The proposition
A theme in many science fiction works, especially newer ones. It is not a bad thing: in older science fiction, portrayals of energy weapons were usually very inaccurate, showing them as simply better because they were more high-tech. This is a surprisingly common misconception: if a category of technology is more "advanced", it must be better than something less "advanced." This is the same logic that has Star Trek medical isolation use "high-tech" forcefields instead of boring old walls, despite the fact that boring old walls don't vanish if you unplug them (and also because you cannot unplug them in the first place).
Real life energy weapons haven't really made it far from the labs just yet... they need too much power, and they generate too much heat. They're expensive, fragile, often require masses of highly toxic, corrosive and explosive chemicals to function, and honestly just don't work very well. High power lasers are still just not high power enough. Particle beams work very poorly in an atmosphere and are too hard to focus over long distances, even in a vacuum. Anti-personnel microwaves aren't as effective as conventional riot weapons, and small lasers that blind people don't sit well with the Geneva Convention. Decades of research still hasn't produced any small arms better than automatic rifles... weapons using chemical propellants are just more effective (that's why they are used), and barring a technological breakthrough, this situation isn't going to change any time soon.
Speaking of technological breakthroughs, let's look at computers for comparison. The Mainframes and minicomputers of the 1960s through 1990s were massive, heavy, and built like tanks. The software was simple, easy to understand, and because it was expensive to develop, stuck around for long periods, and the bugs were worked out. Uptime for mainframes was often measured in years, and minis were about the same. On the other hand, PCs, tablets and cell phones are cheap, fragile, and when was the last time you had a PC you didn't have to reboot at least every two weeks (if not more often); and forget uptimes of years, we don't even keep the devices that long! Well-designed technology (like slug or kinetic weapons) can be reliable and last a long time; cutting-edge tech (like railguns, lasers and other "gee whiz" weapons) often leave its owners with "razor burn" as they move on to "the next big thing," forgetting how many problems they had with "the last big thing."
Kinetic weapons also have one advantage over energy weapons: the ability to use indirect fire beyond visual range. Due to slower projectile speeds compared to energy weapons, kinetic weapons usually never reach escape velocity, and what goes up must come down. Thus one can rain death upon the enemy beyond the horizon or over large mountains/rocks/hills. Someone, however, is needed to relay information of the target's location back to the weapon.
And to top it off, in the long run, even when the energy weapons will finally reach maturity, kinetic projectiles will still retain one critical advantage: while energy weapons will always need a some form of emitter, however advanced, good old slugs can always accelerate themselves, pretty much indefinitely as long as fuel holds out. Relativistic kill vehicles are the most effective means of destruction possible: after all, any weapon is a means of transferring the energy from the source to the target until the latter breaks.
For the Wave-Motion Gun you need to consider the heat balance, design complex energy conduits, build labyrinthine cooling systems (often with very conveniently placed exhaust vents), etc., each of which would waste some of the energy as per the laws of thermodynamics. For the RKV you can simply hook the reactors to the engines of your slug and say goodbye to your enemies. At the speed of ~0.94 c, the kinetic energy of the massive body exceeds the energy that can be achieved by annihilating it with antimatter of the same mass.
As a technical note, Kinetic Energy = (γ-1)mc^2 where γ (gamma) is the Lorentz-Boost Factor, or γ = 1/√(1-β^2), where β (beta) = v/c. Annihilation energy = 2mc^2, because the matter and antimatter each carry a rest-mass-energy of E=mc^2 and energy is conserved. Mass energy goes in, explodey energy comes out, with the same total energy. Then cancel terms and you're solving for γ = 3, which has β = .943.
Another consequence of physics in this regard is that not only are the more exotic kinetic weapons longer to build up more speed (railguns, coilguns, and so forth), but the ships carrying them must be more massive so as to do themselves less damage in firing weapons off due to conservation of momentum. That is, a very light ship with a big projectile will accelerate itself backwards when it fires, which is known to be detrimental to things like pilots or internal structure. Therefore, massively large ships may be justified as what amounts to artillery pieces, even though more mass is detrimental to fuel efficiency and rate of acceleration. This may imply that kinetics-based ships are a valid example of of tiered scifi fleet.
