Fire! Fire!
Advanced Explosive and Thermal Rounds for GURPS
By Peter McFerrin	mcferrin@uti.com

Many times have I been annoyed with the wimpy damage that TL9+ explosive shells do. Since 
it’s not always feasible to put a micronuke inside a 20mm bullet (let alone rational), I have 
explored (with the help of MA Lloyd’s GURPS Vehicles revisions) two possibilities in 
explosives.
Plasma Cannon (TL9)
A classic anime weapon (and my favorite gun in Doom II), the plasma cannon shoots solid balls 
of plasma. It is not the fusion beam of TL12; instead, it works in the following manner:
A bullet-shaped projectile covered with fissile materials (usually hyper-enriched uranium, but 
possibly thorium or plutonium), with a core of fusible elements (tritium/deuterium; 3He is too 
valuable for use in weaponry), is wrapped in a high-strength steel sabot. 5 charged particle 
beams (or lasers) blast the back and sides of the bullet (the sabot has strategically placed gaps 
allowing the beam to get through), causing plasma to form. This beam is directly behind an 
electromagnetic coil gun, which stabilizes the plasma through the powerful magnetic field it 
exerts, which also ionizes the plasma. The force of the beam blasts the projectile forward, aided 
by the electromagnetic coils. By the time it has left the barrel, the steel sabot is almost 
completely slag, which aids its armor penetration.
The effects of such a round are incredible. The plasma, which is still burning hot, gives it the 
effects of a fusion beam (but not the explosive concussion). Additionally, the liquid metal of the 
sabot causes the target to lose 1 DR for every ten points of damage before subtracting DR, much 
like a flamer. Being a (relatively) low-speed projectile as compared to a fusion beam, a plasma 
round has all of the disadvantages and advantages of a shell.
To design a plasma cannon, build an electromagnetic gun (minimum bore size is 40mm at TL9, 
30mm at TL10, 20mm at TL11, and 10mm at TL12+). Only manual repeater, autoloader, and 
automatic mechanisms are allowed. It would be wise to make it low-powered, since kinetic 
energy is meaningless to one of these rounds; a barrel of Medium or short is also preferable, 
since most “plasma cannon” are shorter-barreled weapons. Double the effective weight; this is 
the heat shielding required. Find cost based on this weight. Take the square root of bore size and 
cube it; this is the minimum output of each of the five C-PAWS or lasers. Build all five weapons 
(rainbow lasers can be used, as well as X-ray lasers at TL10), and find the cost of them as one 
weapon. The plasma cannon is composed of both weapons; add their costs together and multiply 
by 4 at TL9 (2 at TL10, no modifier at TL11+).  The rate of fire of the whole apparatus cannot 
exceed the cyclic rate of the beams; power consumption is for the entire apparatus—add them 
up.
Plasma rounds are built exactly like HEC rounds, and do the same amount of damage; however, 
this is categorized as Special damage, and has the above-mentioned effects. The cost modifier is 
x200.
Self-contained Plasma Rounds (TL10)
Laser crystals make it possible to fire a plasma round from a normal EM (or at TL11, gravitic) 
gun. No beam is necessary; instead, many, many laser crystals are installed in the sabot. They 
have an additional x1.5 cost modifier.  This type of round would simulate the “plasma 
torpedoes” of Star Trek.
Excited State Explosives (TL10)
Vastly superior to normal explosives, but not as powerful as nuclear or antimatter weapons, 
excited-state explosives use the power of highly unstable chemicals; when these unstable 
chemicals can no longer be contained, they release breathtaking amounts of energy (read: 
explode).
There are three main types of excited state chemicals. Suspended atomic hydrogen (SAH), in 
which a large amount (~30%) of hydrogen is kept in the monatomic state, is the most common. 
Free radical hydrogen (FRH), like SAH but with 100% monatomic character, is also common.  
Metastable helium (MSH), in which helium is excited to triplet state (which creates enormous 
potential energy), is used where a nuclear weapon could be used, but would be overkill.
These are essentially normal explosives, with some exceptions. In order to maintain these 
obscenely high amounts of energy, large quantities of electricity are required. When in storage, 
excited-state rounds are usually attached to a continuous power supply.  In the field, each round 
requires its own power cell.  As a general rule, for each 120 dice of damage a SAH round does 
(192 for FRH, 720 for MSH), assume 0.1 kJ are used up every second.  For example, a TL11 
MSH round doing 1440 dice of damage would drain .2 kJ per second; at TL11, a C-cell (9000 
kJ) would last 12.5 hours, and the shell would then explode.  Using MA Lloyd’s realistic power 
cell rules, a TL11 C-cell is 1.08 lb., so the end result from an excited state round may not be 
worth the energy expended in storing it.  Therefore, although excited state explosives are 
extremely useful, shells using them may not be built until war seems very much imminent. Any 
excited state round, regardless of damage, is LC –1, and security around them will probably be 
tighter than that around nuclear weapons—it takes a lot more to trigger a nuke, while a feather 
can set one of these puppies off.  However, excited state weapons never have fusing problems—
assume that any serious contact will explode them. It is not possible to build HESH, SAPHE, and 
APEX rounds with excited state explosives.
For X values, SAH is 15, FRH is 24, and MSH is 90. For H, SAH is 5, FRH 8, and MSH 30. 
Multiply CPS by 30 for SAH, 50 for FRH, and 200 for MSH.

Dislike my ideas?  Harass me at my school address, lancer35@imsa.edu