I installed one for a friend some years ago, never did understand why he wanted one.
Anyrate, get the .750" diamer one for a standard 20" barrel.
Liberally coat the inside surface area of the adapter, that will contact the barrel,
with red Locktite gel. This will act like a bedding compound, increasing the adapter's ability
to get a grip to the barrel. And YES, use RED, it will hold up to the barrel's heat build up.
Keep the Locktite off the hardware, remove excess gel with a cloth & toothpick.
Wait 4 hours before shooting.
Removal will be easy, no marks or scoring of the barrel.