New 20" barrel, now it short strokes

[Dracomeister]

OK Gentlemen, what did I do wrong? My DPMS LR-308 (16" carbine length) was running good just not accurate enough at long range. Bought a new Anderson 20" barrel on the cheap to try out, changed from carbine length to rifle length gas tube but used the original gas block. Tried it out today and never got a complete cycle. I ran two different factory loads and two different handloads, no joy. Slopped it with oil, checked the BCG, everything looked and felt good. I thought maybe the gas port was too small or the original buffer was wrong. Any ideas? Thanks

[Rsquared]

Show us a picture of the inside of the upper. We've seen this before, where the gas tube might not be long enough. It should extend to about halfway into the cutout for the cam pin.

[Dracomeister]

It looks like the gas tube extends into the gas block about 1/4" - 3/8" with the bolt in battery but doesn't make it to the halfway point of the cam pin cutout.

 

[jtallen83]

Looks to be on the short side.

[guruofhotrod]

Excuse the bad pic but, this is what you should be after:

 

 

[blue109]

Guessing that barrel requires an AR10 gas tube. I assume you are using a standard AR15 tube currently?

[98Z5V]

Armalite AR-10 rifle length gas tube will fix that.  The Armalite part, not a copy.

[blue109]

2 minutes ago, 98Z5V said:

Armalite AR-10 rifle length gas tube will fix that.  The Armalite part, not a copy.

Important...Because many retailers call anything 308 related an AR10 part.

[survivalshop]

 Fulton Armory sells the longer AR 10 Gas Tube . WE have seen other rifles lately that function with the Gas tube in that position .

  Did you change the Bolt with the new Barrel or use the same one that was used with the 16" ? Check Head Space .

  Check the Chamber for roughness , it may also show up on spent brass .

[Dracomeister]

Thanks, I didn't realize the AR-10 used a different length tube. I'll have to order one since my LGS is firmly grounded in all things not Armalite but then call everything an AR (Hey, what can I say, we're all Hillbillys around here)! Had the headspace checked and it was good (cases grew 0.001" - 0.003" when fired), the chamber is pretty rough but I'm planning to lap it and the barrel so that should go away. I'll get the new tube and try it out then let you know what happens. All in all, I'm pretty happy with the barrel's accuracy, firing single shots it was sub MOA with my 168 Gn Match loads!

[unforgiven]

These guys are good to deal with : https://dsgarms.com/filterSearch?adv=true&cid=0&q=armalite gas tube&sid=true&isc=true

[392heminut]

I hope your Anderson barrel works out better for you than mine did. It had a tight chamber that had issues even after I switched to a small base sizer die. I could live with that (I have a friend who has a 308 chamber reamer) but when I tried to sight it in at the desert shoot a week ago it wouldn't group good enough to even attempt to adjust the scope. Barrel nut is torqued good and scope is properly mounted, as far as I can tell the barrel is just junk! No way in hell I'll buy another Anderson barrel!

[Dracomeister]

Well folks, I picked up an Armalite rifle length gas tube and it was exactly the same length as the other one. Put it in anyway, test fired the rifle with the same results. Went back to my gunsmith who checked the gas port, supposed to be 0.0890" mimimum to 0.0960" max, actually drilled at 0.0760". Drilled it out to the minimum 0.0890" and it "kinda-sorta" worked most of the time, went back and drilled to 0.0960" and now it works like a champ. Chamber and barrel were also pretty rough but I lapped them with JB's and it really cleaned them up. It almost seems that Anderson Parkerized the chamber and bore when they did the barrel.

Hemi Nut, sorry you are having issues, I've only fired 50 rounds through mine (Remington 150 gn factory, Winchester 150 gn factory, 150 gn handloads, and 180 gn handloads) but getting MOA from the RP factory loads and the 180 gn handloads (different POI but really tight groups). The Winchester factory rounds were about 2 MOA and the lighter handloads were shotgunned!

My thanks for all the suggestions, there is more knowledge on this site than I can imagine! Soon as I get the barrel broken in I'll post some "Group" photos.

