bwinters,
Quite a few different powders work in the .257, but I have loaded for around a dozen rifles and around 46-47 grains of either IMR4350 or H4350 with 100-grain bullets, or around 44 grains of the same powders with 115's, have been accurate in just about all. Or at least I can't remember any rifles where those loads didn't provide sub-inch accuracy.
Some +P brass is heavier than standard brass, but I don't know if all is. Dunno why they made it heavier, because the standard brass from Winchester was always plenty strong for even very warm handloads.
I have had some recent Remington brass that was so soft the above loads (which aren't much more than +P, if any) resulted in difficult extraction. Never had that problem with Remington brass in the past, but there it is.
My wife and I have killed several pickup loads of big game from pronghorns at 400+ yards to cow elk with various bullets from the .257. For average deer-sized the 100-grain Ballistic Tip works very well, but for anything bigger the 100 TTSX and 115 Partition provide plenty of penetration. In my experience the 115 Partition kills quicker than the 100 TTSX, on average.
