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Originally Posted By Tiribulus: @November5 If you see this, what's the rationale behind this, if you don't mind my asking. I would have thought putting the seam on the bottom. To be clear, that's a question, not a challenge. Originally Posted By Tiribulus: [b]Originally Posted By November5: Wrap [the barrel extension shim] tightly around the extension, with the seam to the right or left side of the rifle (NOT the top or bottom) If you see this, what's the rationale behind this, if you don't mind my asking. I would have thought putting the seam on the bottom. To be clear, that's a question, not a challenge. the reason I put the seam on the side is most of the pressure from loading the bipod, laying on barricades, sandbags, etc. is up and down, not side to side. Does it REALLY matter?? Probably not, but seems to make sense to me. |
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Originally Posted By November5: This post is from @desertmoon ------------------------------------------------------------------------------------------------- A very painful lesson I learned from positional alignment along the fore and aft axis of port alignment is this: if you move the gas block back more than a few thousandths, you can run into the issue of setting the gas tube back into the carrier key by that same distance. When I made the mistake of doing that on my big Mega, I had the carrier key colliding with the gas tube enough to actually pretension the barrel. My cold shot would be something like four to six inches from the rest of one very strange group. If it weren't for a very odd experience at Fort Benning back in the 80s, I don't know if I EVER would have figured it out. Replacing the gas tube and the block with a unit that needed no alignment fixed the issue immediately. I went from this: https://www.AR15.Com/media/mediaFiles/173129/15_1-345370.jpg To this: https://www.AR15.Com/media/mediaFiles/173129/15_2-345371.jpg The Benning incident that clued me in to the gas tube having that much influence on a barrel stemmed from the 20 ( or whatever...Ranger, help me out ) to 600 meter pop up range qualifications in Infantry School back in the 80s. The day I had to qualify it was sometime in December or January and it was raining cats and dogs. My qual rifle was an old...old...old....XM16E1 with triangular hand guards. Everyone was wet as could be and everything we had was soaked. It was about 40 or 45 degrees out. I'm in my foxhole when the targets start popping up and I take aim and squeeze the trigger on my first shot and POW! The water just EXPLODED off of the gas tube. It literally superheated right off the tube. Sooooo....for every shot I took I would have to raise up my head and BLOW the steam away ( the rain was comnig down that hard ) each time. I still managed a 35/40 score.....but I was pissed....I would have had expert had it not been for my bobbing and weaving above the sights with each shot. It made me realize how incredibly HOT a gas tube gets with one shot....and also HOW FAST IT COOLS BETWEEN SHOTS. The weirdness in my groups with my Mega was VERY indicative of something reacting to temperature over time. Changing the block and tube fixed the issue immediately. The key thing HERE is that when I check the original alignment of my ports, the gas block was .037 thousandths of an INCH forward of being in true position over the barrel port in the FORWARD direction. Sooooo, I milled the block BACK .037 INCH. In my job, I work with SI Units (metric) and I think in MICRONS. Can you see where I got into trouble. To add insult to injury I was using a straight Wilson Gas tube along with the Wilson block. I fucking Hubble Space Telescoped my gun. Add the possible over length of the straight gas tube with with over length (almost 40 thousandths ) of the block position and who KNOWS how over long I had that system. I never thought how proprietary their parts might be versus Syrac or Odin Works. Lemme tell ya...I learned my lesson and learned it well. THE MORAL OF THE STORY: When attempting to align the block and barrel port positions to "true" make sure you aren't setting your gas tube too far back. I have learned you can reshape and remove a bit off of the ball end of the tube....enough to prove my theory which incited me to replace the gas system....and the gun will run. Your best bet though is to shim forward a bit but try to keep from "milling backward" if you can. I will add some more gas tube/key info. My Craddock barreled build shot fairly well at 100 but wasn’t great 1”-1 1/4” with 77 otm’s. When I took it to 600 it went to shooting patterns. I cleaned it well after that session and started going through my checklist. One item on my list is testing the gas key/tube interface. With the bolt out I slid the carrier in and CRASH! The key hit the tube and wanted to stop. A little pressure and it slid over the tube. Not good. I pulled the hand guard and loosened the gas block. By moving the block small amounts I could tune out some of the impact but not all. So I began looking at the barrel nut. The nut on this rail is quite long and the gas tube passes through one of 4 ports in the nut. The nut is also thick for this hand guard designed for a can to ride inside. My next observation was the angle of the gas’s tube as it passed through the port. I didn’t measure it but could tell it was steeper than it should be. When I pulled the gas tube the wear on top of the tube from impacting the key confirmed my suspicions. With this info in hand I went to my local shop and found a new hand guard that had a barrel nut the gas tube passes over vs through. With that installed the key/tube interface was greatly improved. This morning I shot my last 14 rounds of Berger 77 OTM Tacticals loaded over Varget. Attached File I think this can be improved upon. My next steps will be to try a new gas tube and to check the gas key alignment. If anyone know the size and a source for a gas key pin to check alignment please let me know. |
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Originally Posted By SpeyRod: If anyone know the size and a source for a gas key pin to check alignment please let me know. |
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On new builds I lay the barreled upper upside down and slide the carrier, without the bolt, back and forth to check the interface with the gas tube. I want the carrier to mate around the tube with no contact, either on the sides or bottoming out in the back. if it catches on the tube, I can usually bend the tube in the right direction with a bit of pressure back where the tube exits into the reciever. and yes, I do have to leave a millimeter or so of space between the back of the gas block and the front of the barrel journal to account for the missing handguard cap on most barrels |
Don't be so open-minded that your brains fall out.
General education should not be mere job training, but training in how to be fully human.
General education should not be mere job training, but training in how to be fully human.
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Originally Posted By November5: https://www.brownells.com/tools-cleaning/bore-barrel-tools/gun-barrel-tools/barrel-nut-alignment-gauge/ Thanks! Ordered. Very curious to see how the alignment is. |
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Update on the gas tube alignment issue. I received the alignment gauge from Brownells. I stripped the upper down to the installed gas block and tube. With the gauge installed in the gas key I inserted the bolt and found the tube was still high by @ 1/16th +-. Next I pulled the gas’s block and tube to check the key alignment with the upper. It was perfect. Now it was down to the gas tube as the issue. With it reinstalled it was definitely hitting the front of the barrel nut which was changing the angle of entry in the upper. I don’t have an extra tube so my option was to adjust the bend of the tube to “tune” it. Before doing that I traced the tube on paper to track the change. After some judicious bending I reinserted the block and tube. All I can say is I got very lucky and the alignment was spot on. In the photo below it looks a little off side to side but that is due to the bold. Floating in tolerances. Attached File Upper trace is pre bending, lower after bending. The beginning trace was shows almost no bend in the tube. Attached File Now the only question is if I am going to blow the gas’s tube out because I bent it. We shall see. Thoughts? |
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you're fine. As I said, I bend gas tubes into place all the time. Your original tube with no bend is odd. All gas tubes I have seen are cantilevered like your final version |
Don't be so open-minded that your brains fall out.
General education should not be mere job training, but training in how to be fully human.
General education should not be mere job training, but training in how to be fully human.