I am trying to piece together a 450BM upper for hunting season and could use a little help. I have a 16” Bushmaster barrel on the way and need to know what size gas block it requires, 0.750 or larger? I understand that I’ll need a modified upper receiver and BCG for 450/458. Is there anything else that I need to be aware of before I start the build? Will a standard or heavy buffer work fine? Thanks guys!

Can’t help with the gas block, but pretty sure you need a standard buffer. My Beowulf chokes all over the place using anything heavier than that.

Get that thing running right and you will never again pick up a 12G in this state during November. I promise.

Whoever you bought the barrel from, should be able to tell you what gas block it uses.  It could be .750, .875, or .936

Tony

Thank you for the input, I've been hunting with a 12G for years, can't wait to get it dialed in.

Pretty sure you're going to be needing a .750 sized gas block. Where'd you get the Bushmaster barrel from? I would call them to ask what size the gas block journal is.

I have not seen a 450 barrel with a .936 journal, so it leaves the other 2 sizes mentioned previously.

I used a Satern barrel. It has a Mid-length gas system with a .875" inch journal. I used an SLR fixed gas block, a Tromix compensator, and Tromix modified Lancer magazines with the Tromix aluminum magazine follower. Carbine weight buffer from Aero Precision. The barrel came with a fitted bolt, and I installed that into an AIM Surplus EXO BCG I had. Shoots real nice, but I am am contemplating going to an A2 fixed stock just because I don't really need the adjustability a carbine stock has.

Do NOT go to a standard A2 stock.  For two decades whenever we have heard of someone saying the .458 SOCOM kicks like a mule and it will rattle your teeth, it always turns out to be either an extremely recoil sensitive person or the person is using the A2 stock with the standard butt plate which is a meat grinder.  If you use the A2 stock, get the Kick-Eez  recoil pad or you'll rue the day you switched.  Personally, I like the Magpul CTR stock with the Limbsaver pad or the Magpul MOE rifle stock with the extended, ..700 or .800 inch, recoil pad.  It is harder than the Kick-Eez A2 stock but it still does a better job than the standard MOE butt pad.  I wish Limbsaver or Kick-Eez made a pad for the Magpul MOE stock because I would be all over that like a duck on a June bug.

My 450B prefers a wolf extra power spring and a spikes heavy buffer.

Good advice here.  I started with an A2 stock and HATED it.  No better description than "meat grinder"....hahah, pure truth.

To clarify, I should have said A2 style....as in fixed length. I have a CTR with the extra thick butt-pad....and being a Magpul fanboy that replacement would be a MOE fixed stock.

I received the barrel today, I got a great deal on it from the EE. 16" Bushmaster barrel with 1/24 twist. I appears that I'll need a 0.750 gas block. Thanks for all the input. Ive got a few extra BCGs so I guess I'll just order a 450/458 bolt, any recommendations?

You only need to know one word for anything big bore AR related parts, TROMIX.  There is a hell of a lot more to making a big bore bolt than enlarging the bolt face and dehorning the extractor.  Get a TROMIX bolt and as long as the barrel extension is not mucked up (and it won't be with a Bushmaster barrel) you will be problem free.

+1 for Tromix

Also have big bore barrel extensions