The first step is to reduce the rim of the shell to a thickness of 1.5mm (or thereabouts) using a flat file. The shell is passed 'back and forth' over the file until the rim is the correct thickness. This is a quick and simple procedure. During the trimming procedure rotate the shell a quarter turn every few 'passes' to ensure the rim is evenly reduced.
Checking the rim thickness. It should be 11.5mm
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