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I’d be interested in the extension with wrench flats and not teeth. I’m curious if teeth were used for a reason
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Because flats, squares or hexes look wonky when they're not timed. The 12 scallop design looks fine no matter how it's oriented. It also allows a smooth shoulder with a smaller maximum diameter/thinner profile. It's designed to be removed or installed with 12 point wrenches or sockets, 11/16" or 17mm in this case.
The reason for the reduced diameter section of the barrel just behind the muzzle on the S&W pistols, Kel-Tec pistols and others is for barrel removal while still having sufficient diameter to engage the barrel bushing. It serves no other purpose, and that purpose ceases to exist when there's an adapter that has to be removed before the barrel can come out anyway. I turn away the "bulge" and thread behind it with very fine pitches (usually 40 TPI)
The threads on the S&W barrel show some chatter because I used a tool post milling attachment I made with a single point thread mill rather than turning them with a conventional static cutter. It allows me to better control depth and skip the undercut, since I rotate the chuck by hand with the half nuts engaged while the thread mill cuts. I use the same process for cleaning up damaged receiver threads like this Kimber receiver I just did, in which the thread crests were absolutely mangled: