98Z5V Posted November 15, 2011 Report Share Posted November 15, 2011 On my side, I found another gas block that's drilled all the way through - the YHM lo-pro. I need to look at the rest of my stuff to see if there's more. Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 15, 2011 Author Report Share Posted November 15, 2011 Ok I have now ruled out the:1: Gas tube2: Gas Block3: Barrel extension4: Upper ReceiverI am leaning toward not 1 but 2 incorrect machined barrels...Does anyone have a DPMS 16" and 20" barrel not mounted that could give me some measurements or direct me in the right direction to find these?I am getting nothing out of DPMS.. Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 15, 2011 Report Share Posted November 15, 2011 Sorry to say, but I have no barrels on hand at this time. I would think that any brand DPMS pattern .308 AR barrel of the same gas system length as yours should be able to give you the pertinent dimensions you need to compare to the barrels in your possesion. Quote Link to comment Share on other sites More sharing options...
survivalshop Posted November 16, 2011 Report Share Posted November 16, 2011 Wouldn't any AR barrel be the same ? They all have the same gas systems in them ,be it carbine , mid or rifle , 5.56,300 BLK , 6.8, 308.The difference would be barrel/extension dia. & chamber length . We are using the same parts of the gas system , so the distance should be the same ,no ? Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 16, 2011 Report Share Posted November 16, 2011 This is a topic that I wish was definitively addressed somewhere. I wish there were a chart that covered this. If there is, I've never seen it. What I have found from reading, and my own builds, is that, based on my purchases of Noveske barrels, what they refer to as a .308 carbine length gas system, uses the same length gas tube as the 5.56 barrels that they sell as a midlength gas system. So does that mean that a 5.56 carbine length gas system would use the same gas tube as a .308 pistol length system? I also have read that the Armalite .308 rifle length gas system puts the gas block approx .250" further forward than the rifle length gas system for a DPMS .308. I don't know if they use a proprietary gas tube or make up the length difference in the gas block. I guess this is just one more place where the lack of a mil spec for the large platform AR's makes things confusing sometimes. Does anyone know how many different length gas tubes there are commercially available? What about these SBR barrels, do they make proprietary gas tubes for them? I also have ordered rifle length gas tubes that were straight with no bends in them, while others that I've ordered have had bends made during manufacture. I'm not sure if that is just a manufacturer preference or if they sell gas tubes specifically designed for bull and lightweight barrels that have different gas blocks. It always appeared to me as though the .750 ID gas block oriented the gas tube in the same place in relation to the bore centerline as the .936" ID one does? Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 16, 2011 Author Report Share Posted November 16, 2011 Here is an update, I have removed an AR-15 rifle length barrel and compared to the rifle length LR-308 barrel.There is 5/16 inch diffrence, from gas balock location to end of barrel where it mounts in the upper.I think I have been sold "second's" barrels, not only a 20" but also a 16", what are the odds, only me.I have attached pictures, the barrel on the left is the .223-5.56 and the one on the right is the .308-7.62. Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 16, 2011 Report Share Posted November 16, 2011 I posted earlier that I have read that the Armalite barrels GB location is approx. .250 plus that of the DPMS. Is there any possibility that you have an Armalite pattern AR barrel? If this were hypothetically true, do you think that a slightly longer gas tube might solve the cycling issues you've described? Quote Link to comment Share on other sites More sharing options...
imschur Posted November 16, 2011 Report Share Posted November 16, 2011 if you line up the gas block holes with the stop on the barrel extensions do they line up? I dont think you can compare lining up the extension ends. The .308 bolt is bigger and longer.Anyhow comparing the two different barrels might not be a valid test. After all the gas tube references from gas block to the carrier tube.Perhaps Jgun is on to something. Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 Man I don't know anymore....Barrels are stamped DPMS Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 If you go back and view the first pictures I posted, of the under sider of the upper with the gas tube installed, and compare that to the last pictures I posted or the barrel coparison, the diffrence, would suggest that the barrels are not correct.I am going to turn an additional 5/16" from the barrel where the gas block mounts, re-drill the gas port hole and re-assemble and test fire..... Quote Link to comment Share on other sites More sharing options...
imschur Posted November 17, 2011 Report Share Posted November 17, 2011 This one hurts my head, I cant imagine how you feel Quote Link to comment Share on other sites More sharing options...
imschur Posted November 17, 2011 Report Share Posted November 17, 2011 We needs to get some good barrel/upper/gas system dimension for reference at the site here. Would make this stuff easier to sort out. Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 17, 2011 Report Share Posted November 17, 2011 Fishco, Before you irreversibly alter any parts, let me ask you. Do you have a .308 AR of any pattern that functions? If so, what gas system length is it? Bear with me here if I don't go back to review your previous posts. What is the gas system length You are trying to install? It's DPMS pattern, correct? I think I have a couple of gas blocks and gas tubes on hand. I have an Armalite pattern carbine length gas system gun on hand, along with my DPMS pattern, rifle length gas system rifle (my Kaiser). Is there any way that I can give you measurements from any of the above that will allow you to determine if your barrel is in fact out of spec? Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 Jgun....Yes, Rifle, Rifle, Yes........I have positively confirmed its the barrels, they are off 5/16".... Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 I think that who ever made these barrels did not allow for the diffrence of the bareel extension, I think they were in AR-15 mode and should have been in LR-308 mode...Have no fear, I will trim the barrel back and re-install and test.I will be back in a few days and post the results, wish me luck! Quote Link to comment Share on other sites More sharing options...
98Z5V Posted November 17, 2011 Report Share Posted November 17, 2011 .308 barrel extensions are longer than AR15 extensions - what you have looks right. They're consistently off, but by the length of the extension. What you need to compare, between those barrels pictured above, is from the mounting flange on the extensions, to the gas port location. That is gonna be the measurement that tells you if you've got Armalite-machined barrels or DPMS-machined barrels - I think Jgun hit it. I can't really see something Armalite-based coming out of DPMS, but DPMS isn't the ones making their own barrels, either - that's sublet, to others. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted November 17, 2011 Report Share Posted November 17, 2011 if you line up the gas block holes with the stop on the barrel extensions do they line up? I dont think you can compare lining up the extension ends. The .308 bolt is bigger and longer.^^^ This. Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 17, 2011 Report Share Posted November 17, 2011 Fishco, Don't know if you've already started cutting, but if not, perhaps these measurements will help you to determine if in fact that's what you need to do. I measured the distance that the gas tube protrudes past the back face of the barrel extension. (which is what you will be changing if you remove the material to move the gas block back). On the Armalite pattern (Armalite extension and bolt) .308 carbine gas system gun, the dimension from the end of the gas tube to the face of the barrel extension is 1.035" and on the DPMS pattern, (DPMS extension and bolt) rifle length gas system gun, the dimension from the end of the gas tube to the face of the barrel extension is .910" I also measured a rifle length gas tube that I bought from Fulton, for a DPMS pettern rifle length .308 build I'm doing, and the OAL is 15.156" (that's OAL, not to the gas port hole in the gas tube). Let me know if these measurements are of any help or if some others would be useful. If your gas tube is in fact mislocated, you should be able to tell by seeing if it's way off that .910" measurement. Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 jGun,Not cutting yet, was going to start this afternoon, do you have a measurement from the end of the barrel extension to where the gas block butts up to where the barrel is turned down, or to the gas port hole. Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 The next gun build I do, I am going to measure and spec out every single piece that I can, so to help the next poor guy like me!I could get my 14 year old to build an AR platform rifle with his eyes closed, but this one has stumpted us.....We built it and it didn't work :-(What should have been lees than a one hour build, has turned into a week process, oh well at least I will learn from it! Quote Link to comment Share on other sites More sharing options...
imschur Posted November 17, 2011 Report Share Posted November 17, 2011 The next gun build I do, I am going to measure and spec out every single piece that I can, so to help the next poor guy like me!Please do!!! Quote Link to comment Share on other sites More sharing options...
survivalshop Posted November 17, 2011 Report Share Posted November 17, 2011 As 98 said , you have to measure /compare from the seating flange of the barrel extension . Looking at your photo's , as said , the extension on the 308 is longer ,but the distance from flange to gas port should be the same . Your photo's have a misleading comparison between the two .Measure from the extension flange to the gas hole ( where it seats on the upper receiver )of both barrels (same gas system lengths ,of course )What is the length of the gas tube you are using , by its self ? The one in my hand is close (15 1/8 " ,with tape measure )to the same Jgun has . Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 17, 2011 Report Share Posted November 17, 2011 On my DPMS pattern Rock Creek barrel, I'm measuring between, 13.350"and 13.375" from the back face of the barrel extension to the back shoulder that locates the back face of the gas block on the barrel. I'd be inclined to say that the shorter dimension is more accurate. If your going to start turning the barrel down, I'll tell you, the back face of the barrel extension is flush with the inside face of the upper receiver on mine (Kaiser) don't know if it would be the same on a DPMS upper. If it were me, I'd remeasure the existing gas tube to barrel extension dimension and compare it to that .910" I got, along with the present dimension from the barrel extension face to the GB shoulder, You know, it's hard to put the material back on once you turned it off and if you take too much and the gas tube is too deep into the receiver you may run into problems. Quote Link to comment Share on other sites More sharing options...
Fishco Posted November 17, 2011 Author Report Share Posted November 17, 2011 Jgun,From the end of the barrel extension to the end of the barrel where the gas block butts against it I have: 13 9/16".................from the stop lip of the barrel extension that butts against the reciever.....12 5/16" .................. from the back of the stop lip on the barrel extension away from the reciever..12 3/16"At this point I don't think I have anythiing to lose in turning this barrel back 5/16", it doesn't work now.... Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 18, 2011 Report Share Posted November 18, 2011 From what you've posted I'm reading that your dimension is approx. +.200" from my measurements, is that correct? I don't know what your thoughts are on this idea, but, If you were to take a rifle length gas tube, which only costs $10 so it's not a big deal if it doesn't work, But if you drill a new gas port hole .200" further forward, and then drilled a new, roll pin hole in the gas block so that you can locate the gas tube .200" further back, You should be able to accomplish the same thing as turning the barrel and redrilling the gas port .200" further back. if you do decide to drill the new gas port in the barrel, you'll have to make sure that the new hole doesn't Nick the rifling but is fully in the grooves. If you tried my idea You might have to use some sealer so that the extra hole in the gas tube doesn't allow gas to leak out of the back of the gas block where the gas tube goes in. Quote Link to comment Share on other sites More sharing options...
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