Lets get this straight from the start, and the heaver the buffer on a correct stroke, the more felt recoil the rifle is going to have. Why, the buffer is going to crash into the back void wall of the receiver extension to cause the dead blow effect stall of the buffer at back of stroke, and is really the recoil that your feeling in a 223 AR rig.
Note, correct stroke is that the bolt will lock back on the bolt catch (catch in front of the bolt face) when the last round is fired out of the mag.

So why do I bring this up, the harder the felt recoil/the jarring of the buffer off the back of the receiver extension, the more is going to effect the mag in the mag well, and the rounds in the mag.

So lets work this backwards.
First make dam sure that the tip of the mag catch threads is flush to the mag catch button face.  This at least has the mag catch retaining the mag in the well the best it can.

Next, we have a can on the rig, which will cause the bolt to unlock faster than normal (more gas pressure through the gas port from back pressure of can), and we need to bring that bolt unlock back into check.

So if the solution is to go to a H buffer, then we just increased felt recoil/ the whole mag jarring problem.  So instead of slapping in a heaver buffer to increase the Jarring effect, you could go with a increased recoil tension to slow the bolt unlock instead. Or in some causes if we need more than just a stronger tension recoil spring, just go to a adjustable gas system to reduce the amount of gas that is going through the gas system to get the bolt unlock back in check.

As for mag, even brand new mags should be pulled apart to CLP clean all the parts.  Not only does this allow you to clean any derbies out of the mags from assembly, but allows you to check the mag follower skirts for molding edges that may be casing the mag follow to bind up on the body.

Next, and stop trying to use the mag as either a bipod leg, or a hand grab point.  
Both will cause the mag to cant in the mag well, and will cause nothing but problems.

From here, service life on a  springs is around 3K rounds, and since you have 2.5K on the rig now, problem could be just weaker springs, that is allowing the bolt to unlock faster over just the can.  Take hammer spring, and it not only to give the hammer enough force to strike the FP, is part of the math on the unlock like the recoil and buffer weight as well.

So short of a mag that you can rasp the bottom of it off a table and the mag fountains the live rounds out from a problem in the mag, should have you covered on resolving the double live feed from the rig.  Also, when loading a mag correctly (enough room in the top round to push it down for almost 2/3 of the way of loading another rough, make sure to rasp the back of the mag off the back of your hand to seat all the rounds to the back of the mag.  During recoil, the rounds will be kept to the rear, leaving enough room from the tip of the bullet to front of mag to not cause stack rise problems.

And since I brought up the gap between tip of round to front of mag, make dam sure your not long loading the rounds to long, and they don't fit correctly in the mags to begin with.

Stranger things have happened... you "might" be on the ragged edge of limp-wristing your AR.  Cold, possible cold and gummy lube, and a less then sturdy grip.

I read about one guys 308 AR not feeding right... he keep sending it in... manufacturer kept sending it back with and A-Ok rating. Turned out the guy was 170lbs.. the tester was 230lbs.

So as "luck" would have it... the owners "stance" ( ? ) was a contributing factor to the issue short stroking issue.

Kind of a tolerance staking.

Not saying yours is this issue for sure... but I would certainly test fire again... with a tight grip, and less lube and clean BCG.

Looking forward to the results.

That shite will get you killed in Country!!!!!

At the 300 round counts on a M-16, you need to at least pull back on the charging handle enough to add some fresh CLP into the carrier gas ports to flow it to the back carrier gas sections (use the FA to lock the bolt back home on that live round), so as you continue to fire the weapon in adverse condition, the Fresh CLP will migrate through the rifle so the cleaning agent in the CLP can dissolve any fouling that is trying to build up and choke the rifle out.  Throw a can into the works with high back pressure, and would step the added CLP down to around the 200 round count instead.

Once you do get out of the fire fight, them break the weapon down to clean and relube it correctly, including using a chamber brush with CLP to clean the chamber.

Hell, with your round counts, would not even take a Pig, Duece or M-240 that long without a barrel changes, much less breaking out the LSA  on the second barrel change. M-249 maybe for that round count, but would still be lubed along the way.