The Contender because of its design has always been limited to cartridges that generate about 40,000psi on the breach face. More and more wildcatters were pushing that limit and even exceeding it so T/C came out with the ENCORE which can be chambered in 99.9% of the cartridges out there today. The Encore can shoot in the 60,000psi on the breach face range so you can have a 3006, 300WinMag, 7MM Mag, 257Roberts, .243Win. None of those can be chambered in the Contender. Lastly the barrels can not be interchanged between the Contender and Encore frames.
The Encore comes in 28g, 20g, and 12g. The Contender used to be chambered in .410 but I haven't seen one advertised in ages. I'n sure if you really wanted one Fox Ridge or Ed's could get one for a price.