You can use your H4350 but I wouldn't choose either of them for a 257 Weatherby. I use H4831 in my 25-06. I'd use H1000 in a 257 Weatherby.I have a 257 wthby that I am starting to reload for. Looking at the manuals I have, I found several IMR4350 recipes. I do not have the IMR on hand, but have a pound of H4350 on hand. To my understanding the IMR is slower than the H powder. Could I use the H that I already have safely?