What are some ways to make combat a more deadly affair?
I'm trying to bring my next campaign into the modern world, so you can bet there will be firearms and explosives of some kind. How do I really distinguish how much more deadly these weapons are than in previous editions, where pistols and grenades are quantified down into 1d8 (which is still just a drop in a pond, at later levels)?
I'd appreciate some ideas. Anything, no matter how ill-thought-out, will be considered.
You have to decide for yourself what is balanced, and how balanced do you want the game to be?
Are you okay with new PCs being made ... a lot? Because if the issue is PCs are too aggressive, there's better ways of fixing things.
D20 Modern has rules for guns, grenades and explosives. Guns do 2dX damage (2d4 for a Derringer up to 2d12 for an anti-materiel rifle), grenades usually do 4d6 damage (save for half), and dynamite does however many dice you want, and possibly more with a skill check! A rocket launcher or a tank's main gun do 20d6 damage.
For guns I think a 4e-style mod would work better. Maybe they don't need such high base damage (stat 'em like a crossbow?) but allow Dex to apply to damage and not just to hit. Then you need to stage encounters to make guns deadly. If the enemy are starting in concealment (darkness, in thick woods) or maybe have a lot of elevation and shoot from far way, by the time PCs get to them they'll be half dead. Especially if there's difficult terrain or worse!
Unfortunately I've never seen WotC come up with good rules for smoke grenades or covering fire. Come up with such rules. Use them. Anyone rushing machine guns is going to be moving slowly and taking significant penalties to hit or even damage due to fear.
D&D 3.x (and d20 Modern) have rules for traps, as in how much damage should a Fireball trap deal. (It's 2d6 damage per point of CR.) So a bomb hidden under the floor is a trap, and if the PCs are 10th-level, having it deal 20d6 damage is perfectly reasonable. (And note, that's one CR 10 bomb, not the entire encounter!)