Thundergun from hell

[Vwporschegt3]

First off I just have to say there is such a great wealth of knowledge here and I appreciate everything that's been posted in here.

  Dad bought this upper and I have spent the last year dicking with it trying to make it right.  Having done some aggressive research I came across the gas tube length and have determined, among some others , what the problem seems to be with this gun... and I believe it directly kings to the length of the gas tube.

     Currently the tube does not project into the cutout with for the bolt retainer , so an armalite gas tube will be on order.... secondly the gas port on the 20" barrel was only .069"....  port was opened to .095" by me and it started cycling  to what I would say is 80% of the time. 

       I'm a reloader and have been shooting my own reloads as well as about 2 boxes worth of winchester through it. Same results . My loads are 25.5grns of 4320 with sierra 150gr. Bullets. These are all hand metered loads so I know they are extremely consistent. 

          This was a "budget build" so the recoil system is what came out of the utg ar308 a2 stock box.  I have been reading that I may need to adjust the buffer weight to 5.4ounces? And what spring would be ideal?   The rest of the lower is dpms parts on a 80% lower. I have a second lower configured exactly the same and get the same issues across the board.

    Any and all advice is welcome. I understand from the get go this is a hodgepodge . Thanks guys

[Vwporschegt3]

Btw the issue is short stroking

[Armed Eye Doc]

Can you give a parts list of what you have in the rifle?  Please be as specific as possible. 

A picture of the gas tube and where it ends would be helpful as well.

Can you weigh your buffer, measure your spring and measure the inside depth of you buffer tube?

I am not the best at diagnosing these, but this info would be helpful for those who are.

[Vwporschegt3]

Will do I'll get the info in a few hours

[Vwporschegt3]

Here is the gas tube. I dont have a scale to weigh the buffer. But again this is a thunderguns.com upper 20". I'm going to replace the gas block with an adjustable and the gas tube with the armalite that is .325" longer. 

     The stock on my lower is the leapers utg ar308 stock it came with the "308" spring and buffer 

   The lower is a gen 1 dpms pattern 80 percent with dpms lower parts kit.

   I'm using magpul sr25 20 round mags , I dont think I'm having a mag problem since the problem seems to be with short stroking.  I also modified the ejector since it was the flat type, I rounded the edges and have yet to test it. 

       I will weigh the buffer on monday and will be able to tell you the weight then 

Edited March 7, 2020 by Vwporschegt3

[Armed Eye Doc]

Your gas tube is extremely short.  It should extend to the middle of the cutout.  I think you are on the right track by replacing it. 

[98Z5V]

Read this thread, and look at the detail on parts that this guy posted, and look at the responses he got - this is the level of detail needed here, in order to figure out what's REALLY going on with your gun.

[Vwporschegt3]

Just to round this one out.. armalite tube and adjustable gas block brought this gun in to harmony... after tuning the block it cycles a locks open on last round with just about anything I can throw in it..   it has become extremely  reliable .

If anyone ever buys a thunderguns 20" rifle.length gas upper and decides to run a utg 308a2 stock...  check your gas port size for one, the buy a gas tube and adjustable block... this is just as reliable as any ar15 I've owned now 

[Armed Eye Doc]

13 minutes ago, Vwporschegt3 said:

Just to round this one out.. armalite tube and adjustable gas block brought this gun in to harmony... after tuning the block it cycles a locks open on last round with just about anything I can throw in it..   it has become extremely  reliable .

If anyone ever buys a thunderguns 20" rifle.length gas upper and decides to run a utg 308a2 stock...  check your gas port size for one, the buy a gas tube and adjustable block... this is just as reliable as any ar15 I've owned now 

Great to hear that.