Ok, the seller got back to me and to paraphrase, you have to provide an adequate length of 3/4" pipe and drill a small hole in the threads of the pipe so that the set screw on the wrench end can be "locked" into place. He also recommends clamping the barrel as opposed to gripping the upper receiver or the handguard. He's selling handguard adapters so you can use a standard AR-15 handguard and barrel vise jaws sized to fit the 15-22. I'll probably pick up one of his wrenches, it looks like a cleaner and more professional version of the one I made with a dremel