I had a day off and ran some experiments with 300 blk cast loads.
Mold - Lyman 2660410 , WW + tin, sized 0.309", HF Flat Black PC, sized 0.309
Weapon - AAC MPW SBR (9")
Powder -IMR 4227
COL 1.822
Section 1: Speed testing (~4 shot averages)
14 Gr. 1424 fps
15 Gr. 1536
16 Gr. 1659
comments: speeds were consistent, no evidence of leading
Step 2 - accuracy testing
50 yards
poor "groups" at all above loads
Step 3
downloading
12 gr. (cut holes, won't cycle)
13 gr. (cycles good, locks bolt back on last round-group opening up)
12.5 gr. (cycles good, bolt not locked on last round, group intermediate to 12 & 13 gr. 3 shot -1.25")
Conclusions:
accuracy load is closer to 12 gr, however not cycling - wish I had a bolt to check it - Ruger Ranch.....or a can.
It was getting dark when I finished, so I will resume testing at 12.5, not too worried about bolt not locking back for bench work.
13 grains will likely be my goto for close work on the steel targets
lighter loads are tighter groups
nice to see that the hotter loads did not lead
it will be a long time before I cut holes @ 100 yards with this boolit...
I'm thinking about flaring the cases a little and trying some sized, coated, loaded - someone wrote that it tightened groups.
I will chrony the light loads.
Final conclusion...SBR is fun! Need massive supply of WW....