Barrel nut should be installed so the top void between the two spines is dead center of the upper receiver gas tube channel. Hence use a drill bit the size of the upper receiver gas tube channel to confirm that voids are TDC for both the barrel nut, as well as the delta assembly

On the delta assembly once you have confirmed that the barrel nut top spines void is indexed correctly, slip the gas tube  through the delta ring slot to keep its slot TDC, slip the delta spring around until it slot is TDC, then slip the C clip around until void is TDC to allow the gas tube pass through all three, as well into the upper receiver gas tube channel; which will allow you to install the gas tube in the FSB; with the gas tube still floating in both the delta, and the upper receiver gas tube channel.



Now with the Gas tube installed, remove the bolt from the carrier and with just the carrier with key, dry fit this in the upper receiver to the gas tube (gas tube enters the key when the face of the carrier is about 1" back from the touching the face of the barrel extension).   If the gas tube needs to be tweaked to align with the carrier key, bend/tweak the gas tube over the center of the barrel; not the end of the gas tube that is protruding into the upper receiver.

If you are working with a gas block, install the gas tube to it, and measure from the barrel shoulder edge to the center of the gas port, then the back of the gas block to the center of the gas passage in it, to determine if the gas block should be tight to the barrel shoulder, or if you should leave a gap between the barrel shoulder and gas block for say a front hand guard clip that may not be in play.  Now mark the center of barrel gas port on the barrel shoulder, and using the center of the gas tube, install the gas block on the barrel to align the block gas passage with the center of the barrel gas port.   Now you can break out the carrier with key to check the alignment and if needed tweak the gas tube over the center of the barrel to align the end of the gas tube with the carrier key.
Note, do not start loosening and side canting the gas block on the barrel to align the end of the tube with the carrier key.  all this will do is end up blocking the flow of gas out of the gas port into the  block passage.

Bottom line, seems you are not tweaking the gas tube over the middle of the barrel to get the correct key to gas tube alignment.

Yes, gas tube is easy to tweak over the center of the barrel with your hands to get the end of it aligned cleanly with the carrier key.

Just make sure that the gas tube is floating through the delta and upper receiver gas tube, and not binding up on something in line there.


Hence we tweak the gas tube to align with the works cold, but still need it to float as the gas tube heats up and slightly distorts, so it still entering the taper on the key correctly as well.

Don't snug the tube down, let is free float through the center of everything once you have it correctly indexed with the carrier key.

This allows the slop of the carrier in the upper receiver channel, and even the entire tube slightly distorting when it heats up, to still allow the end of the tube to find the entry of the key.

Granted that you may be on a semi only rifle, but in a full auto rifle where you are glowing the tube from time to time, it's real important to have the entire tube free floating with no binds isntead.

Bluntly, the DI system works very well over a gas piston system, since unlike the gas piston system that flexes the barrel/muzzle downward at full bolt unlock, the flex in the gas tube as it free floating only has the slight tension on the front of the barrel downward until the bolt starts to unlock, the end of the tube clears the key and the barrel spends much less time being forced downward instead.
Note, both are a very small window time frame with barrel disturbance, with the time the bullet passes the gas port until it leaves the barrel, but with the gas tube floating/not bound up against anything, this allow for the slight flex of the tube before the bullet leaves the barrel; with less barrel harmonics disturbance.  Also to note, the DI system has the carrier pushing back off the bolt to unlock itself, while the piston system has the carrier being pushed off the front of the gas block isntead.


To really understand the difference, have to think in the terms of full auto AK-47 (milled receiver), verses the stamp receivers of the AKM's.  With the sold milled receiver, the rifle holds pretty steady since the receiver takes the force of the piston off the gas block on the barrel with no receiver flex, while the AKM with the receiver flex, you get a lot of receiver movement transferring back into the entire gun instead (think sloppy sewing machine when you used to play race car on your mom's machine).  
If you pull the retarder on the AKM to mac jack it, it almost feels like the receiver is trying to snap in half during a mag dump with the amount of receiver flex that is increased with the faster cycle rate.