It was all about what is readily available in AR-15 barrels and there was/is a tremendous number of manufacturers of 9mm (.355") barrels whereas there is a complete lack of .357" AR-15 barrels.
The new 8.6 Blackout, .338" bullets, looks promising. Subsonic 200 - 300 grain bullets will surely bring down a lot of critters with some serious authority!
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