Researching 450 Bushmaster

[DMW1116]

I’m building my first upper (still) and started wondering about a big bore AR15. I have a BCM blem stripped upper. If I wanted to turn it into a 450 BM upper, what parts do I need?

A 450 BM barrel and magazines are all I know for sure. Does it have a different buffer, buffer spring, or bolt? I’ve seen some people use a different upper with larger ejection port. Did I push myself out of 450 BM territory with a standard upper?

Is an adjustable gas block necessary? I see a few subsonic ammo offerings and they seem like a nice option for versatility. I plan to reload most of my ammo. Will subsonic loads cycle the action or are they meant for the various bolt guns I’ve seen? Can FMJ 45 ACP projectiles be used or is the diameter/velocity not compatible?

 

[mavracer]

Everything related to the lower is the same I run my 14.5" 450 on the same SBR lower as my 12.5" 5.56.
The standard port is an issue you can open it up but then the door won't close.
You might look at 350 Legend also.

 

[sevt_chevelle]

You are gonna need to open up the port on that bcm upper, it's not a big deal, I've done it. Open it up right and the door closes.

The bcm upper uses an undersized extension for a thermal fit with the barrel, that's a good thing.

I have two 450 bushmaster barrels from Satern. Both are sub moa with hornady black.

The bolt carriers are the same, I use AO precesion and mircobest.

Western has load data for 230 fmj bullets.

All my AR builds get the A5 buffer system.

 

[jmorris]

Yep, ejection port is larger, I use Colt GI mags with a 3D printed follower on top of the original one and yes a 5.56 bolt won’t work.

The using .452 pistol bullets thing really didn’t work out for me, probably why I prefer the 458sc.

 

[JWF III]

I agree with all posted, but will add my experience with my newly acquired Bushmaster upper. Shoots great with Hornady Black. I gave Sub-X a try wanting to use it, hunting suppressed. (The suppressor is still in NFA jail for a few more months.) The Sub-X would not cycle the action. Gas port alignment checked out. I tried messing with the buffer. 1 weight at a time and it never would cycle. Even with an empty buffer.

I will be trying again once the suppressor is paroled. Hopefully it will add enough back pressure. But I’m not counting on it.

Wyman

 

[mcb]

As I stated in your 350 Legend thread, the 450 BM upper is different but only around the ejection port and you can do it yourself if you're handy.

In addition to the slightly modified upper the 450 Bushmaster obviousl need the barrel, bolt specific to the cartridge Magazines are optional. There are 450 BM specific magazine but they are not strictly required. 20rd 223/556 magazine will often work without modification, especially the straight 20rd magazines. I made several of my 450 Bushmaster magazines by converting NHMTG straight 20rd magazines with a 450 SOCOM single stack follower from calegalmags.com. They hold 7rd and have been very reliable.

If the barrel is ported correctly than standard carbine or rifle buffer and recoil spring should work. Increase buffer weight if desired to mitigate recoil if you upper is slightly over gassed. An adjustable gas block is only need if you upper is severely over gassed.

There are lots of good ammo available from 450 Bushmaster and its a fun cartridge to reload for.

My favorite load is the Barnes 275gr TSX, you can get that bullet for reloading and if you look around some of the smaller loaders are offering it in loaded ammo.

This is my personal 450 Bushmaster. Started with a factory 450 Bushmaster upper and an old DPMS A2 lower. It's been a very fun gun.

 

[earlthegoat2]

Though I personally have never had any luck with standard 223/5.56 mags, if @mcb says they work, I believe it. I have heard you shouldn’t use PMags or any other mag with poly feed lips.

I like the center feed modified modified Lancer mags and the C-Pruducts Duramag.

Supposedly, 450 BM is best run on carbine and mid length gas systems. Since that is all I have ever used I cannot verify this. As pictured above, Bushmaster themselves made a rifle length barreled 450 BM upper but I am unsure of how it is gassed.

Ideally, you would have a 1:16 twist or faster to run the heavy subsonic bullets as well. I believe this is why Ruger twists their 450s in 1:16 and the limited 450 SS No 1 is 1:10 twist.

I used an H1 buffer in my carbine but ever since I built this one with a rifle buffer system I am reluctant to ever go back to a carbine buffer. It may just be the whole rifle build but the rifle has much less perceived recoil impulse than I ever experienced with the carbine system. This rifle is also 1.5 lbs lighter than the carbine system gun. It’s personal preference in the end.

I like the Faxon BCG and the Tromix lightweight barrel. However you can buy a bolt and barrel from Tromix as well if you want the bolt matched up. It may or may not make a difference but some folks like that.

More recently I got a barrel custom made by McGowan so I could get ultra lightweight and fast twist.

 

[DMW1116]

While that doesn't look too daunting, I don't think I'll be modifying the ejection port. Since I don't plan to hunt with this rifle, a 450 BM seems excessive both in terms of power and cost to shoot. This comes after looking at load data and pricing some projectiles and brass. Those big 45 caliber cases eat powder and lead like a fat kid at a candy store. It's the same reason I don't have a 45-70 in an affordable single shot package. A semiautomatic would just get away from me. However, this is all good information should I change my mind in the future. I see it does require its own bolt. The build cost is about the same as anything else with the upgrades I want, like ambidextrous controls, upgraded trigger, etc., so that's not an issue.

 

[jmorris]

Though I personally have never had any luck with standard 223/5.56 mags, if @mcb says they work, I believe it. I have heard you shouldn’t use PMags or any other mag with poly feed lips.

Pmags won’t work because of the rib molded down the center, 450 and 458 are single stack right down the middle.



This is the follower I use on top of the Colt 5.56 follower for 450.

.450 Follower by ferrellm

(SCALE UP MODEL x10; saved in centimeters.) This is an insert to convert a standard metal AR magazine to accept the a .450 Bushmaster cartridge. Simply slip it in at an angle and rotate upright. This also works with the .50 Beowulf and I believe it will work with the .458 SOCOM. A standard...

www.thingiverse.com

Its unnecessary for the 458.

 

[mcb]

Pmags won’t work because of the rib molded down the center, 450 and 458 are single stack right down the middle.

View attachment 1167685

This is the follower I use on top of the Colt 5.56 follower for 450.

.450 Follower by ferrellm

(SCALE UP MODEL x10; saved in centimeters.) This is an insert to convert a standard metal AR magazine to accept the a .450 Bushmaster cartridge. Simply slip it in at an angle and rotate upright. This also works with the .50 Beowulf and I believe it will work with the .458 SOCOM. A standard...

www.thingiverse.com

Its unnecessary for the 458.

I even tried using the little 10rd mag Pmag, The 10rder is short enough and straight so I could remove the rib down the front with a good file. The problem is the feed lips and walls are not stiff enough and when loaded with 4rds of 450 Bushmaster the feed lips and in turn the side of the magazine had spread far enough to make it very difficult to insert into the gun. It would feed once inserted but it was too hard to insert to be worth the effort to convert.

 

[CraigC]

If someone would develop some load data for the Lehigh copper solids, it would be a legitimate dangerous game cartridge.