Building an AR 450 Bushmaster

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Re: Building an AR 450 Bushmaster

Postby Hoot » Thu Apr 12, 2018 9:20 am

Smithjdsr wrote:Got this one

http://smileysarmory.com/product/18-450 ... ss-barrel/

must have been the last one. I hope stainless is good. A hunting rifle probably won’t be shot as many rounds, so cleaning may be less of an issue, but stainless should be nice for field application. (Unless I decide I love Bushy, get some dies and start shooting it more...) Next step: set up the mill and open up the port on that upper...


That'll get it done! Hard to beat an SS barrel.The picture looks like the barrel extension is matte blued or Manganese Phosphated. Might as well go for the least expensive 450b BCG as it'll need some break in anyway. A slippery against a rough surface, the slippery suffers. There's still the benefit of ease of cleaning though. Not to sound like "Debbie Downer". I'd still get the Fail Zero BCG. You'll love it in the end!

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Re: Building an AR 450 Bushmaster

Postby Bmt85 » Thu Apr 12, 2018 9:53 am

To distinguish between the two, ty using BCG for Bolt Carrier Group. Lol

As to the stainless, you should be fine. Melonite will last longer, but at the speeds and pressures we are normally seeing with this cartridge, it should take a long time before it's shot out. In my opinion, if I was going to purchase one of BCA's barrels, I wouldn't do Melonite. I haven't heard amazing things about them. With an uncoated/untreated SS or 4140/4150, you can smooth them out a bit if they are rough. When Melonite treated, you really can't, it's basically locked in. Besides, you will probably end up building another one before it's shot out anyway, a few of us around here can atest to that . ;)
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Re: Building an AR 450 Bushmaster

Postby Smithjdsr » Thu Apr 12, 2018 8:46 pm

So I’m looking at the diagram of the enlarged ejection port on the first page of this thread. What is the measurement from top to botton? As I understand it, the top can stay where it is and the bottom is what gets miled out. The angle is easy enough and the length looks the same. With a good X-Y vice I can get a smooth cut along of the bottom but I need to know how wide to make it.
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Re: Building an AR 450 Bushmaster

Postby Hoot » Fri Apr 13, 2018 4:59 am

Smithjdsr wrote:So I’m looking at the diagram of the enlarged ejection port on the first page of this thread. What is the measurement from top to botton? As I understand it, the top can stay where it is and the bottom is what gets miled out. The angle is easy enough and the length looks the same. With a good X-Y vice I can get a smooth cut along of the bottom but I need to know how wide to make it.


Two things. In the original thread back about 7 years ago, I realized that the image with the dimensions I measured is upside down. The dimensions are still valid, but the image is inverted. The other thing is; I have seen upper receivers specifically made by 3rd party vendors, for large calibers like the 450b and they only mill out the front 1/2 to 2/3 of the opening. Think about how the case emerges as the BCG travels rearward and an empty case pops out just behind the front of the ejection port. As it travels sideways out of the receiver, it rotates like a whirly bird, with the rear half passing out through the same forward portion of the port as the front emerges. That might support only milling the front half wider.

That being said, consider that Bushmaster could have done the same but chose to mill out the entire port length. That may wind up being your best approach as the cases dont rotate so quickly with hand ejection to clear the chamber of a live round. My vote would be to open up the entire length between the radiused corners.

We have a search engine as well as a Google Smart Search. There are many threads about opening up the port.

Reading is time well spent.

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Re: Building an AR 450 Bushmaster

Postby Smithjdsr » Sat Apr 14, 2018 12:33 pm

So far, so good! A question about the gas block? I searched here and someone asked, but I didn’t see a clear answer. Do I need an adjustable gas block? I’ve used M4 style before but open sights on this one will be strictly backup, so I’ll use a low profile. I will use a Matrix handguard https://matrixarms.com/products/foxtrot-handguard (I like the non-indexing barrel nut). The barrel was listed as mid-length gas system. I assume included paperwork will specify size for the gas block? But does it need to be adjustable? I don’t have dies -yet- but chances are...
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Re: Building an AR 450 Bushmaster

Postby Bmt85 » Sat Apr 14, 2018 6:34 pm

You don’t need an adjustable gas block. You can if want, but not needed. For my 450 builds I went the route of heavier buffers (Slash’s XH) so nothing to really tune.

As far as BCA’s gas block diameter, I looked at there site and didn’t see anything specified, but their other 450B barrels and uppers are spec’d .750. So I assume it will also be .750, but just to be safe, wait until it comes in and measure it.

I have a couple of the Matrix handguards, and honestly they are a really good option for inexpensive. Yes, they don’t need indexing, but it doesn’t have a way to stop it from turning or shifting. I haven’t had one move on me, but I haven’t put one on a 450B either. Lol
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Re: Building an AR 450 Bushmaster

Postby Smithjdsr » Sun Apr 15, 2018 10:29 am

Bmt85 wrote:I have a couple of the Matrix handguards, and honestly they are a really good option for inexpensive. Yes, they don’t need indexing, but it doesn’t have a way to stop it from turning or shifting. I haven’t had one move on me, but I haven’t put one on a 450B either. Lol


Is blue or purple Loctite ok for a barrel nut, or is it a no-no? Also, I’m just realizing that my torque wrench is the old “needle displacement” style (my buddy with a click-stop had the nerve to move and take it with him). Mine seems to work fine for a lawn mower head gasget, but do I need to go get a newer one?

Confession: I “Jim-Bobbed” the ejection port. I won’t effect function, but not so pretty. I’m gonna build it, since this one is a budget model anyway. I have another upper, but if (when) I like the Bushy, I’ll buy one already sized correctly and put that 20” fluted barrel on it.
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Re: Building an AR 450 Bushmaster

Postby Bmt85 » Sun Apr 15, 2018 2:57 pm

Spec'd torque for an AR barrel nut is something like 30-80 lb/ft. Most shoot for a round 40-50. As long as your needle type torque wrench is semi-accurate, it shouldn't be an issue. To be honest, my cheap build's all use the Matrix handguards, and I haven't used a torque wrench on them. I first clean the barrel nut with brake cleaner and a rag. Then I install it, torque it a bit, remove it, wipe the threads of the receiver and nut, install it, torque it a bit more, remove, wipe, then do the final torque down. You don't need to crank the crap out of it. Also don't torque the crap out of the screws, it's only aluminum. Torque one a bit, then move to the next, when you come back to the first one, apply a little more torque.

As to loctite, I haven't used it and honestly don't think it's going to do much, except make a mess. If you really wanted to, I guess you could, but I would try it without at first.

I opened up the port on a forged upper for my Tromix build, was doing a semi lightweight build. Used a dremel. It's not perfect, but actually looks pretty good, and functions just fine. My last two 450B builds I decided to go with side charging uppers. I like them a lot.
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Re: Building an AR 450 Bushmaster

Postby Hoot » Sun Apr 15, 2018 8:53 pm

I would never loctite a barrel nut. Quite the contrary. I put copper bearing anti-seize on them and use the tighten/loosen, repeat, to get them to >30 <80 ft lbs, depending upon where the gas tube hole aligns. Never had one come loose and that included my 300 WSSM AR 15 upper. That thing made a 450b seem like a 6.5 Grendel by comparison. Great for clearing out the shooting benches on either side of me. :o

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Re: Building an AR 450 Bushmaster

Postby Smithjdsr » Tue May 01, 2018 10:11 pm

Image

Handguard showed up...
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