Step one.

Remove the bolt from the carrier, and dry fit the carrier with key into the upper receiver.  The key should not be binding in the upper slot or U slot of the charging handle, and when the face of the carrier is about a inch off the face of the barrel extension, the gas tube should enter the key cleanly.  How cleanly, with the carrier up against the face of the barrel extension, a light shake should be all that is needed to have the carrier walk back to the full rear position with the muzzle upwards.  If the gas tube needs to be indexed tweaked to align with the carrier key, tweak the gas tube over the center of the barrel.  Also to note, the end of the gas tube should be loose in the upper receiver.  If it's rock solid tight in the upper receiver gas tube channel, then you did not index the barrel nut top spine/top channel with the upper receiver gas tube channel and this is what is causing the bind of the gas tube in the channel.   If the gas tube is correctly indexed, but the front of the carrier will not touch the back of the barrel extension, then you got one of the gas tubes that is slightly too long for a normal build.  Hence there are some gas tubes out there that are about a 1/2" too long, and they are barrels that the gas block is placed about a 1/2 forward of a normal barrel.
Or, when you installed the gas tube in the FSB/Gas block you did not get it all the way in, and have the Gas tube cross pin driven through in front of the tube instead.

Hence the end of the gas tube should look like this against the key releif slot.




Next with the bolt in hand (outside the carrier) shove the Firing pin all the way forward, then break out a caliper to measure the firing pin protrusion out the bolt face.  The correct protrusion should be .028~.032".


Last test with the B/C, install the bolt without the cam, push the bolt all the way back, and when you go to pull the bolt forward, the only friction you should have is the rings only.  If the tail of the bolt is binding in the carrier channel for it, post a photo of your bolt tail.

Last test, bolt back in the carrier, install it into the upper and with your finger in the back of the carrier, walk it all the way forward and back.  When the bolt goes to cam lock and unlock will be slight friction, but if you feel the bolt lugs binging entering and exiting the barrel lugs themselves, then double check the barrel install into the upper receiver, being that the bottom top lug of the barrel extension is dead center on the gas tube channel/center line on the front take down lug/ hence the barrel did not slip in the upper receiver barrel socket when you tightened the barrel nut (with the barrel now rotated in the upper receiver instead).

Now lets move onto the FCG, and make sure that you have all the spring installed correctly , including the hammer spring with is legs sitting on top of the trigger pin.



Now pull the extractor off the bolt, and if you find a O ring or D fender around the extractor spring, remove it and place it some where for save keeping.   Hence most bolts now come with an extra tension recoil spring, and when you add the O ring as well, way to much tension on the extractor, and it causes problems with the extractor being able to climb over the case rim at loading, causing the bolt to not fully lock up correctly.  Hence when the bolt is not fully locked up face of the carrier tight against the back of the barrel extension, the back of the tail is not outside the back of the carrier's channel for it, and when the hammer comes down, it does not drive the FP first out the bolt.  What happens is the FP is driving into the back of the carrier, which now drives the carrier forward to lock up the bolt/get the carrier tight against the back of the barrel extension, and what energy is left after, push the FP into the primer without the needed energy to ignite the primer instead.

Also with the B/C back together, hook a spent case on the extractor and cam the ejector into it bolt face channel a few times. The ejector should be under major spring tension, but the ejector should not be binding up as it moving in and out of it bolt face channel.


As for the last of the  run down, make sure to clean the rifle correctly, including using a AR chamber brush with CLP by hand to scrub the chamber during cleaning, that the upper bearing areas are CLP lubed before you put the rifle into service. Also when you go to charging the rifle, DO NOT WALK THE CHARGING HANDLE BACK FOWARD!!!  
Either lock the bolt back and hit the bolt release button to allow the bolt to charge forward full force for a clean mag strip and bolt lock, or if you want to use the charging handle, then pull it all the way back and let it go.


As for brand new rifle never been fired, only a single round is loaded into a mag, it charged with the empty mag in the well, then the rifle fired to make sure that the bolt locked back on the mag catch (catch in front of the bolt and not just under the carrier holding the B/C back that way instead.

If the bolt locked back on the single round, then next two rounds are loaded, and them fired once by one with the rifle self loading for the second shot.  Why only two round, we want to make sure that the rifle is loading the second round fine, and that is there is a Problem with the FCG and you get a run away with the rifle firing the second round as soon as the bolt locks up, it only two round that get dumped; and not an either mag isntead.

At this point and the rifle past both above tests, then you fine to load a full mag afterwards.

So to sum it up, with the round not firing, the bolt is not fully locking up at loading (face of the carrier tight against the back of the barrel extension for the reason give above), or you installed the hammer spring incorrectly instead.

Easy to check the gas tube, hence use a piece of rubber tubing on the end of it inside the receiver, and blow about 100lbs of air through it.  You should be getting a clean flow of air out of the gas port to the barrel bore. Also to point out, with the reduction in gas port size from the gas tube ID, if you put some CLP around the FSB to barrel and even gas tube to barrel, will have enough back pressure to see if you have gas leaks between the block to barrel and gas tube.

Short of piston B/C that a rod will push the carrier back to unlock the bolt, what DI B/C for a gas tube use type B/C does not use gas rings for the bolt?????  
If you mean that you are using a Mcfarland gas ring instead of the 3- C type gas rings, then keep in mind that the Mcfarland gas ring can be tight as hell in the carrier until it mates in, and can cause some problems as well.

If you are using a piston type B/C, with a gas tube, then here is your problem instead.

Bolt catch is catching the bottom of the carrier, and not the the front of the bolt instead.

Quick double check on just pulling the charging handle all the way back,  last few inches of the CH should not increase in tension, and face of the bolt should stop about 1/8" to 1/4" in front of the back edge of the ejection port.  Also to point out, with empty mag in the well, catch should rise up and when you release the charging handle, bolt should be retained back via the bolt catch on the face of the bolt.

If the above it fine, then suspect that either chamber/upper fouled to the point of causing problems, or gas tube to gas block/ gas block to barrel leaking, key to top of carrier leaking isntead.

On the Gas block/FSB, CLP around the tube to block, and block to barrel seams front and back, and via  piece of rubber tubing, pressurize the gas tube to about 100lb of air via air compressor with blow tip.   If you have leaks, you going to see it pretty fast.

On the key to carrier, CLP around the bottom of the key to carrier, hold the bolt inwards, then 100lbs of compressed air directly in to the front of the key to check for leaks between the key to carrier. Or, since you do have a working B/C in another rifle, put it into play on the problem rifle to confirm that is a carrier leak on the question Carrier instead.

As for fixing a key leak, pull the key bolts and throw them away.  Now using a lapping plate and 400 sand paper, lap the bottom of the key straight. Now use the straight key bottom and lap it into the top of the carrier next.  Both surfaces cleaned up, few drops of loctite to ask as a gasket when it dries between two, install the key with new key bolt torqued to 37 in lbs ( 3.08ftlbs), then peen metal from key, into the side spines of the key bolt heads.