An energy rifle, in Sidh definition, is a rifle-sized directed energy particle-beam infantry weapon. In the Imperium's 27th century setting, energy rifles of various models have largely replaced ballistic firearms as standard infantry weapons.
The first energy rifles were introduced in service in the 2220's, shortly before the Age of War and saw widespread proliferation over the coming centuries of warfare.
Energy rifles operate by projecting self-containing bolts of superheated plasma sourced from heavy metal particles. A laser beam first creates an ion channel, reducing atmospheric resistence (though this is obviously not necessary in airless environments). A mass of heavy metal nanoparticles is then drawn from a source into the reaction chamber, energized and projected down this ion channel at velocities up to 5% of the speed of light. In this respect, the weapon works very much like an ordinary linear accelerator, i.e., a railgun, except for firing an energized mass of particles rather than a solid metal projectile. The metal plasma bolt contains itself by means of pinch effect, retaining integrity over long distances before it's energy levels diminish sufficiently. Due to the extreme velocity by which the bolt travels, it stretches out greatly enough to cover the entire distance between the target and the weapon, effectively appearing like an instant-hitting beam rather than a bolt.
The most obvious advantage of the energy rifle is the lack of ammunition. That's not to say the weapon never needs reloading - the particle source, a canister containing roughly 3.5 kilograms of heavy metal (usually uranium, osmium or tungsten) nanoparticle fluid, needs to be refilled after roughly 10 thousand shots in a standard infantry rifle - which makes the weapon's ammo supply likely to outlive the user on the modern battlefield. Likewise, energy rifles need to replace their power cells after 200-500 shots (depending on model) unless connected to a power source by means of an umbilical cable (usually to the micro-fusion core in the backpack powering the user's powered armor suit). Rapid fire can also melt the weapon's heat sink, which will also need replacement. To avoid this, energy rifles have been designed with a fire-rate limiter that prevents the weapon from firing above sustainable rate.
The energy rifle is an immensely-powerful weapon for an infantry firearm, many would even say - overkill. The effects of a particle beam just 2 millimeters wide are comparable to a 25-millimeter autocannon shell or a small grenade. This beam can burn through up to six inches of hardened steel, and even a near miss is bound to cause severe burns. Furthermore, particle beams are virtually unaffected by gravity and wind, hitting exactly where the gun is pointing at distances up to 5000 meters (in heavy energy rifles; most models have an effective range under 3000 meters), beyond which the beams rapidly dissipate due to losing energy. The beam carries considerable electric charge as well as kinetic and thermal energy, temporarily disrupting unshielded electronics in close proximity of it's passage and impact.
Such massive firepower does come at a certain cost, however. The most obvious drawback of energy rifles is their massive heat output, requiring bulky cooling systems and restricting sustainable rate of fire. A typical energy rifle only fires roughly at the same rate as WWII-era semi-automatic rifles like M1 Garand or SVT-40. This makes energy rifles rather poor at providing suppressive fire, requiring Imperial soldiers to use different weapons for that role.
In most models, the fire-rate limiter can be overriden to enable rapid-firing of the weapon, but the massive heat buildup will quickly render the weapon inoperable after just a few bursts and may result in a violently-explosive catastrophic failure, so using the override feature is recommended for emergency uses only. The brilliant-purple beam is very visible on the battlefield, requiring eye protection for the user to avoid being dazzled by his own weapon, revealing his location and making any efforts of camouflage across a large spectrum largely useless after first shots have been fired (hence the relative lack of effort invested in camouflage by the Sidh military). Furthermore, the massive thermal emissions from the beam tend to ignite any flammable substance or surface within a foot of the beam's passage, starting at times undesirable fires.
Because of the weapon's sizable cooling and recoil reduction systems, energy rifles tend to be rather bulky and cumbersome weapons (by unaugmented human standards, at least), the average energy rifle weighing at around 32 kilograms and being longer than most regular infantry firearms. This weight, the heavy recoil, and the dangerously-high heat output makes energy rifles very uncomfortable and hazardous even for an augmented user to fire without powered armor - though then again, most were never designed to be fired without one.
