Quoted: My RRA 16" is throwing spent cases with marks on the cartridge rim face that concern me.  The marks dig into the brass slightly in a straight line across the lower 1/4 of the rim face.  Some actually result in small points of brass extending out from the edge (caused by the brass being dug out).  The point of brass is generally in the same line as the rim face mark.   I don't have a camera with enough resolution to show them to you. No other problems -- seems to shoot fine and otherwise extract correctly. I am using an MGI buffer with a standard carbine spring.

Your upper is probably over-gassed.
I'll assume since you have an extra heavy buffer and defender that you should not otherwise need that you probably had FTE's in the past.
Upgrading your buffer and increasing the extractor spring strength has fixed the symptoms, but not the problem.
Now you just have a strong enough extraction system to start pulling rims off the cases.
You need to reduce the amount of gas getting into the action.
This could be done several ways, an adjustable gas tube, adjustable gas block or reduce the size of the gas port on the barrel by fitting a bushing.

Quoted: Actually, no.  I did not previously -- to my knowledge -- have extraction problems.  I got the RRB to reduce the recoil and the "d" thing came with it. I have taken the bolt apart and cleaned it well.  I have also removed the "d" thing.  The extractor works much more freely now.  Hopefully, I can get it out the range this weekend and see if it did any good. I think that since the brass is frequently getting mortised with a short burr sticking out from the rim, that it is ocurring on the ejection cycle. That is not to say that I might not have a gas problem.  I've noticed what appears to be some erosion at a small place on the FSB.  Perhaps that is normal...

MGI also makes a very good adjustable gas tube.
If it turns out that you are getting marks from the extractor pulling on the rim, that is a sure sign of excessive gas.
Excessive gas is also why it recoils so much.
Tap less gas into the action and everything gets better.

Quoted: Tell me how...I'm still learning.

YOU ASKED FOR IT...
...so here it is.

Randall's description of AR operation and how everything works in harmony.
I have not written this up in a while and some day I really need to build a whole page dedicated to it.
This is just off the top of my head.
Much of it comes from Rick McDowel (competetion specialties) when I was first learning AR's & from Tweak along the way and my own experiences mixed in along the path to enlightenment.

Ok, starting with a cartridge in the chamber, hammer back.
Trigger lets the hammer fall.
Hammer hits the firing pin, driving it forward.
Firing pin drives the primer (and attached cartridge case) forwards in the chamber until the shoulder in the chamber stops the shoulder on the cartridge case.
This distance could also be called headspace which is the distance from the bolt face to the head of the cartridge when fully seated in the chamber.
The firing pin continues forward to ignite the primer.
Primer flash ignites powder charge, instantly creating great pressure within the cartridge case.
Cartridge case expands first outward towards chamber walls (path of least resistance) where pressure holds the case in place and then the case stretches backward until the case head is stopped against the bolt face.
Here is WHY long headspace makes cases fail!
Bullet begins movement down the barrel, first encountering the throat.
Here is why you want a throat DIAMETER closely matching the bullet.
More about throat dimensions can be found here: www.ar15barrels.com/data/223-556.pdf
Loose throats do not control the bullet and keep it as straight while engraving into the rifling.
Now the bullet has obturated and engraved into the rifling and it's accelerating rapidly down the bore.
As it passes the gas port, gas begins to flow into the gas block where it turns and heads towards the bolt carrier via the gas tube.
The pressure is still high in the barrel, usually 15,00PSI+ until the bullet leaves the muzzle.
Just as the bullet leaves the muzzle, gas escapes around the base of the bullet.
Here is why a proper crown is important.
Gas is traveling about 5x faster than the bullet when it leaves the muzzle.
An even crown releases gas all the way around the bullet at one time.
An un-even crown lets gas go on one side first.
This can tip the bullet just slightly sideways at the moment the bullet is released into the air.
This is a very important time in the bullet's flight.
Now, remember, high pressure gas always follows the path of least resistance, which is now out the front of the barrel instead of into the gas system.
Barrel pressure drops immediatly.
During the bullet's travel down the bore between the gas port and the muzzle, we had a metered amount of gas fed to the action.
This gas does the following:
Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand.
This is the area inside the bolt carrier where the bolt lives.
Gas expanding here forces the bolt carrier back AND the bolt forward.
Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt.
For a short moment in time, these forces are about equal.
Ideally, this is while the bolt lugs are unlocking and before the extractor starts pulling on the case.
The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring.
All of these effect timing, that's why we have different heavy carriers.
The next thing the carrier encounters are the cam surfaces against the cam pin.
Of course we know that the cam pin goes through the bolt.
Rearward movement of the bolt carrier causes the bolt to rotate.
(pay attention here, this is the meaty part)
Here is where timing comes into play.
Let's make a couple assumptions here before we continue.
Trust me that pressures in the case hold the case into the chamber, even though the chamber is slightly tapered.
Also trust me that when you release all the pressure out the front of the barrel that the cartridge case will spring back down to size so it's no longer a tight fit in the chamber as it was with the gas pressure present.
Here's where timing comes into play.
We want the bullet to be out of the front of the barrel AND the pressure to have subsided enough that the case shrinks down BEFORE the bolt lugs are unlocked because when the pressure is high, the case WILL try to stay in the chamber.
Now is the perfect time to point out that one sure sign of high pressures are the fact that the case extrudes into the ejector plunger hole on the bolt and the resulting pressure unlocks the bolt while pressures are still high.
This extruded brass gets wiped off the end of the case head, leaving a shiney spot and the brass usually makes it's way under the extractor, later causing extraction problems we will get to in a little bit.

