The adapter nut requires a greater torque than the barrel nut or else it will become lodged inside the barrel nut when you tighten it. It's meant to stay on the rifle, not inside the barrel nut.
Having the conversion nut wrench gives you this ability.
However, if you have a handguard that you want the adapter to stay in forever... Or you purchased numerous adapters for multiple handguards... You could loc-tite them into the barrel nuts and go. You just would be limited to those handguards being permanently used on 15-22s.
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