Energy rifles usually have integrated neuro-syncable optics and targeting system that interface directly with the wielder's nervous system, effectively becoming an extension of one's body and enabling the user to literally "feel" the gun. The weapons support various attachments, such as bayonets and 60-millimeter MSLs (Multipurpose Smoothbore Launchers), which are effectively a hybrid between a shotgun and a grenade launcher/light mortar. In some models, an MSL is integrated into the weapon. Military-issue weapons also have an anti-theft/anti-tampering system in place, recognizing the unique genetic traces of the Sidh species. The said system can be calibrated exclusively to the weapon's assigned user (not normally done for practical purposes), a set of authorized users, or any Sidh in general. An unauthorized Sidh attempting to fire a weapon so fitted will result in the weapon simply not responding. A human or alien attempting the same will meet a fiery demise as the anti-theft system will detonate the power cell, obliterating the weapon and anyone or anything standing within 5 meters.
There are dozens of different models and patterns of energy rifles in existence. Among the most common of them is the Mk.VI Castus (described in infobox), which served as the standard service weapon for Army and Navy through the Reconquest Wars, and is still by far the most widely used model of energy rifle in Imperial armed forces today even as newer models gradually replace it.
Energy carbines are essentially downsized energy rifles, which trade power and range for reduced weight
and compactness. While still quite heavy by unaugmented human standards, weighing within the 8-13 kilogram range, they are just the right size and weight for an unarmored Sidh user, though the dangerously-high heat output still leaves much to be desired. Efforts to reduce weight mean that carbines usually forego integrated grenade launchers, instead having external accessory rails to mount them as add-ons.
While firing only at 1/3 to half-power of a full-sized energy rifle at peak output, an energy carbine still packs a considerable punch comparable to that of a .50 BMG round - more than enough to kill most infantry targets with ease, but somewhat inadequate against hardened powered armor. To compensate for the relative lack of firepower, carbines tend to emphasize rapid fire instead, augmenting their downsized cooling systems with clips of disposable heat sinks. The reliance on a limited supply of heat sinks makes carbines somewhat unsuitable for prolonged combat, so they are mostly used for short missions where this is not expected to be an issue, and where need for a high fire rate and volume of fire outweighs concerns about ammunition (i.e. heat sink) shortages, such as commando raids and VIP takedown missions where prolonged engagement is not expected or desired.
A notable energy carbine model is the Mk.VIIc Lancea (pictured above), commonly used by Imperial Marines, the special operations branches of the Army legions, and also included in emergency survival kits of Imperial Navy escape pods.
Heavy energy rifles
Heavy energy rifles are an upscaled, considerably more powerful type of energy rifle, combining the roles of an anti-materiel rifle and a direct-fire anti-armor weapon.
Considerably larger than the regular infantry rifles, heavy energy rifles can be up to 200 centimeters long and weigh up to 120 kilograms - still a rather trivial weight for soldiers clad in hulking powered armor. While the operating principles remain the same, heavy energy rifles project a much thicker (up to 20-mm wide) particle blast for a considerably longer duration (a fraction of a second as opposed to mere microseconds) at a greater density. This enables heavy energy rifles to defeat much thicker armor plating than their smaller cousins and also produce much greater explosive effect on the target - a fully-charged shot from a heavy energy rifle will easily take out a main battle tank and completely vaporize a man.
Unlike regular energy rifles, heavy rifles cannot be rapid-fired by design, instead having a variable-yield shot feature, allowing the users to adjust the desired output level to meet their needs. Shots at any but the very lowest power setting take some time to charge, up to 4 seconds for maximum charge, requiring more careful aiming on moving targets. A maximum-power blast can burn through several meters of hardened steel, producing a blast comparable to that of a 155-mm artillery shell. The brilliant beam of incandescent particles shines brighter than the sun, igniting every flammable thing close to it's passage.
The Imperial Guard is known to operate a unique hybrid type of heavy energy rifle, the Mk. IX Longinus (pictured above), which combines the best traits of regular and heavy energy rifles, effectively combining charged-shot and rapid-fire features.