I have some Starline in 45-70 and 90, its hard. I anneal it before I load it for the first time and every time I load it. Now saying that all of mine is at least 10 years old and they very well could have changed and softened it up years ago. It was discussed on a few BP forums so Starline was aware. If you would like to shoot some heavy cast postells send me a PM with a shipping address and I will be glad to send you a bunch ready to load. What I would send you drop at .460 and are lubed in a 460 sizer.
Do you know what pressure that rifle will be happy with?
Edit, I just went and looked at what you bought. Interesting rifle for sure. I dont believe you could fit the long Postells in the mag, It would be interesting to see a chamber cast and see if a bore rider bullet would chamber..
Good, I rolled hard on the 77 grainer .224 blems. They showed up yesterday in two medium flat rate boxes.....
One of the yayas I shoot with bought big on the 338 250 grainers too. There was some very good pricing from Larry, almost refreshing....
With the free shipping right now I ordered a few hundred 7 and 8mm's last night...
Its been awhile but RMR had Hornady 75gr BTHP match. 224 cheap i bought 3,000 of them. I haven't even started on them yet