In order to understand the differences between the various types of upper receivers, it’s important to know what was going on with the REST of the rifle at the same time.

From World Guns, Modern Assault Rifles

With rapidly growing presence of US troops in Vietnam, in 1966 US Government makes the first large purchase of the Ar-15 / M16 rifles, ordering 840 000 rifles for US Armed forces, worth almost $92 millions, and in 1967 US Army officially adopts the XM16E1 rifle as a standard “US Rifle, 5.56mm, M16A1”.

m16a1
During immediately following years, a number of negative reports apears from Vietnam. M16A1 rifles, issued to US troops in the Vietnam, severely jammed in combat, resulting in numerous casualties. There were some causes for malfunction. First of all, during the introduction of the new rifle and its ammunition into the service, US Army replaced originally specified Dupont IMR powder with standard ball powder, used in 7.62x51mm NATO ammunition. The ball powder produced much more fouling, that quickly jammed the actions of the M16 unless the gun was cleared well and often. It also had different pressure curve, resulting in increased stress on operating parts of the gun. This pitifully combined with the fact that the initial M16 rifles were promoted by the Colt as “low maintenance”, so, for the sake of economy, no cleaning supplies were procured for new M16 rifles, and no weapon care training was conducted fro the troops. As a result, soldiers did not knew how to clean their rifles, and had no provisions for cleaning, and things soon turned bad. Another cost-saving measure on the part of the Army was to give up with cromium plation of the barrel bore and bolt group, which made these parts much more sensitive to corrosion and rust that originally designed.

After several dramatic reports in US press and Congressional investigation of the troubles, several actions were taken to remedy the problems. The 5.56mm ammunition was now loaded using different powders that produce much less residue in the gun action. The barrel, chamber and bolt of the rifles were chrome-lined to improve corrosion resistance. Cleaning kits were procured and issued to troops, and a special training programs were developed and conducted ever since. Earliest cleaning kits could be carried separate from rifle only, but since circa 1970 all M16A1 rifles were manufactured with the containment cavity in the buttstock, that held the cleaning kit. At the same time (circa 1970) the new 30 rounds magazines were introduced into service instead of the original 20 rounds ones, to equal Soviet and Chinese AK-47 assault rifles, which had 30-rounds magazines from the very beginning.

So, now that you know the history of the entire m16A1 rifle, the A1 upper receiver differences should be obvious.

The A1 original upper receivers would not have the forward assist or shell deflector but will have a fixed carry handle. Mainly, the rear sight will only adjust for windage and not for elevation.

original a1

Original Colt M16A1 Upper Receiver

Click here to learn about the A2 Upper Receiver

In order to understand the differences between the various types of upper receivers, it’s important to know what was going on with the REST of the rifle at the same time.

From World Guns, Modern Assault Rifles

In 1981, Colt developed a variation of the M16A1, adapted for the SS109/5.56mm NATO cartridge, and submitted it to the military trials as the M16A1E1. This rifle differed from the M16A1 by having the heavier barrel with faster 1:7 rifling, a different type rear sights (adjustable for both range and windage), round handguards instead of triangular ones, and by replacing the full-auto fire mode with the burst (limited to 3 rounds per trigger pull), to preserve the ammunition. It was officially adopted by US DoD as the “US Rifle, 5.56mm, M16A2” in 1982, which still is the primary infantry rifle for US Armed forces and a number of other armies and law enforcement organizations.

M16A2

M16A2

As you can see, the M16A2 upper receiver underwent some changes from the A1, namely the addition of the forward assist and the ability to adjust for elevation as well as windage on the rear sight.

Click here to learn about the A1 Upper Receiver

In order to understand the differences between the various types of upper receivers, it’s important to know what was going on with the REST of the rifle at the same time.

From World Guns, Modern Assault Rifles:

By 1996, the two newest versions of the M16 appeared, the M16A3 and M16A4. These differ from the M16A2 by having a removable carrying handle, with the upper receiver being fitted with a Picatinny-type accessory rail. Otherwise the M16A4 is similar to the M16A2, while the M16A3 also replaced the infamous three-round burst mode with a full auto mode. The key advantage of both the M16A3 and A4 rifles is the ability to quickly mount and re-mount a wide variety of optical, red dot or night vision / IR sights with MIL-STD 1913 (Picatinny-type) compatible mounts. The M4 carbine was also upgraded to “flat top” configuration, which is now standard.

A3 Upper Receiver

The barrel nut has a couple of functions.JB-BarrelNut-2T

For starters, it secures the barrel to the upper receiver. The barrel indexing pin fits into the indentation in the upper receiver and the barrel nut threads over top of the barrel, onto the receiver, in order to keep the barrel affixed to the upper receiver.

Secondly, the barrel nut allows the handguard – whether free-floating or drop in – to attach to the upper receiver. After securing the barrel nut to the upper receiver, most free-floating handguards attach to the barrel nut via allen keys or a company-specific wrench.

Finally, in the case of direct impingement, the barrel nut p_100009657_2holds the gas tube in place to allow the AR-15 to cycle properly. When a bullet is fired from the AR, the gas forcing the bullet down the barrel is pushed through the gas port, through the gas block, down the gas tube (which is secured to the upper receiver by the barrel nut) and helps to cycle the next round

Image4

Fixed A2 Front Sight Post with Integrated Gas Block

Fixed A2 Front Sight Post with Integrated Gas Block

Gas blocks serve an important role in the function of your AR-15.

Whether rail height, low profile, or part of a fixed A2 front sight post, the gas block holds the gas tube in place, which is key to the cycling of your AR.

Gas blocks are precisely positioned over the gas port in the barrel of the AR. When a round is fired, the bullet travels down the barrel, propelled by gas. Once the bullet passes the gas port in the barrel (a tiny hole in one side of the barrel), the gas is forced through the gas port contained by the gas block, through the gas tube, and back into the upper receiver to cycle the next round in your magazine.

Rail Height Gas Block

Rail Height Gas Block

Low Profile Gas Block

Low Profile Gas Block

Also called: forend, forearm, foregrip. drop in

The handguard on an AR-15 is located over the barrel of the rifle. It can come in a variety of lengths (usually 7″, 9″, 12″, 13″, 15″, and 17″), are available in drop-in or free float, can be made from a variety of materials (polymer, aluminum, and carbon fiber to name a few).

Some handguards require very few tools and know-how, while others require more extensive knowledge of AR modification and tools.

Most free-floating handguards will require a barrel nut and barrel nut wrench for installation.free float

Drop in handguards will require a delta ring assembly, end cap, and either a rail-height gas block or fixed A2 front sight post.

For an AR-15, the upper receiver is the part of the firearm that houses the charging handle, bolt carrier group, and in some receivers also the forward assist and dust cover. Newer models of the upper receiver may also have the charging handle integrated into the receiver, rather than as a stand-alone part.

Most upper receivers also have a brass/shell deflector. However, some competition-level receivers do not.

Conversely, some upper receivers in an A1 or A2 configuration have fixed carry handle on top, while other receivers like the A3 or A4 do not.

The upper receiver also connects the barrel and handguard to the lower receiver, as well as connecting to the lower receiver to complete the firearm.

A2 Upper Receiver with Fixed Carry Handle

A2 Upper Receiver with Fixed Carry Handle

Upper Receiver with Integrated Charging Handle

Upper Receiver with Integrated Charging Handle

Upper Receiver with No Forward Assist, Dust Cover, or Brass Deflector

Upper Receiver with No Forward Assist, Dust Cover, or Brass Deflector

Traditional Upper Receiver with Dust Cover and Forward Assist

Traditional Upper Receiver with Dust Cover and Forward Assist