Vit 320... not affected by the REACH rules, clean burning, easy to meter.... only thing missing for you is that it isn't a high volume powder.
I use 4.4gns behind a 158gn hardcast TC bullet in a .357 case for my Marlin, but i only shoot that at 25m, although i'm sure it will reach 50
You may need to tweak the load a bit for consistency at 50m.
HTH