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New Member, and troubles with my Homebuilt AR


Elmer75
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Hey All,

This is my first post on the 308 forum, but I've been reading here for almost a year.

It all started with an innocent 80% AR15 project back in early 2020, which morphed into "needing more".  One thing led to another, and one day I found myself machining an 80% AR308 DPMS lower.  I knew then that I had a problem.  At the time AR parts were pretty hard to find, prices were high, and I bought what I could find.  The 308 went together pretty well, but I'm having failure to feed issues with my setup.  Here's what I have:

CBC 18" barrel with mid-length gas.  Gas port is 0.070"

CBC supplied BCG (make unknown)

5D Tactical 80% DPMS lower

Daytona Tactical DPMS upper (5D Tactical uppers weren't available at the time)

5.4 oz buffer, 5.25" long, and has a loose weight inside the buffer

308 buffer spring from ??

A2 buffer tube, 9.75" inside length

JP Enterprises adjustable gas block/FSB

Mid length gas tube that ends right in the middle of the bolt cam notch

Barrel nut tightened to 45 ft lbs

I'm firing American Marksman 147gr M80 ammo.  Groupings are consistent at 100yds.

As far as I can tell everything is set up well.  The gas tube ends where it's supposed to.  The adjustable FSB is wide open, and the gas ports are lined up right.  The gas tube is clean and clear.  The bolt key engages the gas tube well.  But the BCG barely moves after a round is fired. Once in a while it will start to eject the shell, but always hangs up in the chamber.  I haven't once had a clean feed.

My gut tells me the gas port is too small.  There is very little gas residue inside the upper or the bolt after 30 rounds.  If the problem is the gas port, I have no idea how large to make it.  I read the Forum article on Barrel Gas Port Size, but there is very little info on 18" mid-length barrels.  What little there is makes me think my gas port is too small.

I appreciate any feedback you care to offer.

 

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9 minutes ago, Elmer75 said:

It all started with an innocent 80% AR15 project back in early 2020

This is how it always starts here...   No offense, but 80s SUCK if you don't get the matching upper for them.  At any rate, I've pinpointed the first problem...

9 minutes ago, Elmer75 said:

CBC 18" barrel with mid-length gas.  Gas port is 0.070"

That gas port diameter isn't right.  For an 18" midlength gas gun, you WILL NEED 0.080"~0.085" of gas port diameter, or it's NEVER gonna run right. 

I haven't read the rest of your first post yet, to see what you're running for a recoil system, or if your gas tube is even correct length, but I'l get to that.

You need to fix the gas port diameter in that barrel, very first.  I'll check back later, after I read the rest of your first post. 

 

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25 minutes ago, Elmer75 said:

5.4 oz buffer, 5.25" long, and has a loose weight inside the buffer

308 buffer spring from ??

A2 buffer tube, 9.75" inside length

Numbers are a little off, but...  Buffer weight is right, buffer length should be 5.200" for a rifle buffer in a .308AR.  Let it ride, let's see what happens.  Internal depth on the receiver extension should be 9 11/16", not 9 3/4", but let's see what happens - they made the buffer too long, and made the extension deeper?   To account for that?  Again, let's see what happens here.  For the spring, with everything else looking "close enough", then just buy the REAL Armalite EA-1095 spring, OR the Sprinco Red spring, and run the gun. 

After you drill that gas port diameter up.  Besides that gas port diameter, the spring is your only questionable part.  Don't let that thing slide, go quality, and do it right. EA-1095 or Sprinco Red.

25 minutes ago, Elmer75 said:

JP Enterprises adjustable gas block/FSB

Run that MF'er WIDE OPEN until you get this gun figured out.  Otherwise, you're just fucking yourself if you try to run it closed down.  Max it out.  Test the gun.  Dial it down later, after the gun functions. 

Edited by 98Z5V
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Thanks for your input.  I'll drill out the gas port first thing.  I had a feeling....

The buffer and buffer tube came from 2 different sources, so I don't thing there was any compensation going on.