One of the point that many might have missed is the sheer variety of payloads the same guns or missiles can deliver. Your BFG need to punch through the armor of their Cool Tank? Depleted Uranium sabot rounds answers that question. A group of lightly armored infantry trying to rush your tank? Canister shots and beehive rounds will make them think twice about that. Enemies holed up in a building and trying to set up an ambush? An HE shell would level the building and turn whoever inside that building into hamburger. Someone holed up in trenches? Airburst HE rounds would clear them out in no time. The aforementioned HE didn't do the trick for enemies holed up in more elaborate bunkers? Try napalm and white phosphorus. Something you don't like flying towards you? A few guided fragmentation shells with would surely blow it out of the sky. An echelon of tanks passing through a valley? Some cluster ammo would send them a clear message. And God forbid someone who decides to use some more destructive payloads in their weapons, say, a good-old thermonuclear device, some Deadly Gas, or some sort of Synthetic Plague... Yeah, Humans Are Warriors after all, and we just realized there are some weapons that are too efficient at killing what we still need alive, or some other weapons that kills enemies in such gruesome fashions that we've gone too far and we need to put restrictions on them to protect our own kin.
In the world of science fiction, there would be no doubt that artillery pieces and missiles would deliver ever more exotic payloads to combat the plethora of enemies they may encounter. If the setting have Deflector Shields, it will be all but expected to see warheads designed to either defeat or otherwise circumvent it. Against a Horde of Alien Locusts, there would surely be less restrictions on payloads used than if such weapons are used against humans. When an army of robots invade, a barrage of EMP shells fired from beyond the horizon might be the deciding factor on whether a battle is won or lost. Fundamentally changing the type of attacks dealt by a directed-energy weapon means changing the weapon itself (if possible at all), but changing it on a kinetic weapon just meant the next round loaded into the chamber looks slightly different.
The opposition
This is not to say that energy weapons are completely useless, however. At sufficiently great distances, even a relatively small number like 1 m/s can make the difference between a hit and a miss. There are good reasons why research into anti-missile defence is headed into lasers.
Indirect fire becomes a moot point once kinetics need such high velocities to harm the enemy that exceeding escape velocity is not only necessary but commonplace.
- Rebuttal to this particular opposition: Who said "kinetic" is supposed to exclusively mean "kinetic energy penetrators" and "kinetic kill vehicles"? If a directed-energy weapon would be powerful enough to melt/blast through the enemy armor, would it be possible to put something with a similar power output on a more kinetic-based delivery system? If wars in human history have told us anything, if you can effectively kill enemies at a range where they cannot fight back, and manage to keep that range advantage, you win. There's a good reason why guns replaced swords, anti-ship missiles and anti-tank missiles replaced dive-bombers, and aircraft carriers replaced battleships. Unless such warheads and payloads CANNOT exist due to certain in-universe restrictions (e.g. haven't been developed yet, or are in short supply), there would be few reasons on why they are not used. Of course, if the energy output of the weapon needs to be high enough to cause a local extinction event, then yup, I don't think we are going to take the curvature of the planet into account - shooting from orbit would be a much safer bet.
Ammunition will also run out, barring things like Star Trek replicators, whereas depending on how advanced an energy source is available, Bottomless Magazines for energy weapons may be possible. In a protracted war far from resupply, it would make all the difference, and amateurs study tactics while professionals study logistics.
"good old slugs can always accelerate themselves, pretty much indefinitely as long as fuel holds out"? Those last six words are the rub. Storing the necessary fuel for missiles in a manner that is safe for military use may be a problem, as well as being able to store enough. Even high-impulse propulsion like Orion Drive or antimatter would need to set aside space for storage, and those especially would be most prone to disaster, requiring complicated containment systems for safety. Failing to armour up the fuel storage areas is just begging for enemy point defense to Attack Its Weak Point, with obvious results, yet armour also imposes a mass penalty. Less dangerous propellants may be less efficient and thus require more space for fuel. The end result is that the kinetic doesn't really have a lot of mass worth of payload after all, especially after the fuel is consumed.
Kinetics can themselves be complex and finicky. We are reaching the practical limit of chemical propellants for guns; getting any more out of the field may require some hitherto unforeseen breakthrough in chemistry that breaks new ground, with results we may not be able to predict. Otherwise, trying to stuff more propellant, or even a hypothetical more powerful propellant, may lead to ammunition becoming dangerously easy to cook off. Recoil issues may also make it difficult for infantry to use their fancy new guns; workarounds like genetic engineering, cybernetic augmentation or recoil compensators may have their own issues. Even vehicle-mounted versions would have similar issues; even if the oft-repeated saw about battleships being shoved around by its broadsides' recoil has been debunked
, the force from proportionate future weapons is still likely to have similar problems with recoil. Electrothermal-chemical guns and Magnetic Weapons needed to defeat future armour will also require precise, complex engineering that makes the first-generation M16 look robust and reliable; the difficulties real-world militaries and laboratories are having with them are sufficient proof that one should not underestimate the complexity involved. Heat, energy conduits, cooling - all the issues supposedly plaguing energy weapons are all equally applicable to future kinetics. In fact, tests show that current railgun barrels outright melt under the heat. The days of a "simple" kinetic like the memetic AK may be at an end.
