.450 build not cycling

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Re: .450 build not cycling

Postby Hoot » Thu Sep 20, 2018 4:56 am

Al in Mi wrote:might want to bump that load up to about 3gr, 38-39gr seems to be a sweet spot for most guys plus it will get you some extra gas.


What he said!

If using lighter loads exacerbates the problem, that really points toward your original assertion regarding gas volume. And that having been said, using reloads to troubleshoot a cycling problem, introduces another variable to the equation. Not that your weapon should limit you to only using factory ammo, but lets face it, reloads, even using best engineering practices, are a variable. However, if you want to take the test to its ultimate conclusion and I don't mean that in a destructive way, try some loads that are hotter than factory and see if the pendulum swings the other way. You can exceed the "lawyer approved" factory loads without stepping into dangerous territory. Go a little over the line, don't floor it. My experience with my AR 450b is that the current blend of Lil Gun pushing 250gr FTX, hits factory rated velocity at 37gr. I have also shot 38gr loads and though they exceeded factory ammo velocity, they didn't exhibit excessive case head growth (best pressure metric). A properly tuned 450b AR will be very forgiving of ammo choice with regard to cycling reliability. I know. I have experimented with countless combinations of bullets and powders over the years.

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Re: .450 build not cycling

Postby plant_one » Thu Sep 20, 2018 9:22 am

i'd definatly want to tear that upper down and check the gas block alignment and look for any signs of gas leakage. they're easy to find as they leave one helluva carbon signature. If you've got carbon anywhere around your gas block/tube - there's your problem and would explain your short stroking. If you cant find any, and it fails to cycle on multiple lowers - before you get the drill out - CONTACT BEAR CREEK! if you touch it with the drill bit you're pretty much giving up any warranty coverage you might have had.

if not pull the gas block off and check the carbon ring around the port. if the port isnt fully surrounded by the carbon ring then you've got an alignment problem. a "typical" gas block should be just forward of the journal shoulder. if the barrel has a dimple it should line up with the rear set screw on the block.



this is a pic of a barrel thats leaking (from the web) you can see the carbon tracks
Image

similary this is a leak around the gas tube itself

Image

this is one of my origional 300blk upper where i had a block that was leaking - you'll note that necessary gap i was talking about above to get the port aligned right. this is to allow for the metal that holds a traditional "mil spec" style forend up against the block.
Image


this is what the carbon ring under a gas block around the port should look like - this one is almost perfectly centered
Image

here is the carbon ring from a poorly aligned one - and even though this looks like its close to being ok - it was in fact causing short stroking. so you dont need to be off by much. I've seen them bad enough that the gas port is cut in half by the carbon ring before, but my google-fu is failing me today in finding a worse pic than this.
Image



I would also get your hands on a high quality magazine - like a bartz/brenton. if you cant get it to feed right with a bartz mag and factory ammo... send that barrel back to BCA for a QC check. running modified mags when you're having feed problems just adds one more possible complication to things.

hth
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Re: .450 build not cycling

Postby plant_one » Thu Sep 20, 2018 9:23 am

FWIW - any pictues you can share with us will likely be IMMENSLY helpful. even screenshots from the vid you say you have.
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Re: .450 build not cycling

Postby Hoot » Thu Sep 20, 2018 10:07 am

plant_one wrote:...snip...
this is a pic of a barrel thats leaking (from the web) you can see the carbon tracks
...snip...hth


The images present better on 17" or larger monitors using Firefox. Good visuals!

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Re: .450 build not cycling

Postby plant_one » Fri Sep 21, 2018 10:23 am

ya sorry about the image size.. i have very little control over it when pulling them down from google.

just trying to provide as much info as i can to help sort this out - i'm intrigued at this point as we seem to have narrowed things down.



my buddy just picked up a18" BCA with the mid gas and its running like a raped ape for him with factory ammo. his only hiccup was some scarring on the case from a burr on his test mag.
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Re: .450 build not cycling

Postby joegun53 » Fri Sep 21, 2018 10:51 am

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Thanks again everybody. I tore the gun back down and took some pictures. I'll post them below. It looks like I have a small leak where the tube enters the gas block. Don't know if it's enough to matter?? The block to barrel alignment looks good and the holes line up. I don't think I have a mag problem because it loads a new round everytime I cycle it manually but never once has it picked a round up in firing. Hoot, I know I under loaded the reloads but I have been reloading for 25 years and have never or never will start at max or above. I'm still convinced it's under gassed just by the fact things got worse. I will load some up with 38 gr. and see what happens. I just checked BCA's website and it says they inspect barrels 100%. I'm still going to call them and see what they have to say. I got the barrel on sale for $67, so it's not the end of the world if I mess it up. I put a link below for 2 videos I put on you tube. You can see the bolt is not going completely rearward. the videos were taken last week with factory ammo, the pictures were taken yesterday. I'm about ready to fire the drill press up and see what happens.

https://www.youtube.com/watch?v=VxD9yMi ... video_user
https://www.youtube.com/watch?v=nOf6Xr2 ... video_user
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Re: .450 build not cycling

Postby plant_one » Fri Sep 21, 2018 11:11 am

your alignment looks great!

however - thats not a small leak - thats a MASSIVE leak. the tube is minor, but the gas block leak itself is big enough that you're gonna need to scrub that carbon ring off the barrel before you install your new block.

once you get them clean take your calipers and measure the ID of the gas block and the OD of the gas block journal. i'm just curious which is out of spec - even though its almost guaranteed its the gas block itself.
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Re: .450 build not cycling

Postby Hoot » Fri Sep 21, 2018 11:28 am

plant_one wrote:your alignment looks great!

however - thats not a small leak - thats a MASSIVE leak. the tube is minor, but the gas block leak itself is big enough that you're gonna need to scrub that carbon ring off the barrel before you install your new block.

once you get them clean take your calipers and measure the ID of the gas block and the OD of the gas block journal. i'm just curious which is out of spec - even though its almost guaranteed its the gas block itself.


Looks more like a rust ring to me and not out of the question on CrMo barrels. Still, if it were mine, I'd thoroughly clean both mating surfaces and bed them with JB weld. However, if that doesn't solve your problem and you still want to pursue opening the gas port, you will have a fight getting the block off. Nothing that some directed heat and persistence can't overcome. I've had people at the range watch the gas block while I shot to get their opinion of how much is escaping and despite them seeing some poof, the rig still cycled. My 30 RAR leaks like a sieve but still cycles reliably. Its on my list of to-do's this winter for bedding.

plant_one is also correct in determining what is out of spec. Good gas blocks are not cost prohibitive. I took my Bartz upper apart to send the barrel in for muzzle threading. The gas block was so closely toleranced that sliding it off was like slicing through a big log of summer sausage. No way you could rush it and it wouldn't fall off on its own. That's how your gas block should fit your barrel.

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Re: .450 build not cycling

Postby joegun53 » Fri Sep 21, 2018 11:47 am

The gas block is not that tight on the barrel and the tube is fairly loose in the hole as you can see by the carbon. I will measure the 2 and see what clearance is there. I just got off the phone with Bear Creek Arsenal and he suggested I drill the gas hole out in small increments up to .097 thou (shooting it along the way) and that should help. I might try a different gas block first.
Thanks everybody!
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Re: .450 build not cycling

Postby joegun53 » Fri Sep 21, 2018 1:46 pm

I think I will try a different gas block. there's 4-5 thou. slop in the barrel (.748 barrel vs. .753 gas block) and 3 thou. on the tube vs. hole in block. Probably too much.
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