Suppressor for Grendel came in

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Every AA Grendel I have seen has a 9/16x24 thread pattern, with a shoulder. The FAL uses 9x16x24 LH, and this was because the FAL had a very narrow barrel profile as a service rifle, while still needing enough shoulder to support a washer for muzzle devices.

The Grendel thread pattern was determined during the Clinton Assault Weapons Ban period, when the 6.5 Grendel was cutting into new territory with larger calibers in the AR15. The Colt 7.62x39 Sporter II used 1/2x36 IIRC.

Knight's Armament uses 3/4x24 on the SR25/M110/Mk.11 series.

Eagle Arms/ArmaLite Inc. started using 5/8x24 on the 1996-forward AR10.

DPMS started off using plain target barrels without any threading on the LR-308 series because they were developed during the ban as well. After the ban, DPMS started using 5/8x24 threads, like ArmaLite Inc.

There was no thread pitch standard for AR10's, and still isn't to this day. There certainly wasn't and isn't a standard thread pitch for larger bore calibers in the AR15. The 6.8 uses all sorts of different thread pitches from maker to maker, ranging from 1/2x36 for Model 1 Sales, to 9/16x24, to 5/8x24.

There is a discussion of this briefly in Volume II, covering basically what I just said, with the historical context of the Clinton Assault Weapons Ban underlying the technical aspects of thread pitch choices that several manufacturers went with. I don't think I've seen 3/4x28 on a Grendel though. Maybe Bwaites or LR1955 have.

I have a factory AA 19.5" that was originally threaded 3/4x28. It's a Lothar barrel. I was kinda irked at the lack of available muzzle devices for that pitch, so I sent it out and had Tornado Tech rethread it 5/8x24.