In Armorer Courses we teach a simple approach diagnosing issues when a gun isn't working correctly"  If it used to work, and now doesn't, then what has changed?

For process of elimination, start with a thorough cleaning of the barrel, chamber using a chamber brush.  Once clean, lubricate the 7 bolt lugs and rails of the bolt carrier with a quality lubricant like Slip2000 "EWL" Extreme Weapons Lube.  Then try manually cycling rounds through the rifle to see if it works, and if it manually cycles just fine, then try shooting the rifle and see if the problems is gone or is still happening.

I have had 3 rifles (From 2 different LE Agencies) cross my path that used to work, then were having a feeding issue from what I believe is the same as you describe.  What there rifles were doing is that you could load the rifle, locking it into battery, the firing, unlocking and extracting the empty casing, ejecting it, recocking the hammer, but the new live live round would be pushed off the magazine but only go about 1/2 to 2/3rds of the way forward and stop, with the new live round have a long crease on the case.  It didn't make a difference in any of these rifles if it was on the right or left feed ramp, different magazines or ammo didn't make a difference.  I could manually cycle a magazine of live rounds through these rifles with no issues, but when I would fire them they would only fire one live round and the feeding malfunction would occur.

To diagnose the issue, I degreased all parts & ammunition, then coated everything with Dykem metal dye (You could use a sharpie marker), and manually cycled a magazine of rounds through each rifle, and didn't find anything out of normal for friction.  So I went back to the live fire test after recoating everything with Dykem. After firing and having the malfunction occur, what was found to have rubbed off was the bottom of the 2 bolt lugs as the bolt carrier was traveling rearward extracting the empty casing.  So upon closer inspection I found a very slight scuff on the bottom of the collapsible stocks on these rifles (All had M4 type collapsible stocks).  Being that the stocks had a slight scuff on their bottom side, I visually looked at the receiver extensions while they were mounted on the lower receiver and couldn't see any bend or damage.  So I removed the receiver extensions, and compared them to new ones, only to find that in each situation the receiver extension had a very slight upward bend to it (About 1/4" along its entire length).  This bend in the receiver extension was just enough to force the bolt carrier to go back end up and bolt lugs down as the bolt carrier traveled rearward, resulting in the bottom two bolt lugs scraping across the top live round in the magazine and creating a crease that would in turn deform the cartridge just enough so it wouldn't go into the chamber.  In each case we put a new receiver extension (buffer tube) on each rifle, and they went back to running reliably with no more feeding issues present.  Two of these rifles were originally equipped with Sully Stocks, and someone had changed them out for M4 collapsible style stocks of which had been stuck onto something to get bent and cause the malfunction, hence one of the reasons I prefer a fixed stock.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

In all 3 rifles that I speak of, all had a bent receiver extension (buffer tube), where the bottom of the stock had been struck on something that caused the receiver extension to bend upwards (As if you are behind the rifle shooting it).  This upward bend (about 1/4" total) which ran from the front of the receiver extension (buffer tube), to the back.  This malfunction or bend to the receiver extension (buffer tube), had nothing to do with adjusting the position of the collapsible stock on the receiver extension, what it had to do is that the stock had been struck on something which in turn caused things to get bent.  The buffer & action spring (buffer spring) would travel forward and backwards in the receiver extension as normal.  The problem is that the bolt carrier goes back into the receiver extension (buffer tube), and when it's bent, the bolt carrier will get pushed towards whatever direction the bend makes it, and in these rifles the bent receive extension caused the back of the bolt carrier to go upwards as it cycled rearward, and when the back of the bolt carrier went upwards it caused the front of the bolt carrier to go downward of which resulted in the top of the ammunition in the magazine to get creased by the bottom of the bolt lugs.

As for a collapsible stock that has been pinned or fixed into one position for ban states, if you can unpin the stock and remove it from the tube, you might be able to see the ibend along its entire length from front to back (especially if you put it up against another tube or a ruler/yard stick).

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

From what I can see in the videos show a where the empty case was ejected, and the bolt carrier went forward and locked into battery, then the trigger was pressed again allowing the hammer to go forward and strike the firing pin with no bang due to no ammunition was fed off the magazine and chambered.  If you didn't get video of the 2nd round jamming, is there any chance you could post pics of the jammed round, specifically looking for any dings, dents, scracthes, bends in the casing or other marks that would help maybe give an indicator as to where the problem might be?

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

[youtube]https://youtu.be/yH3QiclSHVM[/youtube]
pausing at 00:10 and moving the frame by frame, looks like the bolt face never made it back past the rim of the case in the mag, but even with this, should have stripped the top round out of the mag on the way forward for a bolt over middle of case jam.

Would start by pulling the mags all the way down for a good CLP cleaning.  
When you have a mag body alone, would double check it in the well to see if the mag release assembly is holding the mag all the way up in the the well, hence feed lips should be only about 1/16" from contacting the bottom of the lock home carrier when the mag is locked in the well via the catch.

Since I can see that the mag release button need to be rotated 180* in the receiver socket so the end of threaded section of the mag release button is flushed with the face of the button when the assembly is threaded into it (now a half rotation short of such), would make a pencil mark on the mag release button  for a referance mark on the in for when it reinstalled, and when you have the mag catch assembly out with the empty mag body, would stick the mag body into the well so it tight against the bottom of the carrier, then look inside the mag catch slot on the non ejection side to see where the mag release slot ledge for the body is against the bottom of the channel; as well as taking a look at the mag catch edge of the assembly tab as well.



Hence the tab on the assembly is going to lock the mag into the well, and if the tab is not produced correctly, the assembly is sloppy fitting in the receiver, or even the receiver slot not milled correctly, then the mag is not going to be retained high enough in the well to begin with.

Now back to the short stroking on the rifle, and could be a cleaning/lubing problem with it being done incorrectly/ the wrong solvents in play to cause the rig to foul choke out too fast, or could be a gas leak in the system via the FSB/Gas block or key to carrier that is causing the problem to happen isntead.

Cleaning solvent wise, Something like Sweets copper solvent to just clean the copper out of the actual bore since it does not leave behind a sticky residue like hoppes will. CLP to clean the rest of the rig, including scrubbing the chamber with a Chamber brush with CLP as well.   Once all the Sweets and old fouled CLP used for cleaning have been removed, then you come back with CLP to lube just the upper receiver bearing areas.   The easy way to lube, is a think coat on the buffer and spring before they are reinstalled, then lube the inside and out of the entire B/C assembly, insert it wet into the upper receiver, then empty cycle the actions a few times to migrate the CLP lube off it, to the upper receiver bearing areas.

Also, since I don't have a clue what butt stock is in play, make sure that if a fixed stock, then you do have the correct butt stock bolt in play.  The A-2 stock with end of receiver extension spacer uses a 3/4" bolt than an A-1 stock that does not use the end of tube spacer instead.   Hence when you pull all the way back on the charging handle, the Face of the bolt should stop 1/8" to 1/4" in front of the back edge of the ejection port window (well behind the bolt catch), and the last inch of the charging handle pull (until it will not pull any farther back) should be the same tension as the middle of the pull as well.  If the bolt face is stopping too far forward of the ejection ports back edge of window or the last inch harder than the rest of the pull, then most of the time its the wrong butt stoke bolt in play/ protruding into the back void of the receiver extension to stop the buffer bumper from reaching the back wall of the tube instead.   Other times, if the butt stock is fine, then the hammer tail is wedging against the top of the hammer to cause an last of stroke action bind instead.