I took a machinist's file and opened up the sight gap, trimmed the top down slightly and cut the notch deeper so POA-POI worked to create a perfect combat picture
Don't take this the wrong way I'm just adding some "file lore" to the discussion. I taught jewelry making, silversmithing, pewtersmithing and metalworking in general for over 15 years...a metalsmith lives and dies by the choice and quality of their files.
If the file in the photo is what you used, the correct name for that pattern of file is a "mill" file. I would not have chosen a mill file for this particular job for one very important reason. A mill file does not have a safe edge. In other words it cuts on all four sides. You can grind a safe edge on a mill file by grinding off the teeth on one or both edges. I've done it many times, mostly for my pewter working files. It is best done using a stationary belt grinder as it is easier to keep the edge of the file perpendicular to the face using the belt grinder.
I would have probably used a narrow pillar file for this job. Pillar files usually have 2 safe edges (some have only one) and choosing the proper cut (fineness or coarseness) of the file to use is also important.
Grobet brand files are of excellent quality but are also pricey. Swiss and German files use a numbering system to designate a file's coarseness. The higher the number, the finer the cut. German and Swiss numbers are, sadly, not identical in that a Swiss 6 and a German 6 are not of the exact same coarseness. American pattern files use a word system to describe coarseness. They are from coarse to fine: Coarse, Bastar4 (4="D", yep an illegitimate child), Second Cut and Smooth. American pattern files increase in coarseness as the length increases (Swiss and German do not). So a 10" Smooth cut file is coarser that a 4" Smooth cut file.
For opening up a rear sight such as in this job I probably would have chosen a 4", #2 cut narrow pillar file...or similar. I have well over 100 files in my shop so choice is possible. Were I to do this on a budget I would buy two files, both Nicholson brand, both mill files (though I would make one edge safe on each). I would buy a 4" and a 6" smooth cut, mill pattern file. The 6" MAY be too wide to fit the sight opening the 4" would probably work (I haven't gone to the shop and measured). One last thought and I have seen many very good craftspeople make this mistake, they will use a needle file when they should be using a full sized file. Needle files are not only harder to control but can flex some in use thus effecting accuracy. If I only had needle files to accomplish this job I would use a Barrett file (Triangular in cross section, only cuts on the wide base of the triangle, the triangular cross section aids in the stiffness (lack of flex) with the barrett file.
Hope that helps anyone wanting to do similar work. Nicholson files are the best American pattern files available (IMO) and are very reasonably prices.
In all my years of teaching I found that proper filing technique is one of the most difficult skills for my students to master. Have fun but practice !
