My 35 Whelen, a Winchester 1895, w/24" barrel, I also utilize Reloader 15. In fact when I worked up my loads, 1998, Reloader 15 was so good I have used it for standard loads in a good number of cartridges. I have been looking for some more RL-15, the past year and a half to 2 years, but to no avail, until last week when I found 2# at my local, 2 hour drive 1 way, reloading store. I am good to go, now again, for a while.
Anyway I have settled on 53gr.@2399fps, 54 gave me 2437fps. 52 clocked in at 2365fps. This with the Speer 250SP. since 2009 I have used it exclusively with cast bullets, LBT 250LCFN, with my alloy goes 260gr.
I have tried numerous other powders with the cast, Varget, TAC, Accurate 3100, and RL-19. RL-19 with a granulated shot buffer gave fantastic results, until, of course, I went back to RL-15. Without shot buffer it blew RL-19 out of the water. With my previous inability to find RL-15 I was going to try some BLC-2 with cast as I heard good things about that also. I'm still going to, but now for fun, instead of necessity.
Crash87