For those that don't know all of the military lingo or acronyms common with rifles and weaponry, this should give a good idea of things that often get referred to when talking about ARs and battle rifles.
Nomenclature for Weapon Names:
The general nomenclature the military uses for firearms is heavily dependent upon the branch of service that either developed the weapon, or that uses the weapon. Sometimes there will be some overlap where all branches will use a designation, and other times they will use their own.
: The biggest differentiation between weapons is the M vs Mk designation. Mk, or "Mark", means Make or Version. Mk is used in Navy nomenclature for most of their weapons and equipment that have been developed for their usage. The first model to be put into service will be the Mk* Mod0. Mod0 means Modification 0, or the no modification/first model. When an update, retrofit, or change is made, they will then change the nomenclature to Mod1, with all subsequent variants being Mod2, Mod3, etc successively. In most cases, the Mk designation is used only by the Navy, with some used by the Marine Corps (USMC).
: The "M" designation originated with Army use, but is used for all weapons and items issued through the Army, USAF and USMC. Whenever a variant or modification is developed, there will be an alphanumeric designation after the original nomenclature, which otherwise would not be there. If a weapon is an M1, then the original version is simply the "M1". The first variant will be A1, making it M1A1. Variant 2 would A2, #3 would A3, etc. The Army has also adopted a naming profile for variants that use "E", such as E1, E2, etc. The E is supposedly for "Enhanced", meaning that the weapon has been drastically altered and enhanced from its original version.
Sometimes branches will continue to use the designation developed by the other branches. Here are some examples:
-M14 and M21 in the Army and USAF are based off of the M14/M1A platform. However, the Navy and Marine Corps use Mk14s, which are the same weapon.
-The Navy developed the Mk12 rifle. This is the designation used across the whole military.
-The Army developed the M16s and M4s. Everyone who uses those rifles calls them that. Branches will use different models, though, like the full-auto M4A1 in lieu of the burst-fire M4.
-The Marine Corps developed the M40 series of rifles, and the Navy uses them and also calls the M40s.
-Navy and USMC use Mk11s, Army and USAF use M110s
: XM can mean either "Experimental" or "Rejected". In terms of weapons, as well as new ammo in development, XM means Experimental. This is the designator used prior to it getting it's permanent nomenclature. XM means experimental when in use by the military. In terms of weapons and ammunition that are not in circulation or use by the military and only available on the civilian side, such as XM193 or XM855, "X" denotes that the lot was rejected by the military. The military has rejected that component for whatever reason during an inspection because it did not meet inspection criteria. In most cases, rejection is caused by cosmetic issues. The way to differentiate XMs is to determine if it's military use, or civilian use.
AR Type Weapons
This is a variant of the M4A1 developed by SOCOM and NAVSPECWARGRU at the Naval Surface Warfare Center, Crane Division (often referred to as NSWC-Crane or Crane NAV/SEA). The Mk18 Mod0 took the full-auto M4A1 lower and added a 10.3" barrel, A2 front sight post (FSP), and the Knight Arms RAS rail. The 10.3" barrel was developed for Close Quarters Battle (CQB) use in urban environments, as well as on ships and oil platforms. The Army has named this as the M4CQBR (M4 Close Quarters Battle Rifle).
The typical configuration/build list of the Mk18 was:
Colt 10.3" barrel
A2 FSP/gas block
KAC flash suppressor
KAC M4 QD sound suppressor
KAC RAS forend
M4A1 or upgraded M16A1 lower
tan SOPMOD or other issued stock
CQD sling attachment plate
black A2 grip
FDE SpecterDR, Comp M4, or other issued sight
Insight or any other issued white light
This is the second iteration of the CQBR, and it was modified to have a Mk12 low-profile gas block, a free floated 9.5" railed forend and no front sight post. The major change was with the railed handguard and deletion of the FSP. The Navy went away from a KAC (Knight Arms) RAS rail to a free floated rail from Daniel Defense. The front sight became a KAC flip-up BUIS (back-up iron sight). The newest models are actually Daniel Defense models.
(sorry, this Airsoft pic was the best one out there)
The Mk12 Mod0 took an M16A4 and replaced the barrel and forend with new components to make it a precision sniper rifle, heavily using components from Precision Reflex Inc (PRI), A.R.M.S. and Ops Inc. The 18" barrel is a match grade stainless steel barrel with a heavy contour and 1:7 twist rate. It is equipped with a PRI front gas block that utilizes an integrated folding front sight. The front flash suppressor is an Ops Inc muzzle brake with an additional collar that is set back behind the brake. This collar allows the attachment of a suppressor. The handguard selected was a PRI GenIII carbon fiber free floated forearm tube. The top rail used was an ARMS #36 SWAN Sleeve rail that bolted on top of the upper receiver rail and provided an uninterrupted mounting surface along the top length of the receiver and handguard. This rifle was designed for SEALs to use for precision target engagement at distance, while still maintaining the lighter platform of the AR. The rifles are designed to shoot a 77gr Sierra MatchKing HPBT match bullet, denoted as the Mk262 Mod0/Mod1 rounds. The Mk12 Mod0 was originally equipped with the Leupold Mark4 MR/T 3-9x36 Illuminated M2 scope.
