List of item IDs that can be used to spawn in items using /spawnitem
Sorry for the lazy IDs such as testgun :D

napalmgun - Napalm Launcher
napalmgunv2 - Napalm Launcher MK2
oilshotgun - The Oil Cannon
plantgun - Volitile Light Launcher
plantgun2 - Volitile Light Launcher V2
plasmagun - Plasma Disruptor
poisonshotgun - The Poision Cannon
rocketgun - E.H.P.L.
testgun - Energy Eruptor
test1gun - Energy Eruptor V2
watershotgun - The Water Cannon
acidshotgun - The Acid Cannon
fatlad - The Fat Lad
FGUN - Fr-4G
GGUN - GR3-N4De
laserblaster - L-x Laser Rifle MK2
laserrifle - L-x Laser Rifle
lavashotgun - The Lava Cannon
lightgun - Light Orb Gun
MIRVGUN - P.M.D.D.
laserlance - Laser Pirecer
siphongun - Siphon Beam Emitter
phasegun - Phase Ejector

unstablequantumcore - Unstable Quantum Core
uranium235 - Uranium 235
uraniumcore - Uranium Core
warhead - Bomb Warhead
acidcore - Acid Core
bombcase - Bomb Casing
gunpowder - Gunpowder
quantumcore - Quantum Core
richuranium235 - Enriched Uranium 235
steelbaritem - Steel Bar
siphoncore - Siphon Core

liquidacid31 - Corrosive Acid

acidbomb - Acid Bomb
1timed - Mega 10 Second Bomb
holyhand - Holy Hand Grenade
nuke - Mini Nuke
MIRV - M.I.R.V.
fragm - Quantum Frag Grenade
qgrenade - Quantum Grenade
missile - Missile
cleanbomb - Standard Bomb
plasmawave - (Hidden Uncraftable Item)
poisonbomb - Posion Bomb
QMIRV - Quantum M.I.R.V.
qmissile - Quantum Missile
timed - 10 Second Bomb
waterbomb - Water Bomb
oilbomb - Oil Bomb
lavabomb - Lava Bomb

c4 - C4 Explosive
bombmaker - Bomb Constructor
enrichment - Centrifuge
downgrader - Molecular Downgrader
claymore - Claymore