You are correct the Ruger Flash Hider does not have flats to be removed with the 3/4 Wrench. But there are a couple of ways to remove it. First you need to put the Barrel into a Padded Vise Or the Upper in a Receiver Vise Block.
First method is a Strap Wrench, or by using a T-60 Hex Wrench, or a large flat screwdriver 10-12" that the tip of the screwdriver fits "completely" through the Flash hider ports or a flat piece of steel and turn it off. They are not pinned and brazed! Just a reminder! Looking at the front of the barrel twist it off in a counterclockwise direction. Be sure it is not loaded!!! LOL!