Red dragons literally swim in stars.
Fire Immunity means
Immunity, there, and the harm of stars really is mostly heat, when you ignore radiation. Which probably falls under a special form of Electricity damage, because it's the ionizing that's the problem. Which means a Red/Blue Dragon would probably be utterly safe from everything nasty about stars.
Personally, I think high explosives should be largely Sonic damage because it's the shockwave dealing damage, not the heat. Seriously, it's like these supposedly nerdy people never actually looked into how things cause damage. The fireball of an explosive is a
side effect, not the means of damage. Unless it's a thermoberic bomb, in which case it's still
mostly the shockwave dealing damage, and the fire mostly removes breathable air rather than actually burning.
And I'm going by how heavy crossbows, presumably a counterpart to the steel-limbed ones Knights used, deal 2d6 damage. Most modern pistols have similar immediate lethality to those. The
point of me wanting to add an armor piercing quality to them is so that damage numbers don't have to inflate absurdly to make anti-armor weapons, like .50 cal rifles, which can be used as anti-tank weapons with the right cartridge, and 8 gauge slugs, which are best described by being 2 ounce spheres of lead to the 1 1/3 of 12 gauge for a spherical slug. Mind, this is because the literal definition of gauge is "number of shots able to be made from a pound of lead." The .25 gauge punt guns took four pounds of lead for a full bore slug, while the AA monstrosities took a little over 6.2 kilograms of lead for a full bore spherical slug.
There's actually a formula for gauge, because of the definition being able to be mapped to math. Essentially, the true gauge of a shotgun is the denominator of the fraction of a pound of the sphere of lead that fills the barrel. Because shotguns are optimized for packed shot, this is rarely truly applicable due to non-cylindrical barrels and measuring them by how much of the standard shots they fire per shell.
Also, didn't I
start with complaining about d20 firearms being hilariously underpowered? If medieval siege weapons provide sufficient force to bypass +2 magic bonus equivalence, then a .50 cal rifle ought to do the same. The rifle bullet has
considerably more force at the impact point, thanks to being, well,
pointy. Although an argument for inertia could be made, that just justifies .50 cal as +2 instead of +3. A modern tank shot would thus be +3, if we go by stages of escalation, while a low-kilotons nuke would, indeed, be +4.
Shaped charge nukes, as well as anything in the triple digit kilotons, would have to be +5 because this is literally "put a hole
through a mountain" kinds of force concentration. Very similar to hitting things with mountains, really.
By D20 Modern rules, it only takes 9 pounds of C4 to do 20d6 damage, not 2,000,000 pounds.
A megaton nuke has the equivalent blast of 2,000,000,000 pounds of TNT, by exact definition. C4 is
considerably more powerful, but not a full three or four orders of magnitude. Although this situation with silly damage numbers likely comes from d20 not having any decent examples of blast energy falloff to cover how explosives really act, so damage "has" to be the same for the whole area.