Building an AR 450 Bushmaster

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Building an AR 450 Bushmaster

Postby Smithjdsr » Thu Jan 18, 2018 5:30 pm

I’m thinking of building an AR in 450 Bushmaster. I have successfully built AR’s in 5.56 and .223, but this would be my first big bore.

Background- this year Iowa changed its law to allow a number of straight-walled ‘pistol’ cartridges for deer hunting fired from a rifle. 450B is among the allowed chamberings. I have looked at the Ruger American Rifle, but the word is that its short barrel and flash hider make it extremely loud. At $500 or so, building an AR begins to look way more appealing- and fun.

Some questions, if I may?

My lowers will work, right?
Other parts are standard, like handguard, gas tube, gas block, barrel nut, stock, tube, spring... no need to get special parts for 450B, I think?
I have ‘full auto’ bolt carriers, so I guess I’ll just need to change the bolt itself for 450B.
Where can I get a parts list of the parts that have to be special for 450B?

About the upper receiver? I have a stripped upper. I guess I have to make some modifications? There’s no ejection port Cover, right? And I have to modify the size/shape of the ejection port? How do I do this? To what size and shape?

I don’t really even know what questions I need to be asking?
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Re: Building an AR 450 Bushmaster

Postby Hoot » Thu Jan 18, 2018 9:01 pm

Smithjdsr wrote:I’m thinking of building an AR in 450 Bushmaster. I have successfully built AR’s in 5.56 and .223, but this would be my first big bore.

Background- this year Iowa changed its law to allow a number of straight-walled ‘pistol’ cartridges for deer hunting fired from a rifle. 450B is among the allowed chamberings. I have looked at the Ruger American Rifle, but the word is that its short barrel and flash hider make it extremely loud. At $500 or so, building an AR begins to look way more appealing- and fun.

Some questions, if I may?

My lowers will work, right?
Other parts are standard, like handguard, gas tube, gas block, barrel nut, stock, tube, spring... no need to get special parts for 450B, I think?
I have ‘full auto’ bolt carriers, so I guess I’ll just need to change the bolt itself for 450B.
Where can I get a parts list of the parts that have to be special for 450B?

About the upper receiver? I have a stripped upper. I guess I have to make some modifications? There’s no ejection port Cover, right? And I have to modify the size/shape of the ejection port? How do I do this? To what size and shape?

I don’t really even know what questions I need to be asking?


Welcome Aboard JD!

The only unique parts on the 450 bushmaster are the barrel and bolt. Many manufacturers sell them as a set. The upper receiver will need an ejection port sized to pass the fatter case however. Some upper receivers come already appropriately sized and many do not. Here's the dimensions that Bushmaster actually uses as measured on my first upper.

Image

Let me back up. You will need a magazine that will handle the single stacked fatter cartridges also. A few 556 mags have been reported to work without modification, you can mod an existing mag, or you can buy one specifically made for the 450b.

IIRC, there are how-to's online showing how to open up the ejection port to the appropriate size and not have it look "bubba'd".

Bushmaster uses a standard spring and rifle buffer in their uppers. There are equivalent ones for an M4 style, adjustable stock but I don't have any experience with them. Both my 450b rigs run a standard spring and rifle buffer. A medium size, black, slip-on SIMS Limbsaver recoil pad is a big plus if you do not intend to put a muzzle brake on your rig for reasons you already stated (loud). Without a muzzle brake, you may benefit from a 20" barrel as they get the noise a little further away and the added weight reduces the perceived recoil a little. I'm biased though. I like rifles to have rifle length barrels on them as I don't kick down doors or engage in CQB for a living. ;) Did I mention the additional 100+ fps from the same ammo?

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
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Re: Building an AR 450 Bushmaster

Postby plant_one » Fri Jan 19, 2018 11:05 am

the only proprietary parts you *NEED* for a 450 bushmaster are going to be the bolt, and an upper with the proper ejection port dimensions. otherwise you're very much correct that its going to be like any other ar upper build you've done.

a standard lower will work just fine. if anything you may want to put a H2 or H3 buffer in the stock on a 6 position lower, but otherwise you wont need to do anything. It WILL run with a regular 3oz carbine buffer, but it may (or may not) be rough on your brass. not a concern if you're not gonna reload anyway. as an added bonus, in all but special circumstances (for example: a 300 blk, carbine gas, shooting subsonics, unsuppressed) a H2 or H3 buffer is going to be compatable with pretty much everything else, and will help with symptoms of overgassing in those uppers too.

perosnally i would order a stripped uppper with the proper ejection port in it. its worth the few extra bucks in my eyes for those of us who arent a machinist.


secondly and this is just personal preference, but i much prefer to have a complete BCG & charging handle for every upper i own. that way it just stays with the upper and there's no 'damn i forgot to grab my bolt carrier' things going down. Since you're buying a bolt anyway, adding a carrier doesnt jump the price that much - and as you noted a basic mil-spec "full auto" bcg will do once you swap the bolt. Bargin shop - you can pick one up for around $50 or less on sale. and then you have a spare 5.56 bolt laying around in your parts bin :)

hth and good luck with your build!




ps: yes - we expect pics when you're all said and done too! :mrgreen: :mrgreen:
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Re: Building an AR 450 Bushmaster

Postby Smithjdsr » Sat Jan 20, 2018 3:50 pm

Thank you. Good, sensible recommendations. I’m not MOA shooter, so I don’t really need ultimate precision. My longest deer shot to date is 150 yards (with a .50 muzzleloader), so I don’t claim to be a long-range shooter. I’m leaning toward the 20” barrel to push the noise a little further away and to squeeze a skosh more velocity out of it. If I had a reliable 200 yard rifle I’d be happy.

The barrels I’ve seen seem to be $199 - $300. Does that seem right? Honestly, I might have to do this on a shoestring (you may have heard sixth grade teachers aren’t excessively well compensated) but would one of these kinds of modestly priced barrels do the trick?
https://www.franklinarmory.com/products ... ide-barrel
http://www.selwayarmory.com/glfa-ar15-4 ... tride.html
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Re: Building an AR 450 Bushmaster

Postby Bmt85 » Sun Jan 21, 2018 4:42 pm

That Franklin barrel has a rifle length gas port. Which really isn't needed, especially if you don't reload.
That GLFA is a good price because it also comes with the bolt. GLFA has been brought up a few times recently, and from what I can tell, they seem good. It doesn't say if it comes with a carbine length or midlength gas system. I personally would prefer a midlength, but carbine shouldn't really be a deal breaker.
There are other options out there if you choose not to go with those. Also take a look at Gryphon Trading and Tromix, they make 18" midlength barrels, they run $300 for barrel and bolt
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