trim
刓 centers on cutting or shaving away material to shape or smooth something: trimming, carving, or wearing down by scraping.
刓 combines 元 (origin, head) with 刂 (knife), suggesting cutting or shaping at the head or edge. The exact historical development is uncertain, but the knife component clearly indicates a cutting action.
A knife 刂 is used to trim the top 元 of something, like shaving off the rough edges to make it smooth.
For ガン, imagine a gunsmith trimming a gun barrel with a knife: gun -> ガン, and the knife 刂 shapes the metal.