The point of >=0.94c impacts exceeding the power of antimatter also only applies if it is practical for the society in question to employ kinetics that can achieve such muzzle velocities. Such velocities may not be achievable, or otherwise are only available on Awesome, but Impractical superweapons that are only good for strategic bombardment, not the vagaries of tactical combat. In a world where practical mass production of antimatter is possible but not high-level kinetics - Mass Effect, which has antimatter but even big dreadnought-mounted spinal guns' muzzle velocities max out at 1.3% of lightspeed, comes to mind - having payloads would remain useful. For that matter, with sufficiently low-tier factions, even fusion warheads would be a great boon over lumps of metal. Once again, Mass Effect: Dreadnought guns hit for 38 kilotons, while even a 20th century city-killer weighs in at megatons.
Projectile weapons, by nature of having a projectile at all, can (theoretically) be intercepted. With energy weapons, you can only decoy the targetting sensor or force the beam to go through more armour or shielding. Depending on technology, once evasion is impossible, it may be easier to simply deflect or destroy incoming rounds than spoof sensors or equip better passive defences. Going back to Mass Effect: Payloads would add to the firepower, but no one uses powered missiles any more because strong point defence has obsoleted them. Or consider Honor Harrington: missiles can do .9c easily and are rightly feared in the banned planetary bombardment role for that. However, in the antiship role, they rely on bomb-pumped Frickin' Laser Beams rather than kinetic impact due to the strength of point defence, which can intercept them tens of thousands of km away from the target starship.
Directed energy weapons might also have an edge over projectile weapons on choosing the fire rate and lethality of each attack depending on how they work in-universe. Granted, having output settings ranging from "stun" to "Blow Up Alderran" would be less than likely, but genuinely trading fire rate for lethality and vice versa would be a bonus. Selection between fire rate of projectile weapons ultimately boils down to whether accuracy or covering the area with fire and lead is more important, as the lethality behind each attack would not be affected by the rate of fire; in addition, all ammo of the same weapon, regardless of the type and lethality, would most likely take up around the same amount of space (arguments might be made for propellant usage, although any gun large enough to use separate propellant charges would most likely not be used to fire kinetic-kill ammo thus rendering the point moot). Energy weapons, especially lasers, can trade between fire rate and energy output more easily as long as their capacitors and cooling systems allow.
- When there are no strike crafts and missiles near by, there is no reason for the point defense laser turrets to keep firing at the same fire rate and energy output and then barely scratch the paint of the enemy battleship instead of firing more concentrated blasts that can at do some more serous amounts of damage (granted, Death of a Thousand Cuts would most likely still apply due to system designs, but there's a big difference between that and shrugging off a thousand cuts); conversely, there would also be little reason for an anti-tank laser cannon to continuous firing its powerful yet slow-charging shots when a horde of Mooks tries to Zerg Rush its defensive parameter instead of firing a quicker barrage of weaker shots (same as above in that there will most likely be innate restrictions on the range of power settings, but there's quite a distinction between "Killed 20 mooks in 3 shots and the position is overrun" and "Killed 50 mooks in 20 shots and the position is safe").
The grey area
FTL weapons are impossible under our current understanding of physics. In settings where they exist, though, they can support either side of the matter. Kinetics would dominate if they can be fired FTL while directed energy cannot. On the flipside, if only directed energy can go FTL, the tactical advantages of doing so compared to STL kinetics will compel the former's development.
Soft settings may also allow for Deflector Shields that can completely negate both kinetic energy and momentum, thus making kinetics useless and forcing the use of energy weapons. On the other hand, it is possible for the shields to negate energy blasts, but happily let matter through, making the opposite true when it comes to picking your choice of weapon.
One interesting look at a grey area is in the Star Wars universe. Groups facing possible Jedi interference would use kinetic weapons, as lightsabers could not deflect them like blasters. More importantly, armor technology developed to a point that bullets (or slugs as they are called) were useless against them, but blasters were not, and eventually, most forces gave up on wearing armor; if one's enemy wasn't going to use a weapon that armor protected against, the restrictions it put on movement would be pointless. In some ways, blasters made slugthrowers even more useful; since the former took the armor factor out of the equation.
A slight real-world parallel to the above could be how firearms reduced the usage of body armor that defended against melee attacks, since bullets would easily pierce armor that a melee attack would bounce off of, and now, a combatant is more vulnerable to melee weapons than his pre-firearm counterpart. (Kevlar, contrary to popular belief, does offer some protection against knifes, but other times, it does not.) Of course, where the comparison falls apart is the fact that while slugthrowers and blasters can stand toe-to-toe, Real Life swords and knives cannot with firearms.
