ZDoom

I was typing out a design document for the mech mod I was making and was thinking on what else asides "conventional" methods can make a fictional "gun" and so far found/thought of the following.

• Projectile sput out by a magnetic rail (railgun)
• Projectile sput out by a magnetic coil (coilgun)
• Concentrated beam of light particles (lasers)
• Concentrated beam of light particles that are powerful enough to make a plasma channel (electrolasers)
• Using plasma as the igniter to the propellant rather than the conventional primer (Electrothermal-chemical technology)
• Using radiation as a weapon to fry the insides of an enemy (microwaves)
• Throwing plasma at the enemy using magnets(coaxial plasma railgun)
• Throwing sticky fuel at an enemy and igniting it (napalm)

Still trying to think of other more creative weapon tiers, anyone got any suggestions?

Hm, that's a mighty tough question, as you effectively described most hi-tech firearm methods, but let's look around a bit better! Maybe I could find a gun or two which behaves differently.

- Projectiles that deep-freeze the enemy and cause him to shatter (Freezethrower, Thermal Shok Launcher)
- Projectiles that shrink the enemy (The Shrinker...obviously)
- Projectiles that insta-cook the enemy (Half-life 2 Pulse rifle's Mega Pulse, Zicca's protron weapons)
- Projectiles that suck the enemy inside and compress him like a black hole (Dark matter gun)
- Projectiles that teleports the enemy away (The Banishment Device, The Displacer)
- Projectiles that dissolve the enemy to a stinky pile of goo (I don't know...some kind of acid spray?)
- Powerful electric discharges (Lightning Gun, Tesla gun)
- Waves of sonic force which break down the victim's body and jellifies it (Sonic oscillator)
- Concentrated beam of antimatter which literally erases the poor bugger out of existence (Anti-matter beam)

Nukage tier:
- Rounds reloaded with nukage as propellant.
- Barrel rockets. (open end of the barrel faces down, ignites and burns just enough not to explode)
- Various means of vaporizing nukage that basically give us the chlorine gas weapons they used in WWI.

Anomaly tier:
- Teleport pad launcher. The business end sticks to the surface of the enemy (probably something big) and you teleport (or teleport something) into it to kill it.
- A gun that teleports a bunch of air into the chamber that then "telefrags" the bullet out the barrel. No propellant need be carried (but a bigass teleporter thingy must be so there's a tradeoff).
- A machine gun that feeds from a teleporter connected to the factory.
- A gun that teleports in plasma from the sun.

Biochemical tier:
- Vapour cloud that blocks oxidization of copper-based but not iron-based blood, kills barons and hellknights (and cacos???) but leaves everyone else alone. Not very impressive, though, they'd just spin around and collapse and turn whatever the equivalent a baron has to us turning blue. (Thought of this while considering when we might need to read an actor's bloodcolor.)

What about compressed air bullets?

Spoiler:

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- Bioweapon grenades/missiles containing potent bacteria that cause mortally infected to rapidly putrefy (expand) and explode, spawning a lingering cloud of said bacteria which can poison others. Basically a nastier version of Duke3D's Expander.
- Though partially related to sun plasma, a beam "cannon" that's actually a portal device that opens up to the surface of the Sun. All but the sturdiest of enemies are quickly vaporized. Unfortunately, due to the immense energy coming out of the portal, a very quick safety shutoff is implemented to prevent accidental self-destruction.
- Remote-controlled missiles. I only know of 2 mods that have those, but there may be more.
- Grapeshot. Turn your 60mm cannon into an oversized shotgun.
- Metal storm shoulder cannon. Forget about missile spam, an instant discharge of 50+ bullets oughta leave a good impression.

You could also utilize an airstrike marker as a weapon. Once you target an enemy, an airstrike blows him to kingdom come. Would be an excellent addition to mainly outdoor campaigns, and could be one of the best siege weapons.
Of course it'd be dramatically useless at indoor levels.
Storywise this could create very interesting levels, when the player has either fists or this airstrike marker, but I'm pretty sure programming such a weapon would be a real challenge.

Not really. Via the ACS function CheckActorCeilingTexture, you can make sure the airstrike target is outdoors (in a sector that uses F_SKY1 as the ceiling) in order for it to be successful.

Now I'm just thinking how feasible it might be to use ZScript to move some ceilings upwards, change them to sky textures and spawn some debris...

- weapon that shoot flesh-eating nanomachines that kills and proceed to eat opponents, splatting gibs and blood everywhere but the nanomachines would clean up the mess too
- weapon that change the gravity point of the opponent. Now you stand on the floor, now suddenly sky is the floor so you are yanked to the sky
- once in a while, a weapon that override mechanical or power-armored opponent control, like overheating the heat control system, boiling the opponent inside their armor or discharging all remaining ammo supplies they have at once.

Gravity-based weapons are yummy stuff, but in this case, you'll have to program fall-damage for enemies as well. For meanies like the Cacodemon, it would have no effect.