Could someone inform me what the metal ring that came with my 300BLK Rattler is for? I saw no mention of it in the included paperwork or manual.

It was in a zip lock bag w/ a wrench. Was wondering if this is a suppressor adapter for the tapered shoulder.

The below auction shows the same wrench & ring...

https://www.gunbroker.com/item/879023258

You are correct. The anti-taper adapter is for mounting muzzle devices that require a 90 degree shoulder. The wrench should be a T27 torx for the barrel clamp screws.