Quote Originally Posted by Efrate View Post
Do not mix d and d and real life on a mechanical level. Full stop. Never works. By the rules yes you can negate an assault rifle 20 per cent of the time with chainmail. You are not THAT good as a modern soldier in comparison, and a level 5 practicing expert craftsman can take a 3 round burst in center of mass and be mostly unaffected and healed fully in a few days. With no armor.
you can mix them. you just have to accept that D&D characters are superheroes. Once you accept that your fighter20 can charge at an infantry platoon, dodge half the bullets with supernatural speed and reflexes, more bullets get deflected by his ring of protection, some bounce off his armor because the armor is heavily enchanted to the point that's much stronger than kevlar, and those bullets that actually hit the fighter will bounce off his skin dealing mild laceration because this guy has superhuman (supernatural) resilience, and then he'll cleave in two a tank with his sword because the guy has superhuman strenght and the sword is so powerfully enchanted it may well be a lightsaber... well, once you accept that, you can mix up realism and d&d pretty well.

realism is about being consistent.

that said, what kind of stats to give the soldier? I agree on the level 2-3 expert or warrior with elite stats, because of training and schooling and nutrition. Actually, if we set to BAB 0 the basic level of a middle age commoner, the soldier may well be up to 5th level easily, except for the hit points (I mean, how many skill points do you get going to school for a decade, compared to someone who spent that time as an agricultural manual laborer? the answer can't be just 2).

Prsonally, to model firearms and their capacity to punch through medieval armor, I gave them an armor penetration factor. For a modern combat rifle, that factor may be around 8-10: even full plate is negligible, but stack some enchantment or add natural armor underneath, and it's going to stop a few bullets. A better model would also consider range; for example, the armor penetration decreases by 1 for every range increment. bbut it becomes more complicated.
with this mechanic, modern weapons fare very well against most monsters who rely on natural armor for protection. but it's fairly realistic. take for example a tyrannosaurus, CR 8; bullets would still hurt it pretty well, and assuming he's got a couple rounds before getting eaten, a modern soldier should easily put it down with repeated bursts. A small squad of soldiers could dispatch even a 12-headed hydra or similar big brutes easily.

As far as modern armor, as others have said, it's not good against a sword because it leaves a lot of skin exposed. I'd compare it to a chain shirt or breastplate, but with an armor penetration resistance factor, that would make it useful against firearms.

the rest of the equipment gives some nice perks (gps: automatic success on survival checks to figure out your position. infrared: darkvision, with a -10 to spot because you have limited visual field with it, smoke grenades cast fog, camouflage suit gives +5 to hide), but nothing specially relevant for a battle.

Ultimately, the major advantage of modern weaponry is range. if the soldier has the time to see the monster coming and pelt it with bullets, he can take down anything that doesn't have damage reduction.

however, the presence of magic introduces several weaknesses that can easily be exploited. an incorporeal monster would be immune to bullets (fire damage still works 50% of the times, so explosions ought to be effective). a modern army would have nothing against invisibility (unless infrared detects invisibility). A smoke grenade would probably allow detection of an invisible foe, but nothing stops an invisible wizard from dominating the general, or casting cloudkill in the middle of the barracks.

On the other hand, if you can support your modern soldiers with some low level casters, this changes the game. magic weapon on bulllets overcome incorporeal enemies and most damage reductions. epurate invisibility and similar spells counter most of the magic tricks that would nullify a mundane army. With that, you can take down anything except the tarrasque. and artillery makes minced meat of that one too.

Back to the original question, a challenge rating... difficult to assess, because the strenght of a modern soldier is very situational. Around 5 seems appropriate, but it's much harder if he sees you coming from afar, and much easier if you can use magic to go around it.