They seem to work on whippy pencil barrels.
My dad has one on his Remington model 7. It tightened up groups after he found the sweet spot, and also brought handloads with varying POI a bit closer together.
My concern with it is that it isn't as fixed as something like a muzzle tuner. Shifting it an inch can mess with POI.
Bottomline,they cost something like $10 at Natchez, so you're not out much even it doesn't work.