My first custom RF rifle
Came to me with a Fudd tuner on it. It worked the same way most tuners work. The thing I liked about it was the hole on it was the size of the OD of the barrel. I use to use it so that the face of the tuner was even with the muzzle so that it didn't need to be cleaned. The rifle tuned just fine using it that way. Of course doubters will say it couldn't be so, but it was. At least in 1994 it worked.
Pete