I'm not sure the 257 is over-bored enough to use Retumbo, at least I've never seen any load data for it. A couple of weeks ago I picked up a pound of the new Hodgdon Hybrid 100V, their manual claims to get 3050fps out of the standard 257 Roberts with the 115gr bullet. With the improved case I should be able to up that by about 2.5gr, I'm interested to see what the velocity ends up being. Hopefully, Hodgdon isn't being overly optimistic with their numbers.