It's likely that Mongo will get his flamethrower tomorrow night, so I've got to stat that up. I've also got to stat up the Scientist's laser pistol, and guns in general. I don't like the idea of guns being vastly superior to the standard medieval weapons - all of them will kill you dead if you hit with them. I'm going to move forward with the notion that damage from guns is about the same as other weapons, but you get a bonus to hit, and better ranges for rifles.
I'm not sure how all this will work out in practice. Gunfire in a dungeon seems like surefire way to bring in the wandering monsters, so I'm not going to worry about game balance. Let the chips fall where they may, it's time to up the gonzo factor!
Small Flamethrower: Range 10'. Holds 3 flasks of fuel in its barrel. +2 to hit. Lantern oil will do 1d8 damage (for just the round the flamethrower is used). Jellied napalm will do 1d8 damage per round for 3 consecutive rounds.
For the rest of the firearms (mundane and exotic), the range will be represented as short/medium/long. Short range has a bonus of +1 to hit, medium no bonus, and long range has a penalty of -1.
Small pistol: 1d4 damage. Range: 10'/20'/30'. Easily concealed, 1 or 2 bullets.
Large pistol: 1d6 damage. Range: 15'/30'/45'. Revolver with 6 bullets, or holds clips in various sizes, typically up to 8 bullets
Rifle: 1d6 damage. +1 to hit. Range 100'/200'/300'. Clips in various sizes.
Shotgun: 1d8/1d4/1d2 damage, depending on range. +1 to hit. Range 10'/20'/30'
Light machine gun. As rifle, or burst of 10 bullets at +2 to hit (instead of +1) , 1d8+4 damage.
Heavy machine gun. Tripod-mounted. Bursts of 10 bullets. 1d10 damage. No bonus to hit, but may hit all creatures in 10' area. Range 50'/100'/150'.
Light Plasma Rifle: 1d6 damage. +1 to hit. Range 20'/40'/60'. Holds clip of 8 plasma charges.
Heavy Plasma Cannon: 1d6+4 damage. +1 to hit. Range 25'/50'/75'. Holds drum of 20 plasma charges.
Laser Pistol: 1d8 damage. +1 to hit. Range 60' max, no bonuses or penalties. Holds energy for 3 shots.
Laser Rifle: 1d8+2 damage. +1 to hit. Range 300' max, no bonuses or penalties. Holds energy for 10 shots.