It appears that the port in the gas block is in line with the forward set-screw hole, so I turned the block upside down on the barrel and visually centered the set-screw hole over the barrel gas port - this seemed to leave a gap between the block and the shoulder of .010" as measured by feeler gauge.  Having penciled a line centered on the long axis of the block face, I then penciled a corresponding mark on the barrel.  I then rotated the block 180 degrees, set the .010" gap and tightened it down (after re-attaching the gas tube).  Having said that, prior and subsequent removals of the block on all three of my ARs, even when just set by eye (gas tube parallel to barrel axis), show a gas stain around the barrel port fairly well centered and with plenty of margin, implying no alignment-induced occlusion.  Unless the gas "jet" from the barrel port is sensitive to the shape/alignment of the much larger downstream ports, I don't see that as an issue.

I am not familiar with the fit of handguard caps - all of my ar's are full-float.  How can I tell and what would it affect?  I assumed if the barrel did not come with an A2 front sight, it was not meant to use handguard caps/delta rings.

try setting the gasblock forward a hair, about the width of a hand guard cap, tighten the farthest screw and lightly tighten the port screw, see if it helps.

At this point, I game to try just about anything, but I'm curious what your thinking is on this.

The large port in the gas block may seem big, but it's not going to be able to overcome a substantial discrepancy in alignment with the barrel's gas port.  

I believe the OP said that he'd put the gas block on upside down to verify alignment with the barrel port - and noticed that the block was centered on the barrel's port only if it were about 1/10" forward of the barrel's shoulder.  OP, that 1/10" appears to be very much in line with "enough space for a handguard cap.  Make sure you allow that much space forward of the shoulder when you set things up.  That little space can make a HUGE difference in how much gas actually gets into the gas tube.  

And, OP, I have a few questions about that block.  First, does the setscrew hole line up perfectly with the block's gas port?  Second, if you were to put a small rod through the setscrew hole and into the block's gas port, does that rod appear to be centered on the block's body?  (In other words, can you prove for yourself that the port and screw are properly aligned in the block itself?)  

I'd mark a line on the block for the axis of setscrew/gas port, and one on top of the barrel along its axis and through the barrel's port, to allow for easier alignment by eye.

Here's hoping you get it sorted out and have some fun at the range with this gun!

Not 1/10" but  .010" or 1/100".  And as I said, the gas stain around the barrel port shows no overlap with the block port and plenty of room for greater misalignment before overlap occurs - does this change your recommendation?

Ok - making some headway after the following testing:

       Proven gun with new barrel - fired two rds, ejected at 2:30 and chambered next rd (slightly over-gassed)

       Next, swapped in new gas block/tube assembly - fired 1 rd, ejected at 4:00 and chambered next rd.  (neutral gassed)

       Swapped in new BCG - fired 1 rd, still ejected at 4:00, chambered next rd.

       Swapped in new lower - fired 1, ejected at 3:00, next rd NOT chambered, carrier not locked on empty mag

       Went back to old lower, fired 2, ejected at 4:00 and locked back

My take:  new barrel and BCG are fine, new gas block/tube don't deliver gas well (leak? bad flow?)  new lower buffer system is too "stiff" - which may or may not "break in".  I think I will get a new gas block/tube and look into another buffer spring (CS flatwire spring?).  

What say you?

I misread; sorry.  1/100" isn't enough to mess up alignment, but I'd still ensure that the port on the block was properly centered, and make alignment marks so you get everything as perfectly aligned as possible.

The only CS flatwire action springs I know of are substantially stronger than standard springs, so if you're trying to lower rearward resistance, I'm not sure that's the direction to go.

From Tubb's video (https://www.youtube.com/watch?v=8fMVZbrnvu8), the spring force is similar (or even greater) with the bolt closed, but LESS (compared to regular springs) at the locked-back position with this type of spring (spring rate, free length, number of coils, material, etc. achieve this).  I don't know if that aspect of this type of spring would help or hurt my situation, but if one believes the claim that cs springs provide additional reliability due to more consistent performance over an extended spring life, it might be worth a try.  I know regular springs are cheap and easy to change, but I prefer to minimize the chance of unexpected failure - even if it costs a little more.  It might make sense even if I had to increase gassing to make it function.  It wouldn't cost too much to try it out.

You mentioned having a bit of trouble unlocking the bolt with the charging handle? Look closely and verify if there is any rotational misalignment between the bolt and barrel extension. In other words, the barrel extension could be rotated just a tad.

That issue seems to have gone away with use - I did see signs of light rubbing between the bolt face and the chamber face, but I don't think it is a problem.  The feed ramp extensions seem to line up with the ramps in the upper - does that mean rotational alignment is ok?

Sounds like all that's ok.

