Musket - Muskets appeared around the 16th Century and managed an existence into the 19th Century. These firearms utilized a long, smoothbore barrel for range, were operated from the shoulder utilizing a two-hand hold and fired a spherical metal ball. Muskets were muzzle-loading instruments (that is, loaded from the barrel) which made them time-consuming to reload, forcing lines of infantry (musketeers) to be used in formation. The introduction of rifling brought about the classification of 'rifled musket' detailed next.
Rifled Musket - Rifling was the gunsmithing practice of adding grooves within the bore of a barrel to help a bullet retain accuracy and range once it left the weapon. Many smoothbore muskets were converted by rifling to become 'Rifled Muskets', a sort of interim design between the original musket and the newfangled 'rifle'. The introduction of the breech loading cartridge all but killed the musket as a viable firearm.
Rifle - Rifle is a general term used to represent a modern-day 'long gun' featuring 'rifling' inside the barrel that promotes accuracy at range and is able to be fired from the shoulder using both hands to complete the three-point hold. Rifles include the sub-groups of Battle Rifle and Assault Rifle.
Battle Rifle - The Battle Rifle appeared after World War 2 and was an automatic 'long gun' that utilized a full-power rifle-based cartridge. Battle Rifles formed many of the frontline service rifles of the Cold War and were primarily centered around the 7.62x51mm cartridge in the West. EXAMPLES: HK G3; M14; FN FAL.
Assault Rifle - Classification falls between submachine gun and light machine gun. Assault rifles are today's modern frontline service rifles, having replaced the Battle Rifle used during the Cold War. An assault rifle is categorized by its use of an 'intermediate' cartridge size (that is, less than a full-power rifle cartridge - ex: 7.62x54mmR) coupled with a detachable box magazine. The German WW2-era StG 44 is largely accepted as the first 'true' assault rifle. EXAMPLES: Colt M16; Kalashnikov AK-47; L85A1; HK G36.
Carbine - Carbines are generally shortened forms of existing long guns and can be based on muskets or rifled automatic weapons. The carbine retains the capability to fire a full-power rifle cartridge though this is delivered in a more compact form brought about by shortening the barrel and forend assemblies. Due to the modifications, carbines generally lose accuracy at range though they are intended as close-quarter weapons. Carbines fall between submachine guns and assault rifle concepts. EXAMPLES: KAR 98K; CAR-15; M1 Carbine; M4 Carbine.
Submachine Gun - Submachine Guns are automatic weapons of compact size when compared to their larger rifle cousins. Submachine guns also feature rifled barrels but barrels that are shorter in length to promote the compact sizes required of this weapon class. Additionally, submachine guns generally utilize pistol cartridges which are lower-powered when compared to full-size long guns but carry inherent man-stopping capabilities all their own. The earliest 'true' submachine gun was the German WW1-era Bergmann MP18 which saw extended service into WW2. EXAMPLES: HK MP5; MP38/40; STEN Gun; UZI 9mm; AKSU-74.
Designated Marksman Rifle - Designated Marksman Rifles (DMR) are automatic weapons outfitted with sniper-type optics and issued to squad-level sharpshooters. DMR personnel fall between the standard frontline infantryman and the dedicated sniper element and their weapons are modified to suit the role. Modifications include the installation of optics and a bipod for accuracy at range. Their weapons are generally full-sized long guns firing a full-power rifle cartridge from a high-count detachable box magazine through a semi-automatic action (one bullet fired for every trigger pull). They may actually be modified Battle Rifles detailed above. EXAMPLES: M14; FN FAL; AR-10; HK G3.
Sniper Rifle - Sniper Rifles are operated by specially trained shooters centered on accuracy against targets at long ranges. Weapons are typically very accurized systems with optics, bipods and adjustable shoulder stocks as standard. Traditional sniper rifles are also manually-actuated bolt-action weapons that require the operator to move the bolt handle to introduce a cartridge into the firing chamber. Sniper rifles are nearly always chambered for the 7.62mm full-power rifle cartridge for its range and man-stopping qualities. EXAMPLES: Remington M40; Mosin-Nagant; H-S Precision Pro Series 2000.
Anti-Material Rifle - Anti-Material Rifles owe their existence to the bolt-action rifles of old. The first anti-material rifle was developed by the German Army in World War 1 to counter the threat posed by British tanks ('Landships'). The anti-material rifle is centered on the firing of a full-power large caliber cartridge - typically the 12.7x99mm NATO (.50 BMG), the 14.5x114mm Russian or, most recently, the 20mm cartridge. Their intent is to engage critical components of an armored vehicle, disabling key systems that would render the vehicle inoperable. Additionally, anti-material rifles can engage various other targets including communications equipment and even personnel - in the latter case having a truly disastrous effect on the target. Anti-material rifles have grown in popularity over the recent decades as they become more of a primary fixture on the modern battlefield.