Thanks for the spring recommendations, I just ordered a genuine Armalite EA-1095 spring. It should be here next week.  It's always a bit tricky trying to figure out which parts I really need.  Mil Spec has some real advantages.

I copy you on mis-matched uppers and lowers.  I got lucky, these two actually play together very well.  I waited for months for the right ones to be in stock, and Biden was making noises in the background, so I figured I better get something before I couldn't get anything.  I have an Accu-wedge in the lower which tightens everything up very nicely, with only the slightest gap between upper and lower.  I'm happy with it.

The FSB is wide open.

I'll post a picture once I get the rifle put back together.

Thanks again.

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  • 5 months later...

Hey All,

I finally have a follow up to my 308 project questions.  I drilled the gas port to 0.081", installed the Armalite EA-1095 spring, and the gas system operates correctly now.  I have the gas block set at 1.5 turns out of a possible 3.5 turns, and I get consistent lock-back on the bolt.  Yay.

Now I have Failure to Eject issues.  I ran 5 rounds of American Marksman 147gr M80 ammo.  Each round loaded and shot fine, but none of them ejected.  When I cycled the bolt by hand it grabbed the casing and pulled it out, but it never worked during firing.  I ran a couple misc rounds as well, and none of them ejected on their own, but all of them ejected by hand.  Using the charging handle I can load and eject a live round just fine.

I can see where the extractor pawl has grabbed and pulled on the rim of each casing, tearing a little bit of the brass away.  It appears that the casing is getting stuck in the chamber during firing (casing expansion?), but when dry firing the casing can slide out easy enough.  I checked the headspace with rented gauges when I put everything together, but I wonder if the chamber is too tight/too small diameter.

I also noticed marks running across the sides of casing of both fired and unfired rounds after I chamber them.  They are about 1/4" long, and run perpendicular to the axis of the shell casing.  I get 2 or 3 marks each time I chamber a round.  Could this be from a rough chamber bore?  Does it need to be reamed?

Your thoughts are much appreciated.

 

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On 12/1/2021 at 12:46 AM, Elmer75 said:

CBC 18" barrel with mid-length gas.

I was just re-scanning your specs and came across this.  
I confirmed that their website says it is mid-length as well.
But...  I didn't see above where you took an actual measurement.  You might want to double check that and post it here.  Maybe it's actually rifle-length.

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1 hour ago, Elmer75 said:

What would a mid-length measurement be, and what points is it measured from?

Measure from one end of the gas tube to the other.  If it's all assembled, just measure from the open end of the tube to the gas block and make an educated guess at the amount (probably ~1/2") inside the gas block.

According to my notes (I've only assembled rifle-length systems) a mid-length would be 12-1/16" but I will leave it to @98Z5V to confirm.
Rifle length would be 15-1/2".  It's a big enough difference that it should be obvious without having to get the exact tube length measurement.

For lack of a better way of saying it, I will say, "Everything affects everything else."  All the parts(and gas port) need to be in harmony with the others in order for everything to work together.

Oh, also - a photo of your failure to eject may be helpful to see what's happening.

Edited by RacerX
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It's mid-length gas.  From the anchor roll pin in the gas block to the end of the gas tube is 11-3/4".

There's not much to see of the failure to extract.  The casing just sits in the breach until I cycle the charging handle and pull out the casing.  But attached are pictures of a spent casing, showing the scuffing on the outside of the casing, and the wear/damage to the rim.  You can see a bit of brass that got torn off the edge of the rim by the extractor pawl.

 

20220516_134330.jpg

20220516_134349.jpg

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The bolt locks back, it just doesn't pull the casing with it.

I started with the gas port open all the way. No extraction.

No idea what springs are under the extractor.  It came as a complete unit and I didn't modify it.

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Then you might need to turn the gas down a bit more. That should slow down the cycle and give the brass a bit more time to shrink back down.

I can't shoot steel case ammo in my 5.56 pistol because it cycles faster than the steel case shrinks back down for extraction.

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Okay, I'll give that a try.  Is it possible that the chamber is rough and grabbing the casing? If so, is there anything to do about it?  The marks on the casing make me think the chamber is not very smooth.

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