Seems like nobody has been talking about cast bullets in the 450 for awhile. Been shooting cast bullets for almost 60 years and they can be tricky little devils, but usually in smaller bores. From 35 caliber on up, I have never had much problem getting rifles to hold 2 moa or better @100 with reasonable loads favoring bullets on the heavy side. Handguns can always equally factory loads at 50 yards or less.
Being a late comer to ARs, before I knew it I had two 15s and one 10. 2 223s, 1 300 Blackout (upper) and a 7.62 NATO. What was missing was a 450 BM.
Finally found one (upper) NIB in of all places Cabela's.
Slapped it my R15 lower (also Bushmaster) with a $200 1-4X "tactical" scope. With a box of gold plated factory ammo, off to the range. Once "on the paper" and cooled off, the first 3 cover leafed and the next 5 went in 1.5".
A set of dies, a 300 gr Lee mold and gas checks were quickly ordered.
I have tried everything I've learned about cast bullets. Sized .451,2,3, 4198, 4227, H110, Lil Gun, plain gas checks, crimp on ones, SAECO lube. LPG. The same bullet will shoot twice as good @50 yards from my 1926 Colt New Sevice Officer's Match ! 5" 5 shot groups @50 yards is the best I've done in the 450.
As the 450BM is almost a ballistic twin to the 460S&W, it's interesting to note that my 10" S&W will shoot the same bullet with the same loads into 1.5"@ 50 yards.
I've seen some discussion about blaming the gas port but after 1000s of cast bullets through an M-1 and several 100 thru a Merkel gas operated 9.3x62 rifle , I tend to doubt that.
I have an NEI 4 cavity 300 gr Keith design plain base which will be the next try.
Anybody find the "magic bullet" yet ?