I've seen some made out of semi hard rubber that some guys were trying not sure who made them looked kinda dumb to me
Que Industries makes one that is licensed under the Browning BOSS , it attaches to your barrel witout any threading it uses some sort of collet fixture , its indexed just like the BOSS so you can tune it to find the "sweet" spot.
If you reload I realy don't see any reason to use one of these as you can taylor your load to fit your gun instead of vise-versa.
I know alot of rimfire shooters use tunners ranging from machined titanium weights to wads of rubberbands , but like I said they are using factory loaded ammo so they can't tune their loads.
Anyhow Brownells has the Que Industries brake/tuner for about $180 if memory serves