Well,
It isn't on an Encore, but a T/C Black Diamond.
The barrel on this gun is 19 inches long. We cut the barrel and stock down so it would better fit my sons.
While I do think that with large powder charges one might run into problems with unburnt powder and inconsistant velocities, with lighter loads there are no problems.

We only load 75 grains of 777 and a 250 grain hornady XTP. This load and this short barrel gives excellent accuracy to 100 yards and has a very mild recoil. The boys have shot 3 deer with this gun in the past two years and every shot has been a full passthrough w/ good bullet expansion and good tissue damage.

With the right load, I don't see any problem with a 20 inch barrel.

Hank