First off, cudos to the marine.
Now, as to the "bring enough gun" comments. He lived, one of his attackers didn't and the other had no fight left in him - by definition, it was "enough gun" that time. So the guy ran 300 yards. Deer run 300 yards after chest shot by a .300 Win Mag at times. Doesn't mean a .300 Win Mag isn't enough gun for deer, or that a .30-30 or .243 is way to light.
People are tougher than they are generally given credit for. Most "stops" by handguns that do not involve hitting the central nervous system, perforating the heart or breaking major bones are either from psycological effects or happen several seconds after the person has been shot. Before anyone quotes stopping percentages please think - according to those charts a .357 has the same stopping percentage as a .308 ..... now do you think that data might be sqewed?
A person hyped on adrenalin, not to mention we have no way of knowing what else the robber may have been on, can go a long way in 3 seconds. It's completely possible the guy would have still made it 300 yards after being hit with a .50 AE in the same spot. You want gauaranteed stopping power from a less than ideal hit? Then carry a centerfire rifle - and from the reports that have come back from Iraq and Afganistan, .223 isn't going to be big enough.
It's completely possible the 2nd robber would have never been found had he been hit in the same place with a .22. It's also completely possible he would have been dead when found, and at less than 300 yards. A millimeter diference in the line of the wound channel can mean the difference between bleeding out and walking away.
As far as the bring enough ammo type of comments. Gunfights tend to go last as many shots as it takes till they are over. If you run out of ammo, it's "over". With most modern autos, and the (realitivly) anemic power of handguns, it's completely possible to empty the gun at / into someone who was "stopped" by the first round befroe they have a chance to fall. There was a case in Florida where a cab driver was being robbed and fired back with a .45. He fired 7 or 8 shots (empting the gun) as fast as he could pull the trigger, all hits at 5 - 7 feet and the last round actualy went into the guys shoulder area as he was falling forward (he was dead before he hit the ground and likely before the gun was empty). This compares a lot with the incident with the marine. The first robber haddn't fallen, but was clearly out of the fight.
Personally, from the results of his shooting against the first robber, I'd bet that had the second robber decided to stick around rather than run, several of those other three bullets would have been in robber number 2.