450 bm FTE on every shot.. buffer weight, under gassed?

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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450 bm FTE on every shot.. buffer weight, under gassed?

Postby alclark2 » Thu Nov 10, 2016 8:38 am

450 BM .. every shot FTE

I did several tests this weekend on my .450 bushmaster. As the title states, every shot was a failure to extract. I took several pictures to show what's going on. I've built a few ARs but diagnosing issues like this isn't exactly my forte. I'll start with what I did...
I cleaned the barrel thoroughly
I left the bolt very wet with Rem oil
I used a Franklin armory and a bushmaster mag
I used both Remington green tips and Hornaday factory loads

Here are some pictures of the ammo and mags

450 bm fte issues by [url=https://www.flickr.com/photos/130728319@N05/]

450 bm fte issues by [url=https://www.flickr.com/photos/130728319@N05/]

450 bm fte issues by
450 bm fte issues by Aaron Clark, on Flickr
450 bm fte issues
And a picture of my 100 yard setup just for fun.

I have heard several things... I may need an adjustable gas block. Also/or, I may need to change the weight of the buffer tube.


UPDATE


Just to elaborate... The brass is staying in the chamber. It will eject properly when cycled by hand.

We took a video of the action... We now see that the bolt is not coming back far enough.

https://youtu.be/_O1xVAGVcpw

I took the buffer and spring out of 4 ARs last night.

450 BM 120 grams + (Spring is 1/2-3/4" longer than all of the other ARS)
Sig 84 grams
PSA 87 grams
Unknown 87 grams

I shot the rifle with the most flexible spring and lightest buffer from the Sig AR. We did get enough action to get the brass to stovepipe once.

I looked down the gas tube and barrel with my LED flash light and I can see its transmitting a very clear ring of light. It appears to be wide open.
There is plenty of fouling on the tube. The rifle is definitely flowing. The next step in the gas department is to disassemble and inspect the gas block. We thought there could be a potential leak at that point. This would allow us to check the alignment of the holes. Also, we thought about running a chrono to see what the ammo is actually doing.

The rings on the bolt are misaligned properly. That didn't seem to be an issue.

I've emailed Franklin Armory to ask about any issues with these uppers.

Any help here would be greatly appreciated.
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Re: 450 bm FTE on every shot.. buffer weight, under gassed?

Postby Hoot » Thu Nov 10, 2016 1:05 pm

Welcome aboard Al.

In the video, it look like it's get some gas, just not enough to get the bolt all the way back. The fact that you can hand cycle the spent round tells you what you already probably have determined. The spent cases are not seized in the chamber, though they some times do. With incomplete extraction, the expanded cases can some times jam back into the chamber on the forward stroke. I have an upper built with a Franklin barrel. From the factory, the barrel would not fully extract either. I found the gas sampling hole had to be enlarged slightly to get it to cycle reliably. I must stress that I did not buy a complete upper, just the barrel. That having been said, I'd be po'd if I bought a complete upper they didn't test it at the factory before they shipped it.

Somethings to try: Chamber a round, remove the mag and see if it extracts when fired. If it does and you have one, try it on a known-good, different lower. It should run like a top on a regular rifle spring and buffer. I have no personal experience with telescopic lowers however.

Hoot
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Re: 450 bm FTE on every shot.. buffer weight, under gassed?

Postby Al in Mi » Thu Nov 10, 2016 2:21 pm

looks under gassed to me, might need to open the gas port up.
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Re: 450 bm FTE on every shot.. buffer weight, under gassed?

Postby plant_one » Fri Nov 11, 2016 1:44 am

i would check to make sure your gas block is aligned properly. not hard for one to get turned a bit and cause a partial blockage. get a peice of 1/8" tubing (airline for aquariums) and put it on the gas tube and blow through it. It should blow freely.

just for reference a spring for an adjustable stock should have 34 coils. a spring for a non adjustable (A2 style fixed stock) should have 42 coils. dont worry about length when it comes to springs, easier to identify by coil count and wire diameter.


here's a good chart that shows the various buffer and spring configurations (weight's and lengths, etc) for ar's
Image


yours sounds like a "h2" buffer. nothing a 450bm should not be able to move when other stuff is working right. some of us (myself included) are using a lead slug glued into our BCG that adds as much weight as your buffer has (approx) on top of a standard buffer in the tube, thanks to Hoot's CCWS (cheap carrier weight system) so that's probably not your problem.



if its not cycling with the SIG spring and buffer, its almost surely a gas related problem though. start there :)
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