The 8 inch I have I used on a 16” barrel with front sight base gas block. The orig barrel nut was a little less refined than current models and galled up the screw so I had to use a dremel to get it out. Cut a groove in the screw head to allow extraction. Left a blemish on the guard. Still usable but needs touch up paint to hide it. Figured that was better than drilling a larger hole in the guard and barrel nut.
I’m liking the 10.5” pencil barrel idea.