Lack of gas?

[bentoncity]

DPMS10/308
Built two EXACT identical weapons except one has a 16" barrel & the other has an 18" barrel. 16" version operates 100% like it should. Cycles just fine and stays open on last round. 18" refuses to cycle properly if at all. Swapped lowers, didn't help. Before the weapons were used/fired they were both disassembled, cleaned and the BCG's were oiled with Royal Purple Synthetic gun oil. Armorer set barrel nuts and checked headspace.

Scenario as follows....

• Gas block full open or closed did nothing when firing @ the range. (30 rnds- Federal 168gr BTHP)
  Moved to the shop for the following
• Plugged the barrel from both ends, fed pressured air into gas tube (in the upper) & gas block let air escape when fully open and no air escape when fully closed.
• Pressurized air escapes barrel when fed through the gas tube in the upper.
• Gas block is tight.
• Loosened/slid adjustable gas block & tube down the barrel to check alignment.
• Gas dust marks on barrel indicates alignment with gas block.
• Gas port in barrel is clear.
• BCG holes are clear.
• Bolt gas rings are snug. (Gas powder on backside of bolt rings-none in front of rings)

What hasn't been done/tried/completed...Yet. (had to order up some more ammo)

• Swap BCG's
• Measure gas hole diameters in barrel & adjustable gas block.
• Contact Aero Precision & Superlative Arms for gas port hole diameters

We're at a loss here and looking for ideas or something we've missed/overlooked. Discussion with Luth-AR on buttstock indicates weights/springs is correct for either weapon. No abnormal gouges or wear spots anyplace in the upper or the BCG. Nothing seems to be binding when hand charging or firing.
Parts list in the file below.
Msg is sent to Superlative & Aero Precision.. questions or suggestions welcome.

Dan

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[oli700]

Need to measure the port with a good taper gauge and look up some numbers or call Ballisic Advantage with your measurement .
I built two identical 308 each with BA Hanson premium 16" . One was over gased and the other barely cycles Federal GMM 168's . It happens.
I never did anything with the gas systems I went with buffer system adjustments both run killer now .

 

[bentoncity]

I went with buffer system adjustments ...

??

Dan

 

[v0lcom13sn0w]

i had to enlarge a gas port hole on a 18" BA hanson barrel to get it to run reliably. with an adj gas block you can tune it from there.

 

[turq]

Thoroughly osso paste your chamber with nylon brush.
Rough?
I run melonite coateds and this was important.

 

[oli700]

??

Dan

When I built them I went with an Armalite carbine buffer system . These use standard size AR 15 buffers , longer buffer tube ,7.75" deep, accommodates the longer bolt carrier group of the 308. The Armalite Kit comes with an H3 5.4 oz buffer and Armalite 308 spring .
On the gun that is "over gas" I ended up with an XH 8.5 oz carbine buffer from Slash's heavy buffers and a Tubbs 308 flatwire spring.
The gun that is "under gas" I just dropped to an H1 3.8 oz buffer and the Armalite 308 spring .
If you use the LR 308 buffer system , 2.5" buffer , there are different wieghts to try ..... and springs .

 

[bentoncity]

Thoroughly osso paste your chamber with nylon brush.
Rough?
I run melonite coateds and this was important.

Not a clue what you're stating. My chamber isn't rough if that's what you're implying.

Just to be clear, hand extraction works fine, the bolt isn't cycling like it should after firing no matter the open or closed position of the adjustable gas block. Since the 16" barrel lower operates perfectly, it should operate on the 18" barrel which it doesn't.

Dan

 

[oli700]

Not a clue what you're stating. My chamber isn't rough if that's what you're implying.

Just to be clear, hand extraction works fine, the bolt isn't cycling like it should after firing no matter the open or closed position of the adjustable gas block. Since the 16" barrel lower operates perfectly, it should operate on the 18" barrel which it doesn't.

Dan

How do you relate the problem with the lower ? Or figure the swapping of lowers to troubleshoot the problem? Especially since both lowers/buffers are the same per your description.