[survivalshop]

  If you feel its a little over gased , you can alway install an Adjustable Gas Block . Sounds like you have it taken care of ,but they are out there if needed . 

 Can you measure the Gas Tube you have out of the rifle ?   Below is a partial length guide .

DPMS Gas Tube/AR 15/16

 Pistol             6-5/8

  Carbine          9-3/4

  Mid-Length  11-3/4

  Rifle             15-1/4

Armalite lengths,

Rifle ( 308 ) ----- 15.5 "

Mid length Carbine ( 308 )--12 1/16"

super SASS Carbine ( 308 )--11"

Super SASS< Rife (308 )   ?

Mid ,( 223 )------------------ 11 3/4"

Carbine ( 223) --------------- 9 3/4 "

Rifle ( 223 ) ------------------- 15 3/16"

 National Match---------------  ?

 

Edited April 11, 2017 by survivalshop

[survivalshop]

 We have seen a lot of 308AR Barrels like that lately .

[blue109]

Where are those AR10 numbers from ss? The carbine seems a bit much. Maybe that's suppose to be mid-length tube?

[survivalshop]

9 hours ago, blue109 said:

Where are those AR10 numbers from ss? The carbine seems a bit much. Maybe that's suppose to be mid-length tube?

Good question , I don't remember , I got confusing info on what they call a Carbine , it is really a  Mid length . I think they came from Armalite , but look at Raley's corner , he calls it a Mid length Carbine  , which that long , It would be .

 I changed it on my notes & on this thread . I thought it was confusing also , but it was what I found described in print , they don't have a Carbine length , as compared to an AR 15 or DPMS type 308AR's ( 9 3/4 " ) , as far as I know , who am I to argue with Armalite 

 

GAS TUBE LENGTH

 

·         M-15 rifle – 15-3/16”

·         M-15 mid length carbine – 11-3/4”

·         M-15 shorty carbine – 9-3/4”

·         AR-10 rifle – 15-1/2”

·         AR-10 mid length carbine – 12-1/16”

·         All gas tubes: .180 OD and .117 ID

Edited April 11, 2017 by survivalshop

[blue109]

That seems right. Puts the AR10 tubes about 1/4" longer than their counterparts across the board. 

 

@Dracomeister

Would you mind measuring your old gas tube length and comparing it to the numbers SS listed? It still looks to me like your gas tube is too short. Most smooth running 308s have the tube extending half way through the cam pin cutout. I'm just curious.

 

[392heminut]

Yup, Armalite calls their midlength 308 carbine length.

[Dracomeister]

Blue, just measured the old tube, exactly 15-3/16". I ordered a second Armalite tube (from Armalite online this time) and UPS just delivered it! It is 15-3/8", a full 3/16" longer. I'm taking the other tube back to the shop where I bought it and letting them know (in no uncertain terms) that it ain't an Armalite tube! Now, with the new tube AND the max gas port, I may be over gassed, guess I'll see an adjustable gas block in my future! So tell me, do you EVER get by with just ONE modification on these things???

Edited April 12, 2017 by Dracomeister
Add: just got it installed and it goes to the center of the cutout!

[98Z5V]

30 minutes ago, Dracomeister said:

 So tell me, do you EVER get by with just ONE modification on these things???

It really comes down to whether the manufacturer(s) have their shiit together. With no set pattern to follow, even among the 3 different major types, things are all over the board with parts. Hell, when people say they're making DPMS-based parts, they can't even all be on the same sheet of music!  You just have to watch everything you're buying. You have to really check out full rifles, really be picky about what parts you get, etc.

Honestly, the only complete rifles I'd trust would be the high end stuff (DD, PWS, LaRue, etc.) and less expensive complete rifles that come directly from a major manufacturer (Armalite, DPMS, Windham Weaponry, RRA).

I can't believe there was a gun store gunsmith that said that short tube wouldn't have any affect on the rifle... You have a barrel with a DPMS-based barrel extension on it, and Anderson obviously didn't drill the gas port in the DPMS LR308 position - they drilled it in the Armalite AR-10 position.  Smaller manufacturer, doing whatever they want...  That's what gets us, as we build these things...

