Sorry this wont be as scientific as Hoot's load workups,
but initial testing 450 Bushmaster loads with 300 MP powder With 300 gr XTP bullets were not inspiring.
Started getting pressure signs when things were getting interesting.
39, 40 and 41grain 300MP workups.
*Edit- 300 GRAIN HORNADY XTP MAG
POWDER: 300mp
PRIMERS CCI 450
OAL: 2.11, TAPER CRIMP JUST BELOW THE CANNELURE
Heavy STAB crimp to .475
Hoot bolt carrier lead slug modification, 20" barrel with Ross brake
temp 52 elev 118ft.
DIRTY TEST BARREL, 50 rounds fired previously from last cleaning VERY HEAVY SOOTING FROM THAT OUTING
Standard carbine buffer spring and buffer
Did not yet chronograph loads.
Groups 1.4-1.2",shrinking as the loads went up, unable to specifically measure first group as it shot very low right and spread between adjacent paper targets.
No sooting noted.
Case head Swipe beginning at 40grains, and unusable burred brass at 41 gr.
Primers looked normal at 40grain, flattened at 41gr. Stopped shooting due to pressure signs,
No blown primers.
Left to Right, 39, 40, 41 grain 300MP.
Thoughts,
CCI 450's are definitely a hot primer, I might need to back down to WSR or 400's. Ignition of powder shouldnt be a problem, all things kept same.
I seemed to have Dwell time issues in my rifle 300grainers need a lot of tuning to make them just right, considering to move the gas port forward.
Recoil quite stout IMO compared to 296 or 4227 with 300 grain projectiles. Cheek slap, shooting multiple group will induce flinch.
Stock AR trigger and carbine Lower setup not optimal for tight clusters
.