Badger Ridge has the best sealed 209 setup for the older 700ml. Arrow Head Sporting Goods has a nice one too. Both are much much better than any of the others and they both work with BH209. A couple guys are still selling barrels for them. Occasionally they got some factory take offs too.
I would not waste my time on the 50x475 sabot. The petals are way thinner than the groove depth of the bore. 50x458 will give you more options and cheaper bullets. 54x50 is a good option too if you can find a nice take off barrel. There are several really nice bullets in 50cal that will also work great in sabots.
