I don't think the 340 is that old. Older Weatherbys that were West Germans in the 1960 to 1970 range in the .300,.270 and the .257 had slower twist that did not always work with boat tails. I have something like 35 old West Germans and a few Japs and a lot of the later USA's. You just have to try it and see. I have an older .300 that has the slower twist that shoots 165 grain boat tails into an inch at 300 yards from a solid rest. I wished I could help you with the 250 grain question, but I as you found a sweet spot for the 225's.
Good shooting,
Marcus.
