NO longer hunt Public land with rifles for Turkeys in Fl, as of 2011. Private Only.

If the 20 is Old, chances are it has a long barrel, choked Mod, and will kill Turkeys all day long out to probabaly an ethical 40 yrds.
Try Winchester 2/3/4" #4's up to 6's.. What kind of 20 is it, and how long is the Barrel ?
They are Light, and deadly, throw good patterns, nothing wrong at all with that 20.

Shooting a Turkey at 50 yrds and out with a rifle, would not be very rewarding anyway.