Lots to know and terminology that needs to be explained. Forgive these guys, they have forgotten what it's like to be duffers like you and I.
To start with, there is a metal collar around the barrel just under the front end of your handguard. This is the gas block. It is mounted over a hole drilled in the top of the barrel. Gas escapes the barrel here and is captured by the gas block. Therefore the gas block must exactly line up with the gas hole in the barrel.
The gas is passed back from the gas block via a thin tube along the top of the barrel under the hand guard to the bolt carrier. The gas tube connects to the bolt carrier via a gas key mounted on top the carrier.
When the gas enters the bolt carrier, the pressure pushes the bolt forward in the carrier like a piston (the rings on the rear of the bolt provide a gas seal just like the rings on a piston in your car engine). The bolt carrier moves backwards and a cam pin rotates the bolt which then unlocks it from the lugs in the rear of the barrel extension. The bolt is then free to move back with the bolt carrier and the spent shell case is ejected.
The bolt carrier then moves forward pushed by the buffer spring inside the stock. The bolt carrier and bolt strikes the rear of the next shell in the magazine pushing it forward into the chamber, the bolt enters the rear of the barrel extension, the bolt is compressed back into the bolt carrier, and the cam pin causes the bolt to rotate which locks the lugs together in the barrel extension ready for the next shot.
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OK... that being said, what is most likely happening is that the bolt carrier may not be moving far enough to the rear to allow it to strike the back of the next shell in the magazine so as to properly push it into the chamber. The reason the bolt carrier isn't moving far enough to the rear is either that it is not getting enough gas pressure or that the buffer spring is too strong or is binding back inside the stock. Forget about the ammo being at fault here. Any commercial ammo, 5.56 or .223, will create plenty of pressure. If not, there is something else amiss.
I doubt the buffer spring is too strong, but who knows... or maybe the buffer weights are too heavy, I doubt it unless Bubba has been at work. It could be binding inside the buffer tube if something is installed poorly.
If it's not getting enough gas pressure it can be because the gas block is not properly aligned with the gas hole in the top of the barrel, the gas block isn't sealing against the barrel, or the gas tube is out of alignment.
If the gas tube isn't aligned properly the gas tube and the gas key may not be fitting together properly. If this is the case there may be damage to the end of the gas tube and/or the opening on top of the gas key.
I notice you have a free float quadrail handguard with a forward grip mounted on it. The handguard mounts to the barrel nut (the nut that fastens the barrel to the receiver). The barrel nut has grooves cut around it, or holes drilled in it through which the gas tube passes. If this barrel nut gets turned ever so slightly out of alignment, it will force the gas tube out of alignment with the gas key. Some of these types of handguards are notorious for causing the barrel nut to turn, and this can be greatly aggravated with a forward hand grip. It's like having a wrench attached to turn a barrel nut that may not have been properly torqued down.
If such is the problem sometimes the bolt will scrape the top of the shell dragging it out of the magazine only to be pinched between the bolt and the barrel extension. But more often you should have failures of shells not getting extracted from the magazine at all.
If this isn't the cause it would most likely be feed problems with a faulty magazine. The bad magazine is the most common cause of feed problems, but you have sort of eliminated this. The other cause could be a problem with the feed ramps in the bottom of the barrel extensions. Most ARs these days have what are called M4 feed ramps (two alongside each other) in the upper receiver and in the barrel extension. These may be poorly formed or be missing in either the receiver or barrel extension. You can see these by removing the upper and shining a flashlight up under the receiver (look at the tip of the gas tube while you are in there).
In any case, unless you are set on learning a whole lot about your AR, a professional should be consulted. If it's a new rifle you should contact the manufacturer. But in either case knowing what's probably wrong with it up front will give you an advantage. With even a few shots fired through it you should be able to see any gas leaks in the form of soot. This will even tell you if the gas block is not aligned properly on the barrel when it is removed... don't ask me how I know. :banghead:
