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”.

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.



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 general belief that anti-rotate pins are only necessary if you have weak hammer springs is a myth.
The misconception in the AR world is that the walking of the trigger pin is typically caused by weak hammer springs. Many believe that, as a hammer spring ages and weakens it may no longer exert enough force on the trigger pin to stay in the groove. If it gets out of the groove at all, the rotational forces of the trigger spring can push the pin out.

Hammer pins occasionally break because the hammer spring is new/heavy and the pin steel is weak for whatever reason at the center groove.

However, inconsistencies in manufacturing and materials used can also lead to walking. An out of round hole can show up after 500 rounds on a new firearm.

Anti-rotation pins not only solve pins falling out/walking, but also allows for proper disconnector operation (so you don’t get an off-axis sear and wind up with a full-auto condition), better trigger feel, and more.

This allows keeps everything on axis, keeping the lubrication where it needs to be.

KNS Precision is well-known for making anti-rotation pins. Here’s what they had to say:

I will begin this explanation with a little back ground on this part. We are all machine gunners here at KNS Precision, Inc. and the idea was to come up with a product that would help preserve the $10K-$15K transferable M16s. We recognized a problem with the M16 in that each time the trigger is pulled or the hammer moves back and forth, that the steel pins rotate inside the aluminum receiver. We realized that stopping that rotation would stop the wear. There are “EXPERTS” out there who say that this does not happen and it is a hoax. All one has to do is to use a marking pin and mark the trigger and hammer pins and the receiver. No need to actually shoot the weapon, just cycle it and pull the trigger. You will see the rotation. If you are familiar with M60 Joe, he does a lot of work on M16s and he can verify that this is a legitimate issue. The Gun Store in Las Vegas can also verify that the rotation is an issue.

To the next question that comes up. Okay, we understand the use on an M16, but why would someone want to use it on an AR15? At one time I asked the same question myself, but after one range experience with a brand new factory rifle (very reputable company) having the pins walk out on the first magazine and the mess that cause inside the rifle, I understood why. There are three reasons why pins walk on an AR15. 1.) Improper Installation of the pins/springs 2.) Oversized/ Out of spec holes in the receiver 3.) Use and abuse wear of the receiver from the pins rotating We have had multiple customers contact us with all of the above issues. Our product eliminates concern with all of the above. I have heard people say that it is the best insurance you can buy for $33.00 . A side effect that was brought to our attention by several gunsmiths is that our Non-Rotate Trigger and Hammer Pins improves the trigger on the AR15 rifles, especially when used with match triggers. What we have found is that by locking the trigger and hammer pins together using the Non-Rotates, it eliminates the slop in the trigger.

I hope that this might help you. If you have any further questions, don’t hesitate to get in touch with me and if I can not answer it for you, I can forward it to someone who can. Thanks!

Gordon Gipson
KNS Precision, Inc
112 Marschall Creek Rd
Fredericksburg, TX 78624
Phone: 830-997-0000
Fax: 830-997-1443


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 defines a “caliber” (as it pertains to ordnance) as, “the diameter of the bore of a gun taken as a unit of measurement.”

In a rifled barrel, the distance is measured between opposing lands and grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. Good performance requires a bullet to closely match the groove diameter of a barrel to ensure a good seal.


The beauty of the AR-15 platform is the ability to use many different calibers in the same lower receiver.

Wikipedia shares the following calibers as possible for the AR-15 platform:

Rimfire calibers

  • .17 HMR
  • .17 Winchester Super Magnum
  • .22 Long Rifle

Centerfire calibers inch measurement

  • .17 Remington
  • .204 Ruger
  • .223 Remington – .223 cartridges may function in a 5.56x45mm rifle, however 5.56×45 cartridges will produce excessive pressure in a .223 rifle.
  • .243 Winchester Super Short Magnum
  • .25 Winchester Super Short Magnum
  • .30 Carbine
  • .30 Remington AR
  • 300 AAC Blackout (7.62×35mm)
  • .300 Whisper
  • .375 Reaper
  • .40 S&W
  • .45 ACP
  • .450 Bushmaster
  • .458 SOCOM
  • .50 Beowulf
  • .50 Action Express

Centerfire calibers metric measurement

  • 5.45x39mm
  • 5.7×28 FN
  • 6.5mm Grendel
  • 6.8 mm Remington SPC
  • 7.62×37mm Musang
  • 7.62×39mm
  • 7.62×40mm Wilson Tactical
  • 9mm Parabellum
  • 10mm Auto

Calibers which will not feed but which are used in rifles where the receiver functions only as a trigger group

  • .50 BMG

For a long time, everyone and their brother wanted a chrome-lined barrel. They’ll pay extra for it. They’ve heard it’ll last almost forever. They’ve heard it’s the best barrel that ever existed.

And they’re only partly correct.

Chrome lining can protect the barrel from heat and pressure. That’s true.

A good chrome lined barrel can still last for 5,000 rounds before you see any degradation. Maybe more.

However, chrome lining can be uneven in barrels, depending on the manufacturer. The barrels can be SLIGHTLY less accurate than a non-lined barrel (you ARE lining the rifling inside the barrel and therefore there is a slight coating over the lands and grooves, dulling them ever-so-slightly.)

Essentially, chrome-lining is one of those leftover processes that “a guy at the range heard from his buddy at work that his dad always told him…” that chrome-lining was the ONLY kind of barrel you’ll want/need.

This is not to take away from chrome-lined barrels. Like I said, they protect against heat and pressure in an AR. It’s not to say that there are zero benefits to using one.

But there are a LOT of other choices when it comes to barrel treatments. See also Meloniting, Gas Bed Carborization, and Cryogenic Tempering