I've got a new build I completed a couple weeks ago that I'm having some issues with. It will not run consistently. It will always fire the first round loaded into the chamber. Most of the time that round will be ejected without issue. Sometimes it will pick up the next round, and sometimes it will not. If it does grab the next round, it has always failed to fire the round. It will leave a tiny mark on the primer where it tried to fire it. I have had a couple failure to ejects as well. I have tried multiple types of ammo and multiple different mags that work fine in my other rifles. I've checked the gas tube and block and seem to be lined up perfectly, so I don't think that is the issue. (I've assembled many ARs before with no issue.) Any thoughts ideas????? Thanks.

16" 5.56 1:7 mid length barrel
Spikes gas tube
Vltor low profile gas block
Mil-spec bcg
Anderson upper and lower receivers
Anderson LPK
Mil-spec buffer tube and 3.0 buffer

Parts details?

Check to see if gas block is on straight and in the correct position

Can you cycle rounds by hand every time by pulling the charging handle back? When you do so does the BCG go all the way forward into battery or does it stop short. Push on the forward assist, does the BCG move forward at all?

What are you using for lubrication?

Sounds like short stroking.   Will it lock back on an empty mag? Will it lock back if you hold the catch in?   Light mark on the primer happens when the round is chambered.

is that buffer heavy enough? assuming 3.0 means an H3 buffer. why?
frequently it can be the ammo itself.

It cycles no problem by hand. Just ran 10 rds through by hand without issue. BCG goes all the way forward like it's supposed too. Not much luve on it, just a fan of frog lube.

I'm running a standard 3.0 oz buffer. Nothing heavy. I've tried Indepence 5.56, Winchester 5.56, and some Perfecta 223. All did the same thing.

Load it w/ one round in the magazine.  Fire it.  Does the bolt lock back?  What you're describing is classic for short stroking.  In the two cases I've seen, the first was an undersized gas port (rare), and the second a gas block that was partially blocking the port on the barrel.  It was in the right place from side to side, but was too far forward.

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So by your description you have tried different ammo.

If you have another AR15 you could use pieces from that to try and diagnose your trouble. First off make sure you lube it real good.

I would swap that upper onto a different lower. By switching lowers you know which half of the gun is at fault.

Not long ago I bought a receiver extension that internally would cause the buffer to bind.  Take out the spring, and drop the buffer into the tube back wards and see if it drop in and out smoothly. if so then yo are good there. Mine would bing or get slightly stuck.

if it is in the upper try swapping out your BCG. I have seen some that leak gas from the gas key.

Try a different magazine too.

If after BCG and magazine are ruled out and you are certain ammo is good you have to remove the gas block and check to see if it is lined up. You will have a carbon ring that will show you. Also I would intall the gas tube into the gas block while it is off the barrel and blow air through. I have seen one tube out spec. causing a restriction.

Lastly what brand barrel are you running? You may have to open up the gas port a little bit.

Thanks for all of the input. I took the gas tube and block off this afternoon and noticed that the hole in the tube and the hole in the gas block do not line up perfectly. The holes were offset by just a hair, but maybe enough. I swapped the tube out with another that I had laying around. Maybe that was the issue. Next time I get a chance to go shoot it I will take my other AR and swap some stuff around if it still has the issue.

If you have another BCG laying around, throw that it there and see if it'll run.

Also when you reinstall it, try to see if leaving a small gap between the barrel shoulder and gas block helps. They were originally made with the handguard cap there, and if you butt the gas block against the shoulder it can cause it to misalign the gas port just enough to give it issues cycling.

Froglube

I knew it.

Take that shit off and use oil, it will likely run just fine.

Seriously. I have had severe problems with Froglube, so did my son.

All, I was able to figure out the issue. I checked the gas block/tube and everything was setup just fine. Swapped out the bcg with a spikes that I have in another rifle and it ran flawlessly. Won't be buying another Anderson bcg after that experience!

I have an Anderson bcg in the 300blk and a buddy assembled a 5.56 with all Anderson parts that function flawlessly.

If the rifle is a fresh build and you did nothing to mate in any of the parts that slide against each other, then if you use less than dripping wet lube in the bcg and upper, and putting a bcg in that has had hundreds of rounds on it resolves the symptoms, then the problem is most likely that the Anderson bcg is still a little stiff and hasn't broken in yet.  Machined surfaces are rough no matter how smooth they look to the eye, and the first few hundred cycles (I did mine dry by hand with the charging handle) mate in all the wear surfaces by knocking off the high spots.  

If you're using some kind of weird lube (I don't buy into the Froglube, etc noise.  I was Hoppes 9 until now, it's Mobil 1 which I buy in 5qt jugs for the 3 cars until I hear a better solution) and it's in the first couple hundred rounds, and the bcg wasn't dripping with lube before you started, I wouldn't write off a $90 bcg.

Spring missing from disconnector.

Vince

Never had to do that with a bunch of different BCM uppers/BCGs. Nor PSA Premiums. Nor Del-Ton.

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I believe you.  I have assembled exactly two rifles and helped my buddy assemble one.  I'm a newbie.  But these things are way less complex than all the motorcycle and car engines I've worked on.  If you have a brand new set of metal parts sliding around they need to mate in before they really get put to use.  Couple rough finished surfaces with crappy lube and there's no way the first 100-200 rounds are going to work as well as they should, especially with crappy lube.  My race bike machinist/mechanic and I had long discussions about this stuff.  I used that 20 year old thought process and ended up with two perfectly functioning rifles, which include the same part the op is complaining about.