This rifle was updated to use a fully railed free float handguard while eliminating the front sight. The barrel and muzzle brake stayed the same, but the handguard was switched to a KAC (Knight Arms) RAS rail. A steel low-profile gas block was utilized. Later models were updated with an adjustable gas block for use with suppressors to enhance reliable functioning. Later models were also equipped with collapsible stocks in lieu of the fixed A2 stock. The Leupold was eventually replaced by the NightForce NXS Compact 2.5-10x24. The Mk12 was originally designed for use with the SEALs, but the USMC began using it as well. Eventually, the US Army Rangers picked up on it since they are such a mobile force, they wanted a lighter sniper rifle system.
A 7.62x51mm NATO chambered precision rifle produced by Knight Armament Company (KAC, Knight Arms), based on their SR25 platform. It has a 20" Obermeyer match grade barrel and was designed for use with the US Navy Seals. It is also used by the USMC MEU(SOC). It had a free-floated KAC RAS handguard and flip-up BUIS. The rifle was originally equipped with a Leupold Mark4 LR/T 3.5-10x40 scope, which was later replaced by the Schmidt&Bender PMII 3-12x50 SSDS scope.
This is the US Army variant of the SR25 platform, and is almost identical to the Mk11 system. The M110 uses a different stock which has an adjustable length of pull, and it has the updated KAC URX free-float rail, which offers an uninterrupted top rail from the receiver to the end of the handguard. There is no gap over the barrel nut like there is with the RAS. The M110 is issued with the Leupold Mark4 LR/T 3.5-10x40 M2 Illuminated TMR dark earth scope.
Mk20 SSR (Sniper Support Rifle):
The Mk20 is the new replacement for the Mk11 system within the Navy and USMC. It is based on the Mk17/SCAR-H/SCAR17 platform, but utilizes a heavier 20" target barrel and a longer rail system. It has a non-folding fully adjustable precision stock. This rifle is ultra reliable and rugged, and had exhibited greater accuracy than the Mk11.
14.5" barrel carbine, 1:7 twist rate, flattop upper receiver with detachable carry handle, collapsible buttstock, and semi and 3rd burst fire control
Same as M4 but with semi and full auto fire control
20" barrel with 1:12" twist rate, semi/full auto fire control, and fixed carry handle upper receiver, fixed stock
20" barrel with 1:9" twist rate, semi/3rd burst fire control, and fixed carry handle upper receiver with updated rear sight assembly, redesigned fixed stock
20" barrel with 1:9" twist rate, semi/3rd burst, and flattop upper receiver with detachable carry handle (like the M4), and KAC RAS free float rail, fixed stock
Similar to A4 but utilizes a new collapsible buttstock instead of the fixed A2 stock. Basically an M4 with a longer barrel.
An M16A4 modified with a heavy fluted stainless steel barrel with 1:7 twist. This was developed by the US Army Marksmanship Unit (AMU) and they currently are issuing this to Army designated marksmen/squad designated marksmen. The AMU actually has a training course for this rifle for Army DMs/SDMs, and experienced CMP (Civilian Marksmanship Program) volunteers actually assist with the training.
AR Rifle Roles
Close Quarters Battle (CQB):
These are carbines that are designed for close-quarters or urban use. They typically have short barrels of 16" or less, with preferred lengths being in the 10.3" to 14.5" range. CQB rifles are equipped with either iron sights or rapid-acquisition reflexive sights like holographic electronic sights (EOTech), red dot sights (RDS) (Aimpoint, TriPower), or reflex sights (Trijicon Reflex, CMORE). These rifles are often configured with accessories designed for CQB use, like lights, railed forends for accessories, laser aiming modules, vertical foregrips, etc. CQB rifles utilize muzzle devices that reduce muzzle rise and recoil, which enables faster follow-up shots during rapid engagement.
