The 12" accuracy is at only 50 yards!
First some details.
AR 16" barrel slugged .457, 1 in 14" twist.
Lee 457-340F, sized .459, Powder Coating (from smoke) and sized again to .4585. 13.7 BHN.
Requires OAL less than 2.030 to reliably chamber.
First function and chrono test, 5 of each (based on Quickload for estimates):
28gr 4227, OAL of 2.020, Lee 342gr
1492fps, pf=509, 1696 ft/lbs, ES-79, SD-31
30gr 4227, OAL of 2.025, Lee 342gr
1617 fps, pf=552, 1990 ft/lbs, ES-82, SD-34
32gr 4227, OAL of 2.020, Lee 342gr
1676 fps, pf=568, 2110 ft/lbs, ES-91, SD-34
The 28gr 4227 had 2 FTE, so pushing below min required with this load. The rest tested fine, zero leading. Zero accuracy.
Next test to get some starting point accuracy, 5 test loads from 29gr to 31gr in 1/2 gr increments, 10 rounds each.
All cycled and brass looked great. Zero leading.
At 50 yards, scoped on bi-pod, they all seemed about the same 8" to 12+" accuracy.
Have read 458socom forum, which does not seem too keen on cast boolits, especially when PC'd, and very few posts on subject. Some say never could get accuracy with lead unless gas checked. I can't see how that could make this much difference. On a somewhat related note, switched from GC S&W 500 to PC and accuracy "increased" 4 fold with Powder coating and no gas check, so I hate to go back to old ways now that I've seen the light. 300 Blackout here has only ever seen powder coated without GC, and sub moa with 155gr Lee at 1850fps results every time.
Could Gas Checks really improve accuracy at 50 yards 12+ inches to MOA? Should I try some PB gas checks (they have helped troubled guns here in the past, before I found Powder Coating)?
Or hopefully, I just picked a mold that is no good for this, and simply need to get another mold, which would be ________? NOE around 350 to 425 in stock preferred.
Also haven't seen 4227 posted as a bad powder for this caliber. Have some Ramshot Enforcer to test if it would make a difference this big.
Giving up Powder Coating would be last resort.
Also have a Lee 457-450F but after seeing the results from similar profile 340F, didn't want to waste time unless it is known to work. Won't be running subsonic for at least a year or more.