.450 build not cycling

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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.450 build not cycling

Postby joegun53 » Thu Sep 13, 2018 3:25 pm

Hey guys, I know this has been beat to death but I want to visit it again. I have built 8 5.56 ar15's with no issues but my .450 is giving me some problems. first I'll tell you what I have and then what's happening.
I have a Sota upper and lower, Sota bolt and carrier. 18" bear creek barrel, mid length with a Sota low profile gas block. Leapers A2 stock with the buffer and spring that came with it, I think it's H3 about 5 oz.I had some early issues with the mags but with the help of this forum I modified them so they are working now. The gun cycles fine manually
with 5 rounds in the modified 20 round mags. I went out yesterday and shot 13 rounds and not 1 round loaded correctly. All ejected with no issues. I took a movie with my phone and it appears to me the bolt is not cycling completely. Like maybe a full inch from going completely back.
I took it apart today and checked the gas hole in the barrel .088. The gas block is aligned properly. The bolt carrier is free with no signs of binding anywhere. I'm thinking of going to an H2 buffer. I don't really want any more recoil but I want it to cycle properly. The buffer is not hitting the stock screws just to eliminate that.
Any help would be greatly appreciated.
Thanks, Joe
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Re: .450 build not cycling

Postby Hoot » Thu Sep 13, 2018 7:23 pm

Welcome Aboard Joe!
I'm by no means an expert on gas systems but I did make a Rifle Length system cycle 200gr FTX bullets. .088 sounds a little small for a 450b using a midlength gas system. I would have expected around .096. Remember, the 450b is running much lower pressure than a 5.56 does. That having been said, there are many other factors that affect the bolt going all the way back. An easy test is to chamber 1 round and insert you empty magazine. Does it lock open after the spent round is ejected and where does that round fall relative to your sight line with the muzzle being zero degrees and the butt being 180 degrees. That is not a firm indication of whether you have enough gas but its a starting point. About how far into your gas key does the gas tube go with the action closed and does the gas tube rub against the key or is it relatively centered? This came up recently in another thread but are you certain your gas block's hole is centered on the barrel's gas port? If you take it off and examine it, you should see the gas stain all around your block's hole, not just off to one side. I probably should have asked this first but are you running the action really wet with lubricant? That really helps it cycle until the abrading surfaces get polished. IE more important when the rifle is new and less once it gets broke in. Can you feel the recoil spring dragging as you pull the charging handle back? I like to put a little powdered graphite down into the tube. HBN, MoS or PTFE powders work just as well.

Adequate Cycling is a gem of many facets, not just gas volume.

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Re: .450 build not cycling

Postby Bmt85 » Fri Sep 14, 2018 5:22 am

What Hoot said, especially running the bcg wet. I’d also like to add a couple questions:
- do you have any other buffer weights to try?
- when manually unloading rounds from your mag, do you feel a lot of resistance, and how scared up is the brass afterwards?
- when you manually cycle rounds in the rifle, does it look like the rounds are slamming into the upper receiver or barrel extension?

I know you said you’ve built multiple 5.56 uppers, so that gives you a good knowledge of the rifle. However, from my experience, most 5.56 barrels are over gassed, especially the bargain type ones. It really has to do with types of ammo available, there can be quite a fluctuation in operating pressures for different ammo. To have less issues, manufacturers will run bigger gas ports. In other words, don’t start freaking out, you’ll get it running. Lol
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Re: .450 build not cycling

