LR308B gas tube

[coryr]

Hello,  I'm new to this forum, hope this is posted in the right place.

I've had an LR308B for a couple of years.  Bought it new, had no problems, enjoy shooting it, but I'm pushing sixty and the weight is getting to me.  I've taken off the bipod, put on a smaller scope.  I've never been able to shoot the gun as accurately as the 18" bull barrel is capable of, so I bought another DPMS barrel, a lighter 16" barrel, to swap. 

I figured swapping one DPMS barrel with another there would be no problem.  But the gas tube off the 18" bull barrel seems to be too short for the 16" barrel.  The problem is not me trying to mix carbine and mid-length gas systems.  The tube is just a small amount short, approx 1/4".  The distance from the gas port to the rear of the barrel extension on the 18" barrel is  about 8 1/8", on the 16" barrel it is about 8 3/8".  The gas tube that I am trying to reuse is about 9 3/4".

I've searched the web for dimensions on these parts but can't find any info.  Has anyone seen this problem before, know what I've done wrong?

Thanks

[98Z5V]

Did you swap over the gas block to the new barrel, or use a new gas block?

More importantly, got pics?  If you can't get them posted, I can help you with them.

[unforgiven]

Welcome from Indiana brother coryr,pics yea. <thumbsup>

[coryr]

Pictures -

First picture is the two barrels, old 18" bull barrel on top, new 16" lighter barrel below.  Yes, had to get a new gas block because the 16" barrel is lighter and smaller diameter.

Second picture shows how far the gas tube extends into the upper with the 16" barrel installed.  Not very far, doesn't look normal to me. I have not tried to shoot the rifle this way.

[98Z5V]

First thing I'd would be shoot a call to DPMS.  Ask them about it.

When you swapped the barrel in there, did you get the barrel index pin fully seated into the groove in the upper receiver?

[coryr]

Contacted DPMS first and they asked if I was using the correct gas system, mid-length or carbine.  Think that would be pretty obvious.

The barrel index pin is fully seated, the feed ramps on the barrel extension and the feed ramp notches on the upper receiver are lined up perfect.

Just contacted the guy who sold me the barrel and he says it was off a DPMS 308 Oracle.  He still has the gas tube so I am going to try that.  I hope it is longer than 9 3/4" when it gets here.  Might try shooting the gun as it is, the only problem would be it might not cycle right?

[Jgun]

Took a look at a couple of mine, (none DPMS receivers), and your gas tube end is definitely short compared to mine, But then you already knew that. I don't have a carbine gas tube on hand, but I'm wodering, just to save yourself the wait and disappointment if it turns out not to be the gas tube, since your in contact with the barrel seller, can you ask him for an accurate measurement of the gas tube he's going to send you? I'm thinking that if it turns out that the gas tubes are the same, you can get started on figuring out how to solve the problem by switching something else (different type of gas block maybe) , or getting a gas tube that IS longer. I have not worked on the DPMS Oracle's, but it seems that DPMS likes to change things around these days to make it difficult to use others components on their guns. They changed the receiver rail heights, and now some of the Manufacturers quad rails dont match up (the ones that have not opted to offer a DPMS specific handguard).

Since you had both barrels out of the gun in the picture, can't you take measurements of both to see where the difference lies? Since you say that the barrel extension lines up properly inside the receiver, I would assume that the distance from the barrel nut shoulder to the back of the extension is the same, but what about the distance from the back of the shoulder to the gas port on the barrel, is it the same on both barrels? You probably already know this, but since you're switching barrels, it would be a good idea to double check the head space before you fire the gun, and yes, I don't see why it should be a problem to shoot the gun with the short tube (although I've never tried that myself), by the way, does the tube even make it into the end of the carrier key?

[98Z5V]

...does the tube even make it into the end of the carrier key?

It should - the gas key almost contacts the front inside part of the upper receiver when the bolt locks. 

[Jgun]

I was giving this a little more thought, and going through the possible reasons that the tube end could be mislocated. The OP says that the back of the barrel extension is located correctly inside the receiver, so if we accept that, there is no problem there. The gas block appears (in the photo) to match up, so if we eliminate that, the only two possibilities become the tube or the barrel. I thought that for the most part, gas tube lengths were standardized, with Armalite having their own lengths, (and some of the short Noveske gas systems I think). Now, unless DPMS has elected to go with a proprietary gas tube to match a different gas port location on the Oracle, That would leave a mislocated gas port on the barrel (unlikely) OR, could they have purposely, chosen to relocate the gas port on the 16" barrel, to change (reduce) the gas impulse by having the gas tube intentionally be shallower in the gas key, and thereby cause it to have a shorter power impulse on the carrier? After all, it is usually agreed that these guns are over gassed. I would think that having a smaller gas port in the barrel would be a more common way to accomplish that goal, but I can't help but wonder why the gas port would be located like that if not on purpose. Maybe it has something to do with timing, delaying the point where gas is directed back to start the unlocking of the bolt?