 

[bentoncity]

How do you relate the problem with the lower ? Or figure the swapping of lowers to troubleshoot the problem? Especially since both lowers/buffers are the same per your description.

Am trying to eliminate easy to try issues 1st. Even though both lowers are identical, MAYBE something was faulty with the 18" lower... swapping the (I know it's operating correctly) 16" lower into the 18" indicates the lower isn't the issue because the 18" still didn't cycle.

I'm confident I have a gas issue but I'm trying to eliminate mechanical issues 1st. I think I'll have to do what v0lcom13sn0w did. But I'm waiting for spec's from the vendors b4 I modify a perfectly good barrel/part & make it worthless.

Dan

 

[oli700]

Couple ways to skin this cat , good luck!

 

[Member 34319]

Am trying to eliminate easy to try issues 1st. Even though both lowers are identical, MAYBE something was faulty with the 18" lower... swapping the (I know it's operating correctly) 16" lower into the 18" indicates the lower isn't the issue because the 18" still didn't cycle.

I'm confident I have a gas issue but I'm trying to eliminate mechanical issues 1st. I think I'll have to do what v0lcom13sn0w did. But I'm waiting for spec's from the vendors b4 I modify a perfectly good barrel/part & make it worthless.

Dan

Whether you take a drill bit to the gas port or the manufacturer does that's what will likely take place to fix the issue.

The difference between sending it back or doing it yourself is the time waiting for shipping and such.

If the manufacturer sends you a new barrel, that's about the only good reason to choose to return the barrel.

Home assembling comes with its own fitting and testing stages. Just like a manufacturer would fit and test their products before shipping them.

Hope you get it running better.

 

[Geardo]

Swapping BCG and triple check GB alignment would be my first steps.

 

[oli700]

Swapping BCG and triple check GB alignment would be my first steps.

I'd drop buffer weight till it ran weak ammo and be happy my rifle is a few ounces lighter and reliable over a wide range of ammo

 

[Geardo]

I'd drop buffer weight till it ran weak ammo and be happy my rifle is a few ounces lighter and reliable over a wide range of ammo

Depends on competition or defense use IMO. I run H2s and flatwire springs in most of my guns for the extra reliability.

 

[oli700]

Depends on competition or defense use IMO. I run H2s and flatwire springs in most of my guns for the extra reliability.

And that depends on how much gas you have to work with. Using an adjustable gas block and tuning for a particular round says competition to me .
None of mine have the same buffer weight , or springs for that matter . Funny all the approaches and schools of thought .

 

[oli700]

Depends on competition or defense use IMO. I run H2s and flatwire springs in most of my guns for the extra reliability.

But I love a Tubbs spring man , never believed the hype till I had one and yeah they are kinda magical

 

[Geardo]

But I love a Tubbs spring man , never believed the hype till I had one and yeah they are kinda magical

Yep. I love me some tubbs + h2. I try to standardize so I can swap uppers and lowers and know it will all work.

 

[oli700]

Yep. I love me some tubbs + h2. I try to standardize so I can swap uppers and lowers and know it will all work.

All my stuff is like a pistol with a pistol gas ,one with a carbine gas , a rifle with rifle gas , a carbine with midlength gas , couple 308 ....so they are each their own . I like standardization though I just can't justify buying two things that are that close to each other .... makes me feel like I'm getting robbed lol . I know a guy with 7 20" FN ar's .... awesome but man I just don't get that ..... he might raise a squad I guess

 

[bentoncity]

Yahoo is on the way.
Barrel port called for #43(.089) drill bit. It snagged going thru, like they stopped shy of going all the way into the barrel. Armorer took it to #41(.096). /a difference of .007 over factory. Cycles like it's supposed to. Just need to put a few rnds thru it to get a final set on the gas block. Thank you oh gun Gods and my armorer..
Thanks to all for ideas/suggestions.

Dan

 

[turq]

I missed the .308 part, and mine are melonite, prone to scratchy chambers.
Very glad that you got it sorted. Gunsmithing problem is like a hurricane can't wait till it's over.