid against the shoulder, I then use a inside veneer caliper measurement to obtain a more accurate reading of how much material to initially remove.

 

I then use masking tape to mark off material I want to initially remove,  making sure circumference of threads is taped evenly so at least .025" less material will be removed than desired, this will allow for final fitting.   I then plumb breech plug true vertical in a vise and with a coarse bastard file  I file level as possible across breech plug face until it is scored evenly and completely, stop, that, and no more.  I then again level as possible, file breech plug face 90 degrees to first cut, this will initially produce a cross hatch effect, I only file until cross hatch is eliminated and return 90 degrees to first cut position.  By repeating this filing process correctly,  I find I can quickly and evenly reduce that material decided upon.

 

I then remove masking tape and use a small machinist's square to check face of breech plug for true.  If uneven, remove high spots until it is true.  I then take a small mill file and ever so lightly chamfer breech plug face circumference to remove any jagged knife edges sticking out.  (not fun to run those steel slivers into a pinky and then break one off deep below the skin surface, I am not a fan of Exacto blade self surgery)

 

I then again run breech plug in by hand and obtain a reading.  If considerable material remains to remove, I repeat the cross hatch filing, but checking all steps more frequently until close.   If close, I will then use a mill file on breech plug face and start using Prussian blue transfer paste on internal shoulder, I use a wrench to tighten breech plug lightly to obtain a true impression.  All the while closely monitoring bolster faces for barrel contact, so as to bring breech plug, and bolster to barrel final fits simultaneously.  

 

When it was the threaded length of breech plug I had to reduce, I have seldom ever, had to remove material from rear of a unmarked center ground barrel face, when I did, it was almost always operator error, for I goofed up and had to go for another flat. ( this mistake can be a real bummer on those barrels with run out, after just getting it so close to right)

 

On final tightening of breech plug, it is well to remember we are dealing with soft steel here, there is tight, and then there is "TIGHT".  If one gets western and over torques, a considerable pre-load can be imparted to those threads.  Now I don't know about others, but I prefer that powder charges alone imparts those big loads to the breech plug threads, not have every shot become a proof load.

 

( I will throw this tid bit in here about hack saws.  A quantum leap in hack saw control for precision cuts can be as simple as reversing the blade for pull, rather than push strokes.  More than one has been  pleasantly surprised when they go from herding cats to leading that blade exactly where they want it.)