357 MagMaxi has to be at the top of the list of the most wanted, there are several ads in the WTB classified on GBO that have had no results, lots of them around, but no one wants to sell. As for rare barrels, 222 Rem, 7.62x39, 7x57, 7x64, 7mm-08 20" Superlight and 300BLK are a few, not very many made compared to the more common chamberings. The 22LR Versa Pack barrel is another favorite.
Tim