Now back to extraction,  normal/correct version:
Pressure subsides, bolt unlocks, carrier momentum continues rearward, pulling the fired (and contracted) cartridge case from the chamber.
As the cartridge case reaches the ejection port, the case pivots on the extractor hook from pressure of the ejector until it is sent flying free of the rifle.
The bolt carrier continues backward while re-cocking the hammer until operating spring pressure or the buffer stops it.
Operating spring returns the bolt carrier forward where it strips another round from the magazine up the feedramps and into the chamber.
Cartridge stops in the chamber, bolt continues forward, causing the extractor to snap over the rim of the cartridge case.
Bolt finally stops against the case head, but the carrier continues forward.
The cam surfaces in the carrier now cause the bolt to lock into battery again.
Now we are back where we started.

Now for extraction, the WRONG ways.
First, too much gas(most common):
The bullet has not left the barrel yet, but it's past the gas port.
Too much high pressure gas is rushing into the carrier, causing it to move rearward faster then desired and unlock the bolt from the extension.
Pressures are still high so the cartridge case is NOT ready to be extracted yet.
The carrier's momentum continues to pull backward, but the pressures in the case actually hold in in the chamber.
This causes a hiccup in the carrier's momentum.
Depending on the severity of the timing, several things can occur:
#1 The (weak) extractor spring allows the extractor to jump over the rim of the cartridge and the bolt carrier continues rearward, grabbing the next round and causing the classic "fired case in chamber, live round behind it" FTE.
The brass shavings under the extractor usually contribute to this one as well.
#2 The extractor does NOT slip off the case, but keeps pulling.
The extractor is strong enough to RIP the rim right off the case.
Same result as above, but MORE brass shavings everywhere from ripping case rims off.
#3 The extractor does NOT slip off the case, but keeps pulling.
During this pulling, the bullet has JUST left the bore, pressures recede and the case shrinks down, allowing extraction.
The rest of the cycle goes as normal, but you have strong pull marks on the case.
Recoil will be higher than normal when the carrier is allowed to travel to the end of the buffer tube and bottom out swiftly against the end of the buffer tube.
In normal operation, the buffer just kisses the end of the tube.

Somewhere between here and the next section, we have proper operation.

Lastly, not enough gas(less common):
The bullet is out of the bore, pressure is subsided, case is extracted and on it's way to ejection.
Depending on the severity of the lack of gas, the bolt carrier may not even get the case out of the chamber before the operating spring returns it forward.
Adding more gas, the case just barely gets out of the ejection port, but the bolt grabs it on it's way forward, classic stovepipe.
Add more gas and the cartridge clears the action, but the bolt does NOT get far enough back to strip a round from the mag.
This is classic short stroking.
You have a single shot action which extracts and ejects, then closes on an empty chamber after you fire it.
In this condition, the bolt will also ride over an empty magazine and close on an empty chamber.
Add some more gas and you will reach the point where it feeds from the magazine and ALMOST works properly, but it still closes over an empty mag.
This is two things, first, poor mag springs are not pushing the follower up fast enough to catch the bolt and second, the bolt is not quite making it back far enough to catch on the magazine follower.
Add just a little more gas and you are back to proper function.

Now, take note, that a lack of gas in a rifle that was functioning fine before can be from several things:
Gas key screws poorly staked and they loosened up, allowing some gas to escape instead of doing it's job INSIDE the bolt carrier.
Gas ring gaps are aligned, gas rings missing or broken, allowing extra gas to flow past them.
Gas block/front sight base is loose, allowing gas to escape before it even gets down the gas tube.
Gas tube "mushroom" is severly worn, probably because it was not properly aligned with the gas key and gas is escaping there.

Please note that failure to extract/eject is a symptom of EITHER too much or too little function of the action.
FTE alone is not enough information to decide what to change to fix the problem.
You need to look for other signs such as the excessive recoil and case rim pulling of too much gas or the short stroking of too little gas.
Unfortunately, many guys who don't understand the magic above always ASSUME that they have too little gas.
What do they do?
They open up the gas port.
Following the examples above, you can see this only makes the problem worse.

Lesson to be learned:
Follow the published troubleshooting procedures.
They are written that way for a reason.

Whew, that was longer than I expected to write.
Hope it all sinks in and you can benefit from it.

I tend to agree with soft brass and high pressures and maybe it's combined with too much gas as well.
The only way you get marks such as you are is for the brass to yield (flow) back into the ejector (round plunger) or extractor (square cut) receesses of the bolt face.
The bolt unlocking while the case is still holding on to the chamber walls is what gives you the wipe marks as the bolt smears or swages the brass back into place while unlocking.

As Dano already said, try some different ammo.