I have a Rossi M92 .357 mag, 20" barrel, an excellent rifle in every way except accuracy. It is equipped with a Lyman receiver sight, shoots a decent group side to side, but strings vertically something terrible. 2" or so horizontal, 8-10" vertical at 100 yards if I try a 10 shot group. Doesn't seem to matter much what I feed it.
Perhaps something to do with the tightness in the barrel bands? There is a fore end band, a muzzle band, and the retaining screw that keeps the magazine from moving forward. I don't suppose any of these are good for accuracy. All are currently a snug/tight fit. Any suggestions?

You have to free the magazine tube so it doesn't bear on the barrel when it heats up, float the barrel bands, and particularly the retaining bit at the muzzle where the mag screw holds to the barrel.
Paco Kelly's site Leverguns has an article on it.

thanks CH! exactly the info I was hoping for.