Edited April 12, 2017 by 98Z5V

[blue109]

I always wait to make permanent modifications until I'm pretty sure it's necessary. I think your port will be too big, but as you said, you can always tame it with an adj block. Take this as your first lesson that most "gunsmiths" don't know as much about these animals as they think they do...And most shops, and even manufacturers don't know the difference between an LR308 and an AR10. We're always learning and sharing in here. It usually takes mistakes to make discoveries.

[98Z5V]

6 hours ago, 98Z5V said:

I can't believe there was a gun store gunsmith that said that short tube wouldn't have any affect on the rifle... You have a barrel with a DPMS-based barrel extension on it, and Anderson obviously didn't drill the gas port in the DPMS LR308 position - they drilled it in the Armalite AR-10 position.  Smaller manufacturer, doing whatever they want...  That's what gets us, as we build these things...

I just re-read this whole thread, and didn't see a reference in your thread here, OP, about that gunsmith and his comments.  That's me mixing up information and threads, on the exact same subject.  That must have been another person in the last couple weeks, with that gunsmith.  We made a recommendation to the guy, based on what he was telling us, and the pics revealed the short gas tube.  Immediate recommendation to replace it with the Armalite gas tube, check operation, and see how the problem was.  Pick off the easy target, and see how it goes, you know, fix the obvious.  He goes and tells his gunsmith that bit of info, and the gunsmith tells him that's not even an issue, and it won't change anything.

Well, I say BS on that gunsmith, and here's why...

It's come up that the carrier key sits further back on the 308AR than it does on an AR15 - it's in the exact same place from the front of the carrier back to the front of the key, but that 308AR bolt is alot bigger than an AR15 bolt, and when the bolts are seated in the carrier, in the firing position, that key is further back on a 308AR.

That's issue #1.

Next, Armalite vs. DPMS (and AR15) gas systems.  Why Armalite gas tubes are longer than standard AR15 gas tubes (and DPMS_based LR308 parts), I have no idea. Why DPMS uses AR15 gas tube lengths for 308ARs, I have no idea...  However, I DO know that Armalite knows their shiit, and they have a scientific, tested, proven reason for making AR-10 gas tubes longer.  Whatever reason that is, you can know that it's a good one.  Pat Raley could tell us all about it, I'm positive.

That's issue #2.

Next, that carrier key overlaps the end of that protruding gas tube, and it's for an exact distance/measurement.  It's THAT WAY so the bolt carrier receives the proper amount of gas, for the proper amount of time, in order to properly cycle the semi-automatic weapon.  Once that bolt carrier assembly has made enough rearward travel, it disengages from the end of that gas tube, and the remaining gas is discharged directly into the (now open) upper receiver.  It's the excess gas you see coming out of the ejection port when your buddy fires his weapon...

That is issue #3.

Now, that leaves us here - with a gas tube that is NOT the proper length, and it's short by 3/16", that bolt carrier is NOT receiving the proper amount of gas, for the proper amount of time, and it's not fully cycling that bolt carrier assembly with the proper amount of force that it was designed for.  It might cycle, but it's NOT going to cycle as it was designed to do...  THAT cannot be argued.  It's science and math.  Done.

Now, this isn't aimed at you, OP - this thread is just a target of opportunity to explain it.  You are on the right track.

I hope that other poster reads this, then prints it, and gives a copy of it to his "gunsmith." 

Thus endeth the sermon...

Edited April 13, 2017 by 98Z5V

[Rsquared]

^^^^^^^^

And a simple thing like that.......is why a young man working for an aerospace company waaayyyyyy back when (at this point) in the 50's, was one helluva fukin engineer. His name was Eugene Stoner.

[jtallen83]

4 hours ago, Rsquared said:

^^^^^^^^

And a simple thing like that.......is why a young man working for an aerospace company waaayyyyyy back when (at this point) in the 50's, was one helluva fukin engineer. His name was Eugene Stoner.

And that begs the question, which rifle length gas tube came first? Is it possible the AR-15 tube was shortened from the original AR-10 and when 308AR's became popular makers just made adjustments for the already in production shorter tubes.