Ok, I tried it again after work today. The jacketed and plated slugs did as well as expected. Surprisingly, the lead bullet groups tightened up a lot. From left to right, 115 grain FMJ, 124 grain plated, 134 grain lead at 25 yards:
https://opeforum.com/attachments/0109211617-jpg.276798/
Took it out to 40 yards, and got about the same results, same bullets left to right:
https://opeforum.com/attachments/010...dr-jpg.276799/
So, it looks like the carbine might eventually get there with lead bullets. I'm wondering what happened to improve the groups with cast. Maybe a good cleaning helped it, I did redo the barrel nut as instructed in the manual. I know some guns need to have the bore broken in with jacketed bullets before they do well with cast, maybe the jacketed and plated slugs helped there. I suppose I should put more of them through it to help break it in. I'll also try the Lee 358-125-RF sized to .358.