You mentioned in the beginning of the thread that you felt some stickiness or resistance when manually cycling the action.  As I said before, I still think there is a chance you have some kind of issue in the receiver extension tube.  In other words the buffer/spring are not moving smoothly over the their entire range.  You' now say you've narrowed it down to the lower, so this would explain the symptoms.   But I still find it strange that TWO rifles would both do this, unless it's due to some defective or out of tolerance part.

I agree that an alignment issue with the receiver extensions could be helping make things sticky, and that it's odd that two guns would have this same problem.

A new gun, whether from a factory of home built, needs to be really well lubricated for the first several hundred rounds.  The FCG, the BCG, the receivers, everything.  This "well lubricated" thing is AFTER you clean the living crap out of the new parts.  New parts and newly made guns ship with preservative coatings, NOT gun lube, and those coatings are not really good at all for running the gun.

Once you get to the "magic number" of rounds for that gun down range, you'll notice a difference in how the gun runs.  You may notice it from one range session to the next, or even during a range session.  But you can even notice a difference after even just a few rounds if you pay attention.  The combination of proper lube for the "new" condition, and the parts "wearing in" together, becoming smoother as surfaces rub on each other, can be pretty dramatic.  You can't hand-cycle a gun to break it in, though that will help identify where you need to lube.  But just shooting it makes a big difference.

The sticky action seems to have gone away, and just based on my previous build, was normal for a new gun.  The lower issue seems to be more one of simply a stronger spring, but since it is supposedly a "standard" weight unit, I just assume it is because it is newer - the reading I have been doing indicates the milspec spring does not do a very good job of maintaining a consistent rate over time.  So I don't necessarily blame the lower, it's just that my possibly sub-par gas system along with fresh moving parts can't handle a "fresh" spring.  And while further break-in may get it to cycle, I'm not comfortable with what may be marginal reliability that depends on factors that can change under different circumstances - I ordered a new block from BTE to try to eliminate that as an issue.  I'm also ordering a CS flat-wire buffer spring, but that is more for longevity - if the new gas parts and more lube/break-in don't achieve good results, I'll open up the gas port some more.

I did clean and lube the action parts (breakfree clp), but forgot to lube the FCG after wiping it down - I hoped that could wait until the first after-use cleaning.  As far as break-in, my prior (first) build did exhibit similar symptoms at first, but only for a few rounds (less than one 10rd mag).  I know some guns might take longer, but there did not seem to be ANY improvement in cycling and I felt like I was just wasting ammo.  I would rather address possible marginal components and do some polishing/cleaning/lubing to shorten break-in and improve future reliability.   But your point is well taken.

I recently did a build with low-end parts. I did lots of gas system trouble shooting to no avail. I finally swapped the spring. That cured 90% of the problems.

What did you swap from and to?  What were the problems that were fixed?  

I switched from a no-name, likely Chinese spring to a standard carbine spring from my local gun store that sells good parts. Failure to cycle, eject, and lock back were fixed. The rifle gets better with use.  Tula still gives it some problems.  I might throw a reduced power spring in it or change the buffer just to see what happens. Springs and buffers are cheaper than time spent disassembling and re-assembling uppers (so are gas blocks and tubes).

That sounds just like what I'm experiencing.  I think I'm on track to to get it squared away, but the "cheap" parts ended up costing me more than just getting quality parts the first time.  I put a little too much faith in "milspec" being good enough - it was a good learning experience though!

The learning is worth a lot, at least to me. I built my cheapo to find out how deeply corners can be cut and still get a good rifle. Below are some lessons I learned. Much of this will be obvious to the experienced folks, but it might help some folks on first or second builds.

1. Check that your gas block and tube are clear as soon as you get them.
2. As soon as you have both your block and barrel, measure the distance between the gas port and the shoulder on your barrel and the hole in your gas block and the edge of the block to find out whether the block should be set off the shoulder on the barrel or flush up against it.
3. Measure you gas port as soon as you get your barrel.
4. Buffer springs seriously impact a rifle's function.
5. The very cheapest parts are risky. The bottom end of the mid-priced parts seems to be where you can find some value and minimize headaches.
6. I'll probably put a front-adjustable gas block on my next build and bore out the gas port on my barrel to something near the larger end of spec so controlling the gas flow will be really easy.
7. If your rifle isn't working right, and you have suspect parts, consider just replacing the suspect parts instead of tinkering around troubleshooting. Start with the cheapest parts first. For example, I had questions about my buffer spring from the get-go. It seemed stiff just handling it. I probably could have saved three hours of time and $20 worth of ammo by buying an $8.00 spring. Your worst case outcome with this strategy is having a few spare parts.

That sounds like excellent advice!  I'm gonna print that out and keep it for next time