It took almost a year, but finally, yesterday I got to the range and finished I think testing these cast bullet 30-30 loads in my same old micro-grooved Marlin 336. Including the data from a few months ago for completeness, here's what I worked up to using BL-C(2) powder and uncooked Cream of Wheat cereal as a case filler on top of the powder charge, under the bullet:
Plus/minus is the std deviation from n of about 4 readings at each level.
COL 2.550", 177 FLP cast bullets. (about BHN 19). Chronograph is about 6-8 feet from the muzzle.
Previously collected data:
23.0 BL-C(2) --> 1576 fps +/- 26
24.0 BL-C(2) --> 1686 fps +/- 23
25.0 BL-C(2) --> 1724 fps +/- 28
26.0 BL-C(2) --> 1770 fps +/- ?
Newly collected data:
27.0 BL-C(2) --> 1849 +/- 29 fps
28.0 BL-C(s) --> 1859 +/- 31
29.0 BL-C(2) --> 1897 +/- 32 fps
I'm satisfied with about 1900 fps, and don't feel the need to go higher even though it ought to be possible to get a bit more yet out of it. This is in the ballpark for the muzzle velocities I see at Hodgdon.com for 170 grain jacketed bullets.
I also shot some rounds from these last 3 groups at 50 yards after I'd chronographed enough. I'd say they're "good enough". No tumbling or keyholing, nice clean holes in the paper, and the grouping (maybe 3-4" at 50 yards) was about as good as I usually do with the 336's iron sights anyway.
No signs of leading at all using this cream of wheat stuff.
Thanks for all of the advice and feedback! I don't know how much I'll actually use this, but I feel some sense of accomplishment at having worked up to this and now I can mostly plink with my 30-30 as much as I want to without having to buy bullets.