Need a .300 Blackout suppressor under 1.5" OD

Griffin Armament Optimus 9, 1.375" diameter. Depending on configuration, it can be 300wm rated. Different configurations determine caliber and barrel length limitations.
 
This, plus it works fairly well. Get the tri-lug adapter too.

I run the Rugged 9mm Tri-Lug on my Obsidian 45, and the fitment on my H&K MP5 is absolutely flawless. Very tight tolerances, minimal play. 👍🏼

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