AR-1

Lore & Background

In the XCOM universe, the Assault Rifle is the backbone of human small-arms doctrine against the alien incursion. Where the shotgun excels in close-quarters corridors of a crashed saucer and the Sniper Rifle picks off targets across a city block, the AR is the weapon a soldier reaches for in the vast majority of engagements. Its three-round magazine and effective range of up to 27 tiles in XCOM: Enemy Unknown (with falloff) cover the distances at which most Sectoid, Muton, and Berserker encounters unfold, while in XCOM 2 the range is 17 tiles with 3–4 rounds per magazine. The weapon's design philosophy in the games mirrors its real-world namesakes: a balanced, full-auto-capable platform that does not demand the precision of a sniper or the mobility of a submachine gun. In XCOM: Enemy Unknown, the AR is available from the start of the campaign without needing research, and in XCOM 2 it is also available from the outset. Upgrades such as the Magnetic Rifle and Beam Rifle push its damage, range, and accuracy higher, but the base AR remains the reference point against which every other rifle is measured. Because class restrictions limit which soldiers can equip the AR—it is not usable by Assaults (shotguns) or Heavies (LMGs) in XCOM: Enemy Unknown, nor by Rangers (shotguns) in XCOM 2—it functions as a common but not universal weapon, simplifying logistics for the classes that do carry it.

In Their Own Story

The alarm in the Avenger's hangar bay still hummed when Lieutenant Okafor racked the charging handle on her AR. Ten rounds. She counted them the way her instructor had taught her in the XCOM academy, back when the sky over Lagos hadn't yet split open with a green fissure. Now the sky over a dozen cities had done the same, and the only thing that had not changed was the weight of the rifle against her shoulder. "Squad, board up. We drop in ninety seconds." The voice over the comms was flat, procedural. Okafor checked her magazine, felt the polymer follow the curve of her palm. Somewhere below, in the flooded parking structure they'd been tasked to clear, a Muton was probably already peeling the door off its hinges. She didn't need a sniper. She didn't need a shotgun. She needed eight tiles of reliable lead and the calm to put it where it mattered. The ramp hissed open. Green light swallowed the world. She stepped out, AR low and ready, and the first Sectoid turned its flat, too-wide face toward her. She didn't think. She just fired.

Reader's Guide

The AR is the baseline Assault Rifle in the XCOM series. In XCOM: Enemy Unknown it is one of the first weapons you research in the Weapons lab; in XCOM 2 it is available from the start of the campaign. No alien-tech salvage is required to unlock it — it is a pure human-tech research slot. It occupies the AR weapon slot, meaning a soldier equipped with it cannot also carry a SMG or LMG in the same slot. All standard soldier classes (Soldier, Veteran, Specialist, Ranger, etc.) can use it without penalty. In the tech tree the base AR sits at the root of the rifle branch. Upgrading it (AR Mk II, and further marks in XCOM 2) increases damage, range, and accuracy. It is the logical first rifle to research if you are not starting with one already. Tactically, the AR is your default mid-range weapon. It outperforms the SMG beyond 3 tiles and is more reliable than a Shotgun at 5+ tiles. In a mixed squad, giving every Soldier and Veteran the AR keeps your fireplan simple: cover with the LMG, pick off with the Sniper, and let the ARs do the sustained mid-range work. It is not the most exciting weapon in the loadout, but it is the one your squad will lean on for the first several campaigns.

Did You Know?

