Ok, I took my newly built corvette rig out today,
Setup: Hoot's old Trident barrel freefloated, New trident bolt, Hoots CCW system, Vltor MUR upper, and a Jard 1.5# trigger, Magpul UBR stock (carbine buffer system), otherwise stock setup.
Wanted to test out the far end of heavier projectiles with the fast twist (1:16) for the first time out.
shooting the 405g Missouris Hard cast, Rated Brinell 18, .458 resized to .452. Weighs 412.1 g with the gas check.
Oal 2.21, rem 7 1/2, New brass. Double crimp (both pretty heavy)
Elevation 784, temp 78 degrees
Started out with IMR 4227 . My previous shortened workup with I4227 showed me that this was still a very viable powder for this beast.
29.0 grain, speeds were ~1550fps, however I severely botched that group due to improper hold as the AR requires a firm hold to keep the gun from jumping the rest. Also, my chrono was not aligned and I did not call a cease fire to correct it. To make matters worse, the Jard Trigger pin walked out due to the recoil by the second round and messed up my rhythm figuring out why my trigger went Mushy.
29.5 grain showed better promise with a 2.24" group overall, which was my best group of the day. speeds were 1599, 1571, 1583, err1 and 1599.
30.0g opened up quite a bit, with a huge 4" or greater group.
30.5 grains, still 4" groups, speeds now in the 1640's.
Switching over to Lil Gun, Right off the bat, at 29.0 grains Lil Gun blasted out of the gates at ~1750fps and gave a 5-6" pattern, (no discernible group).
I continued with 29.5 grains and group was worse,slightly larger than 6" , all chrono loads now surpassed 1800fps, and starting to get a little case swiping but the cases looked fine otherwise.
Edit: My Stock Bushmaster barrel with 1:24 twist had ~1700 fps with ~32.5 grain of Lil Gun. A combination of factors must have driven up the pressure, most notably the increased twist I assume.
Checked for leading and found none significant.
ejection and feeding was robust with all cases, however my Vltor MUR ejection port was not yet opened up for reliable extraction
Thoughts: Me thinks the speeds were too fast for hard cast in this barrel twist, slowing them down may give more accuracy, as 45-70 loads are usually under 1500fps, and most suppliers of cast bullets would optimize for these loads.
There is likely more accuracy to be realized in resized 458 jacketed projectiles, but in reality, the exceptional cost of 458 jacketed is no longer a viable pursuit for me, as .451/.452 projectiles are more plentiful, readily available and cheaper.
Still, IMHO, a 2MOA semiauto cannon with a 405grain payload, still gives a very viable smackdown on anything that may roam God's earth.
Reviewing the loads that others have had with cast bullets in the 450, seem to be completely in line with the 458 load data, I reviewed over here. http://www.beyond556.com/bboard/forum/ammunition/handloading-reloading/big-bore-loading-data-only/4494-458-socom-load-data