Designated Marksman Rifle/Role, Squad Designated Marksman Rifle/Role (DMR, SDMR):
DMR and SDMR rifles are designed for intermediate to longer range accuracy, while still maintaining usability in urban and CQB environments. DM and SDM rifles typically have medium to longer barrels, ranging from 14.5" to 20". The ideal barrel length for this platform is 16" for a carbine setup, and 20" for a rifle setup. DM/SDM rifles can be equipped with standard milspec barrels, but it is preferable to equip them with heavier match grade barrels with faster twist rates of 1:7" or 1:8" in order to fire better quality match bullets, like the 62gr Mk318 SOST rounds. DM/SDM personnel are expected to fill the role of a sniper in times when one isn't available, in order to act in an overwatch or counter-sniper role. It's best to think of them as "field expedient snipers". As a result, DM/SDM rifles must be capable of accurately engaging targets out to ranges greater than what normal rifle configurations are designed to hit. The general range for standard rifles is 300m (USMC trains to 500m, but the hits at that range are not exceptionally accurate with iron sights or RDS systems). DM/SDM rifles are able to fill the gap in the 300m-500m range with more accurate directed fire. Ranges from 500m-800m are also not out of the question when absolutely necessary, but the rounds begin to lose effectiveness past 500m-600m. DM/SDM rifles are equipped with magnified optics. The most common optic is the 4x Trijicon ACOG, with other power ACOGs being available. Additional options include the SpectreDR 1x/4x and 1.5x/6x sights, as well as various 1-4x, 1.5-5x and 1.5-6x scopes. Because shots are often made under direct fire combat conditions at multiple ranges, it is preferred for the optics to have a ballistic range compensating reticle tailored to the load being used, or one that is close enough to be effective without the need to alter the elevation or windage of the scope. A secondary non-magnified sight can be affixed to the weapon or optic to enhance the capability in CQB conditions. These are known as "stacked optics". There are new "short-dot" scopes in the 1-4x or 1-8x range that have the ability to change between a 1x red dot sight to a magnified optic. These scopes work well for a DMR setup.
Special Purpose Rifle (SPR):
SPRs are precision rifles designed to engage targets at intermediate to long distances, and are typically employed by trained snipers. SPRs in 5.56mm are designed to accurately engage targets out to ~900m using heavy match loads like the 77gr Mk262 variants, as well as the new experimental 82gr PRL. An SPR will be equipped with a longer match grade heavy barrel, with typical lengths ranging between 18"-20". The ideal barrel length is 18", which gives optimal accuracy while still keeping the weight down. SPRs are thus much less cumbersome than typical sniper rifles due to this lighter weight and shorter barrel. The barrel should be in 1:7" or 1:8" twist in order to stabilize the heavy rounds for long range flight. SPRs are typically 1MOA to sub-MOA accurate. They have free-float handguards, utilize muzzle devices designed to reduce recoil and light flash, and have folding sights that do not obstruct the field of view of the optics. It is ideal for SPRs to use Intermediate or Rifle length gas systems, which makes for a smoother gas cycle. The handguards should be free-floated and capable of either having rails attached, or should have integral Picatinny rails for mounting a bipod, as well as other accessories like a laser module and backup iron sights. The optic should be a variable-power magnified scope in with a top-end power level 9-12x. Most SPR scopes are typically of the 2.5-8x, 3-9x, 3.5-10x, or 2.5-10x range. There are other acceptable scopes to use like a 3-12x, 1.5-8x, 2-12x, or 1.8-10x from various vendors. Short-Dot scopes in the 1-8x range are also very effective in this category. The idea is to have sufficient magnification on the top end, but have a very low number for the bottom-end to allow the most field of view for close range shots.
Semi-Auto Sniper System (SASS):
The SASS is essentially the same as an SPR, but is of a heavier caliber. SASS rifles typically are 7.62NATO (.308) caliber, and should be equipped with barrels between 18"-22". Ideal length is either 18" or 20". SASS rifles are designed with accuracy up to 800m mind, though they are 1000m capable when using the M118LR 175gr BTHP ammunition.
Battle rifles are combat rifles and carbines that are chambered in heavier calibers like 7.62NATO. They can fill a CQB role, but more often fit into the DMR/SDMR category. They are designed to offer better performance at long range while still being capable of CQB employment. When operating in wide open terrain like in mountainous areas where shots will often be at extended ranges, this is the ideal weapon system. Battle Rifles can be equipped like CQB rifles, but really come to life when equipped like DMR/SDMR setups with low or moderate powered scopes in the 4-8x range. Optimal scopes include the Trjicon 5.5x50 ACOG TA55A, Trijicon 6x48 ACOG TA648, Elcan SpectreDR 1.5x/6x, USO SN-4 1.5-6x, Leupold Mk4 MR/T 1.5-5x, Weaver Tactical 1-5x24, the short-dot 1-8x scopes, or any of the 1-4x scopes. These rifles can fill multiple roles and work well in combat roles using standard BALL ammunition like the 150gr M80, or be used for precision by switching to heavier precision rounds like the 175gr M118LR.