• In XCOM: Enemy Unknown, the AR is available from the very first mission without needing research, whereas in XCOM 2 it is also available from the start.
• The AR's effective range is 1–27 tiles in XCOM: Enemy Unknown (with damage falloff) and 1–17 tiles in XCOM 2, not the 1–8 tiles often misreported.
• The AR holds only 3 rounds per magazine in XCOM: Enemy Unknown, and 3–4 rounds in XCOM 2 depending on upgrades, not 10 rounds as commonly assumed.
• Contrary to popular belief, the AR is not equippable by all standard soldier classes: in XCOM: Enemy Unknown, Assaults use shotguns and Heavies use LMGs; in XCOM 2, Rangers use shotguns. Class restrictions determine prim

Conceptual Foundation and Physical Meaning

The mass attenuation coefficient represents a fundamental way of quantifying how readily a beam of energy—whether visible light, X-rays, sound, or particle radiation—loses intensity as it traverses a material. Rather than expressing this loss per unit of distance traveled, the coefficient normalizes the attenuation by the material's density, yielding a value that describes penetration difficulty per unit mass. This distinction makes the quantity particularly useful when comparing materials of different densities or when the physical thickness of a sample varies. The SI unit is square metres per kilogram, though practitioners working with X-rays more commonly encounter the value in square centimetres per gram, and solution chemists may use litres per gram-centimetre. Historically, this quantity was also referred to as the mass extinction coefficient. Conceptually, one can view the mass attenuation coefficient as a reimagined absorption cross section: instead of assigning an effective interaction area to each individual particle, the effective area is distributed across the total mass of the medium, offering a bulk-level description of how the material resists penetration.

Mathematical Framework and the Beer–Lambert Connection

The mathematical structure of the mass attenuation coefficient rests on a simple ratio: the linear attenuation coefficient divided by the mass density of the medium. This ratio feeds directly into an alternative formulation of the Beer–Lambert law, where the transmitted intensity equals the initial intensity multiplied by an exponential whose exponent involves the mass attenuation coefficient and a quantity called mass thickness, also known as area density. Mass thickness is itself the product of the material's density and the physical path length through which the beam travels. Within this framework, the total attenuation can be decomposed into two physically distinct contributions: a mass absorption coefficient, capturing energy permanently deposited in the medium, and a mass scattering coefficient, accounting for photons or particles deflected from the original beam direction. Each of these is defined as the corresponding linear coefficient divided by the same mass density. This separation is essential for understanding whether a beam loses intensity because energy is truly absorbed or merely redirected out of the collimated path.

X-ray Physics, Interaction Mechanisms, and Public Databases

In radiological physics, tabulated photon mass attenuation coefficients are indispensable tools spanning radiography for medical imaging and security screening, radiation dosimetry, diffraction and interferometry experiments, and crystallographic analysis. The photons involved may be X-rays, gamma rays, or bremsstrahlung radiation. The numerical values of these coefficients are not arbitrary; they emerge from the interplay of several distinct quantum-mechanical interaction mechanisms between incident photons and the target material. These include Rayleigh coherent scattering, Compton incoherent scattering, photoelectric absorption, and pair production, where an electron-positron pair is created in the electromagnetic fields of either the nucleus or atomic electrons. The resulting values have been rigorously evaluated and made publicly accessible through three databases maintained by the National Institute of Standards and Technology: XAAMDI, XCOM, and FFAST. These resources allow researchers and practitioners to look up reliable coefficients for virtually any element or compound at a given photon energy.

Solution Chemistry and Quantitative Composition Analysis

In solution chemistry, the mass attenuation coefficient takes on a particularly practical role. When a chemical species is dissolved in a solvent, its individual contribution to the overall attenuation can be isolated by defining the coefficient using only that species' density and only the attenuation it causes. This is a powerful simplification because, under reasonable assumptions, the mass attenuation coefficient of a given species remains approximately constant regardless of its concentration in the mixture. The total attenuation coefficient of the solution is then obtained by summing the products of each component's mass attenuation coefficient and its respective density, with the solvent included as one such component. This additive property makes the quantity invaluable for quantitative analysis: by measuring the attenuation of a solution across a broad range of wavelengths and comparing it against known coefficients for candidate solutes, one can back-calculate the concentration of each dissolved chemical. The concept is also closely linked to molar absorptivity, with the two quantities related through a simple multiplication by the molar mass of the species.

Gallery