9MAR21 EDIT: Everything below until the middle of page 4 I will leave for educational or nostalgic reasons, however, the Lee 459-500 bullet is NOT good for the 450 Bushmaster. Skip to page 4 and beyond for a bullet that actual works, the Lee C457-500.
Or continue below to see my trial, error, and eventual success. Thanks for reading.
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Having fired everything between 230-300 grain in my 450BM, I figured it was time to try out what the SOCOM fanboys are always raving about. Through seemingly endless internet searching, I could not find a single source document or forum about shooting heavy bullets in the 450BM that didn't end in "The 450BM was designed for lighter, faster loads" which always seemed to be a conversation killer. That being said, I've got my steel toe boots on, let's go bowling!
The gun:
18" AR15 GLFA barrel, mid gas, 1:24 twist, (90% confident of a .451 bore size)
Standard carbine spring
H2 Buffer
FA profile BCG
The bullet:
Lee 459-500-3R 2 cavity aluminum mold
Dropped at 490 grains with straight wheel weights, air cooled, .459
Loaded to 2.3 in twice fired Starline brass. Case mouths required a good bit of flare. Seating depth is required to be long due to the very narrow taper of this bullet, which is designed for the 45/70. This also does not allow us to get a very good taper crimp as the case mouth with the bullet seated at 2.3 is not on a portion of the bullet that measures at .452. My standard crimp is .474", however, I believe these were crimped to .470" and was still not a great grip on the bullet. This bullet requires you to have a magazine that will allow for an COAL of 2.3". Seating any deeper will further reduce the effectiveness of the crimp. I am not set up to use a side-crimp, however, I bet it would be extremely useful here.
The process:
I currently only own a .451 sizer die, so I elected to size first, then powder coat. The sizing process required a layer of Hornady Unique lube, and did require some force. I wiped off the lube and had to mildly sand the bullet base as going from .459 to .451 left a small extruded ring of lead at the base. They were powder coated using standard methods and measured at .452-.453 after PC. I am extremely new to casting and powder coating, so I can say for certain there are probably other factors I left out such as water quenching, age hardening etc.
The good stuff
I fired three groups of three: 21, 22, and 23 grains 1680 after my very first shot at 19gr gave me 807 FPS. I started with a clean bore and did not use a fouler shot.
Group 1: 21gr 1680
1: 994 - case stuck halfway extracted
2: 943 - no extraction, bolt full forward
3: 943 - no extraction, bolt full forward
AMV: 962 FPS
The first two essentially hit the same hole.
Group 2: 22 gr 1680
4: 1059 - case stuck halfway extracted
5: 980 - no extraction, bolt full forward
6: ERROR - case stuck halfway extracted
AMV: 1022 FPS
Tightest group, impact shifted .15" inches up from 21 gr load
Group 3: 23 gr 1680
7: 1188 - case stuck halfway extracted
8: 1111 - case stuck halfway extracted
9: 1123 - case stuck halfway extracted
AMV: 1143
Impact shifted .8" up from 22 gr load. Slightly above subsonic speeds.
Conclusions: I'm not too worried about the rifle not cycling as I have a decent bit of mass in the bolt/buffer that I can remove to help cycle. I have to do this to run 225gr subs in my 7.62x39, so I'll try that out next time. I need to get a .452 sizing die. I think that will help increase the pressure and bit and help with cycling. We'll see if I change my mind on sizing pre powder coat...I probably won't due to the lead movement. I'll probably try one and just melt it down if it doesn't work. I'm going to stick with the 23 gr load and continue to mess with the other variables. In about a year when my suppressor comes in, I may have this load down pat. Let me know your thoughts and I'll keep making adjustments until we get it right. Other powders on hand: LilGun, H110, SWBO
Ryan