Postby joegun53 » Fri Sep 14, 2018 11:24 am

Thanks Hoot and Bmt85 for the quick replies. I'll try and answer your questions in order. I could drill the barrel out to .96 first, that would be easy. Now, as far the the bolt question. On the last round fired it lockes open on the carrier NOT the bolt. It did it every time. I could see it in the movie I took, the bolt carrier did not clear the rear of the ejection port. Out of thirteen rounds fired I think 3 or 4 hit the ground around 4 o'clock 6 ft. away. The other 10 landed on the bench 10 inches from the ejection port. I checked the gas block yesterday and doing some measurements with my caliper it appears to be properly centered over the gas hole. The gas tube had some carbon where it enters the block and wasn't a super tight fit. I had the BCG slopping in oil, it was spraying out with every shot. I will have to take a look at the gas tube alignment in the carrier again but it seemed good. The bolt cycles very smoothly with no obvious hangups. The rounds go in smooth when doing it manually the brass are relatively free of scares except for some mag scratches. I have 3 other .223 Ar15. 1 RR and 2 with Omega adjustable stocks. I'll check and see what buffers they have in them. I might take this one apart and substitute some lighter plugs. I have a lathe. Maybe lower the weight by 1/2 or 1 oz. I forgot to mention that I'm shooting Hornady Black 250gr. I have reloaded for over 20 years and was hoping this sight in would be quick and easy but not looking that way. It's not like I'm going to be shooting this a bunch. I am about ready to pop for the dies, powder and bullets though.
Thanks again guys.
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Re: .450 build not cycling

Postby Bmt85 » Fri Sep 14, 2018 1:49 pm

Well, I say, throw a little loctite in the gas block where the gas tube slides in. Other than that I’d say try a H buffer or standard weight buffer, but it looks like it is undergassed. I personally wouldn’t waste my time on a lathe just to change the weight of a buffer for testing. If you have a lighter one, try that, if not, oh well. I think your going to be opening the gas port anyway.
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Re: .450 build not cycling

Postby joegun53 » Fri Sep 14, 2018 3:00 pm

Thanks Bmt85,
I have 2 other Ar's with Omega adjustable stocks. They have 3.0 oz. carbine length buffers so they won't work with the A2 stock on the .450. The RR Ar has the same 5.3 oz. rifle length buffer that the .450 has. I haven't been able to find a lighter rifle length buffer online yet. I'll drill out the gas hole first and try it. Don't want to go too far the other way by doing both.
Thanks again, Joe
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Re: .450 build not cycling

Postby Hoot » Fri Sep 14, 2018 3:24 pm

joegun53 wrote:Thanks Bmt85,
...snip...The RR Ar has the same 5.3 oz. rifle length buffer that the .450 has. I haven't been able to find a lighter rifle length buffer online yet. I'll drill out the gas hole first and try it. Don't want to go too far the other way by doing both.
Thanks again, Joe


You shouldn't have to go lighter than a standard A2 Rifle buffer. My first lower was an A2 RRA and it cycled just fine with my bone stock Bushmaster 20" 450b. Admittedly, it was running a carbine length gas system so that's apples to oranges but the point is that the RRA lower is a good choice, regardless of which gas system you use. Now you have me leaning more toward insufficient gas port size given your other observations. Sounds like you did your homework and are comfortable working with your rifles. Walk that port size up using #s 43,42 and finally 41 bits, then retest. Still doesn't hurt to coax a little powdered graphite or equivalent into the receiver extension IMHO.

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Re: .450 build not cycling

Postby Hopalong » Fri Sep 14, 2018 7:52 pm

Hoot wrote:
joegun53 wrote:...snip...Still doesn't hurt to coax a little powdered graphite or equivalent into the receiver extension IMHO.
Hoot


What about something like Hornady One Shot, dry lube in a spray form, should work it's way back in there?
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Re: .450 build not cycling

Postby Hoot » Sat Sep 15, 2018 6:10 am

Hopalong wrote:
Hoot wrote:
joegun53 wrote:...snip...Still doesn't hurt to coax a little powdered graphite or equivalent into the receiver extension IMHO.
Hoot


What about something like Hornady One Shot, dry lube in a spray form, should work it's way back in there?


I wouldn't use anything wet as air is getting sucked into and blown out of the tube, increasing the risk of dust sticking to a wet surface.
That brings up a good point. Make sure the stock screw allows the air to escape. If you don't, you may be creating a pneumatic shock absorber. At the speed you hand cycle you may not notice it but at high speed compression, you may.

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Re: .450 build not cycling

Postby Talon_ » Sat Sep 15, 2018 7:57 am

I would put that upper on one of your other lowers before drilling the gas port just to see if it would work.
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