Whenever I tested a weapon and there was no clear winner for which attachment was best, the flash hider won by default.
Use muzzle as reducing the first shot recoil by the most will also decrease the follow up shots as well if you understand what I mean.
You could put it on every automatic weapon in the game and have no issues at all.
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The flash hider is a universally viable barrel attachment.
This guide for all guns up to shifting tides and should be valid for ages unless Ubisoft do a massive overhaul like they did in Grim sky.