So in the time since I posted this thread I found some Mec-gar 17-round 5906 Magazines online for $23 and decided to give it a go.

Upon receiving the magazine in the mail today, I can confirm that said 5906 magazines will fit/function in a 4006TSW with absolutely no modifications necessary, at least as far as hand cycling the ammo goes. It will be awhile yet before I can take it out to the range, so that will have to wait for a future update.

However, I can confirm that the Mec-gar 17-round 5906 Magazines will indeed fit .40 S&W cartridges without any modifications to the feed lips, the magazine will fit securely into the magwell of a 4006TSW, and rounds will manually feed from the magazine into the chamber when the slide is cycled or otherwise locked back to the rear then manually released with the magazine inserted.
Admittedly, my first two attempts to chamber a round did hangup on the feedramp, but a simple tap to the magazine caused the rounds to chamber, and subsequent attempts in which I was more conscientious about pulling the slide fully to the rear then releasing it as well as attempt to chamber rounds with the slide locked back then released were successful, ergo I attribute those first couple of hangups to myself short-stroking the slide.
Regardless, the magazine is at the very least adequate for range use.

Lastly, I was able to squeeze 12 rounds into rhe magazine before my thumbs gave out on me, but I'm confident that with the aid of a mag loader that the magazine could fit at least 13, possibly even 14 rounds of .40 S&W.

As previously stated, once I finally get a chance to go out to the range, I will report back with the results of how reliably the magazine functions, but with the threat of COVID-19 among us, many ranges in area are closed, so don't hold your breath.

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Shooting Comfort is bilateral.