The AR does not use a control feed, meaning that the back of the round is pushed out of the mag with the mag lips guiding the round into the chamber, then during the last of the bolt entering the barrel lugs, the extractor has to snap over the rim of the round.


So on that note, if you have added both a D fender/O ring and an extra tension spring, then right there that is your problem with the extractor tension to great to all the the extractor to snap over.

If the extractor is not the problem, the double check the loads to make sure that they have been sized correctly/ the bullets where not over all long loaded, or that the bullet was over crimped to cause the shoulders to buckle.

Lastly, learn how to use a chamber brush and how to clean and lube  the rifle correctly!!!!!!   The below photo looks like you are dumping media grit into the barrel extension void/the upper is lack of any CLP which would at least dissolved this mess, and  this is going to cause nothing but problems.

Does the bolt carrier assembly go all the way forward and lock into battery with no round in the chamber?  It is hard to tell from the photo angle, but it could possibly be that your gas tube is protruding rearward too far into the charging handle track of the receiver, if this is the situation then do you have too long of a gas tube (I have seen this many times with mid-length gas systems, due to variations in mid-length gas tubes are not standardized for length), is the gas tube pinned behind the gas tube roll pin, or if the gas block is the type that will allow an unpinned gas tube to go all the way through it then you may have the gas block on backwards.  The gas tube should only stick back into the charging handle track about 1", if it sticks back too far then it will not allow the bolt carrier assembly to fully travel forward and lock into battery.  

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

If you are not over sprung, and still having problems with the extractor climbing over the rim at loading, post a photo of your extractor.

Some of them have been known to be incorrectly produced; hence the ramp area of the extractor not angled correctly to allow the extractor to glide over the rim at loading.

The AR has always had a forward assist (the detent that is in the carrier to open the ejection port cover when the B/C comes back),  and it was only later in the series the actual button forward assist was added.

So yes, it does come in handy for more than just silent bolt closing, hence on a fire base nasty fire fight when the round count is getting high (mag after mag) between cleanings, and you don't have time to pull the weapon down and clean it.  In this instance, just grab the CLP bottle that you have in your helmet band, dose the every living piss out bolt at the exhaust ports, close the bolt down on a new round and shove it closed either the FA or the detent in the carrier, and let the cleaning agent in the CLP clean the rifle as you are firing it to keep you in the fight.

The O ring is a band aid at most, an should not be needed.
If you are shooting ammo (read Mk262) that will require a extra tension extractor spring due to slight over function when the bolt goes to open up sooner with the spent case still over pressure bound during the pull), then just install an extra tension extractor spring alone.

Short of that, just make sure that the chamber has been chamber brush scrubbed cleaned,  the gas tube is aligned with the carrier key, and the key not width binding in the upper slot by dry fitting the carrier without the bolt  in the upper receiver.  The key should not be binding as you move the carrier/key without bolt front to rear in the upper, and the last inch forward of the carrier in the upper is when the gas tube aligns with the key to feel if these two are out of alignment/binding up as well.

Also for grins, take a peek inside the receiver to make sure that the pawn of the F/A is not protruding in the upper receiver when at rest to catch the the side of the carrier.