|I freaking love Game of Thrones! HODOR!|
First of all, DR is not always more realistic than AC. If a character is wearing armor with a high DR, this makes him effectively invulnerable. But this isn't actually realistic unless the defender is wearing some kind of futuristic powered armor.
The key to fighting against an opponent in plate armor was to find the weak points, not bust through the armor. Because you couldn't actually penetrate plate except from terrific blows with advantageous leverage - like an direct hit by an overhead blow from a warhammer.
|He will introduce you to new forms of pain|
Otherwise, it was all about slipping the blade between the joints. In practice, knights dressed in field plate would often resort to throwing down their swords and going at it with daggers, hoping to slip the blade into a faceplate slit or whatever.
In fact, warriors who were trained to fight with heavy armor always learned grappling techniques; things like joint locks and throws are still effective no matter what an opponent's armor is like. So even your hypothetical space marine might be vulnerable to crude unarmed combat, if it ever gets that close.
But let's not lie to ourselves. AC gets pretty out of hand at the high end of the damage scale. If a giant is trying to introduce you to the exciting end of his club, plate armor should be of negligible protection. Even though popular depictions of medieval armor as bulky and clumsy are greatly exaggerated, this might be one occasion you want to be completely unencumbered. Because you're not going to parry that uprooted spruce that the giant calls his "club." Nevertheless, AC is completely insensitive to such concerns.
|Everyone's favorite super-hero/cautionary tale.|
So what's the solution? Again, I'm surprised that nobody has come up with this: how about a DR die, or DR roll?
There's a lot I like about this idea. First of all, it gets around the issue where DR can completely shut out low damage weapons. This is probably the #1 issue with DR, in my infinitely humble opinion. On top of that, it does a great job of simulating the effects of big monsters hitting heavy armor by greatly reducing its value in such situations.
But there are still some shortcomings. First of all, the effects of armor can no longer be circumvented with skill. After all, your hit roll goes up over time, but not your damage with the same weapon. The second problem is that plate is still a lot harder to circumvent for a dagger than a longsword. It may be better than a flat DR, but we're only alleviating the issue a little.
One way we might get around these is with a notion of an AP (armor-piercing) die. This could be a die that goes up with a character's level just like their bonus to hit. Or maybe you only get them for being a fighter and having special advantages. You'd roll it alongside damage, and subtract its roll from the DR roll. Any excess of DR is lost, however, which is why it's not added straight to damage.
Of course, this changes things a lot. AC no longer includes a concept of armor - if anything, heavy armor should make you a little easier to hit. Not much, but a little. So let's call it Hit Class, or HC. It can now depend on things like the defender's Dexterity, and possibly things like size, movement speed and distance (especially for ranged attacks). We can also include situational factors, like whether the defender has arms capable of parrying incoming attacks, and the usual stuff like poor lighting and noxious spores.