[coryr]

Thanks for checking your guns and verifying I'm not imagining this problem.  Contacted the barrel seller as you suggested, he measured the gas tube from the 16" barrel at 9.75", same as the gas tube from the 18" barrel. 

I don't know what the name of the shoulder on the barrel where it is turned down for the gas block is named, call it the "gas block shoulder".  The dimension on the barrels that is different is the distance from the end of the barrel extension to the gas block shoulder.  The dimensions around the barrel extension, the "ring" on the barrel that the receiver butts against and the barrel nut clamps on, the index pin, etc are the same on both barrels.  The distance of the gas port hole ahead of the gas block shoulder match on both barrels.  I know the barrel extension is actually a separate part from the barrel, assembled for us by the barrel manufacturer, the part that must be 1/4" longer on the 16" barrel is from the actual end of the barrel (which I can't get to to measure) to the gas block shoulder.

Since the barrel seller said the gas tube would be the same length as the one that I have I did not order it.  I went ahead and finished assembling the rifle.  Fired just five test rounds through it and it cycled OK. 

I may order a rifle length gas tube and try to make a tube 10" long.  I have a lathe.

I would be really interested for someone who owns a 308 Oracle to check how far the gas tube extends into their receiver and to measure their barrel.

By the way, the free floating hand guard no longer covers the barrel up to the gas block, it leaves 1/4" of the gas tube exposed.  picture attached

I already love the lighter barrel, like how the rifle balances, or I would consider going back to the bull barrel.  I know I'll shoot this gun more with the 16" barrel.

Thanks for all the great replies and help.

[Jgun]

I guess your photo of the 16" with the gas tube exposed between the FF tube and the back of your gas block is the giveaway that the dimension between the barrel nut shoulder and the gas block shoulder is longer on the 16 than it was on the 18.5" bull barrel. As I mentioned in my other post, I'm thinking that DPMS must have done that on purpose, I just am not sure why. I would also think that the possible effects I mentioned, delayed bolt opening, and reduced gas force acting on the carrier, may be the reasons. If It's functioning as is, and the barrel was using that length gas tube originally, I don't see any reason why you should bother to cut down a mid length tube, besides, you will be cutting off the larger dia at the end which seals the tube to the gas key, and you may end up with less cycling force working on the carrier. If it were me, I'd try to get a look at a factory 16" Oracle setup, just to confirm that the gas tube protrudes the same distance into the upper on that as it does on your's. I don't think I'd bother changing anything until I did that. Good luck, and please let us know what you determine.

[Robocop1051]

If it works, it ain't broke.  :auto:You know what they say...

And if it ain't broke.... <thumbsup>

[coryr]

All good input.  Think I'll just shoot this rifle the way it is and enjoy it.  If a problem shows up in the future I'll revisit this.  Thank you all.

[tlcoy1911]

Can you show a pic of the markings on the barrel?

It looks like that barrel is the Armalite lenght.

DPMS barrels are 3/16" shorter from the barrel ext shoulder to the gas block shoulder (than an AR15 and Armalite) so they can use the AR15 gas tubs.

Armalite uses the standard AR15 lenght from the barrel ext shoulder to the gas block shoulder, but that means you have to use a longer gas tube.

You could just order an Armalite gas tube.

[98Z5V]

I've never been able to shoot the gun as accurately as the 18" bull barrel is capable of, so I bought another DPMS barrel, a lighter 16" barrel, to swap. 

I figured swapping one DPMS barrel with another there would be no problem. 

OPs info, in the first post - both are DPMS barrels.

[tlcoy1911]

Just because the barrel was on someone else's DPMS receiver don't mean its a DPMS barrel

[98Z5V]

I'll wait for clarification from the OP, but he seems pretty sure he bought a DPMS barrel.  My assumption that he knows what he bought, is just as good as your assumption that he might have bought an AR-10 barrel by mistake, despite his own statement...