I may well be wrong on this, but was of the impression the reason behind these industrial shells construction was so they "Would not" fit a regular 8ga shotgun. Pressures may well be too high for most. Also, "IF" the machinist was knowledgeable enough about guns to know this, I also would call him a "Bad Machinist" for making the mod. No one should do work which could prove to be hazardous. I suspect he simply didn't realize the difference & figured the owner knew what he wanted & did the work.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra