In this post I’ll go over all the specific parts of the Upper Receiver and Barrel with all associated parts.  Click this post or the ‘continue reading’ link below to see the full details.

Upper Receiver

Much like the lower, my basic understanding is that uppers are all pretty similar.  There are a few variations of design, but all function the same. Here are some of the variations:

A1 Style Upper: Has a fixed handle on the top.

A2 Style Upper: Has a fixed handle just like the A1, except that it has a rear sight with elevation and wind adjustments.

A3/A4 Style Upper: Sometimes called ‘flat top’. It has an accessory rail on the top for mounting any type of scope, sight, or A2 style removable handle. The A4 variation is effectively the same receiver, except that it has M4 style feed ramps that compliment the feed ramps on an M4 barrel. This actually rather confusing, so I’m dedicating a whole post to the M4 variation.

Forward Assist – This ‘button’ more / less serves a mechanical function of applying forward pressure on the bolt.  My understanding is that in certain combat situations, dirt, dust, and gunk can clog the upper and prevent the bolt from closing properly against the back of the round.  If this jamming occurs, you can slam the ‘forward assist’ to jam the bolt into position.  I’ve talked with AR range shooters who have never had to touch this button, and I don’t expect to have to use it either.  There are some variations of the Upper that don’t have the forward assist at all.

Dust Cover – Pretty straight forward.  It snaps closed… and when the bolt is moved forward or backward it pops back open.

Charging HandleThis works similarly to racking the slide on a semi-automatic pistol.  The charging handle allows you to bull the entire Bolt Carrier Group backwards and release it loading the first round into the chamber.  If you want to lock the bolt in the ‘back’ position, you similarly use the charging hand to pull the BCG back and engage the Bolt Catch.

T-Marks – A lot of listing for uppers mentioned having or not having ‘T-Marks’… WTF is that?  T-Marks are small white location indicators that are integrated into a Picatinny Rail system.  They’re used to note the location of an accessory should you decide to put it back after a re-location.  They simply count the number Picatinny Rail spaces… utilize them however you’d like.  Oh, and these are T marks because they’re on the TOP. A quad-rail hand-guard can have T-Marks, B-Marks, L-Marks and R-Marks.

Delta Ring Assembly


Includes Delta Ring, Barrel Nut, Weld Spring and Barrel Snap Ring.  This kit serves 2 functions: 1) Secures the Barrel to the Upper Receiver 2) Provides an attachment for the Hand Guard.  Some Hand Guards require a special type of assembly and are typically included with that Hand Guard if needed.

Hand Guard

Hand Guards or sometimes referred to as the ‘Forearm’, attaches to the Upper / Delta Ring assembly and serve as a ‘forward position’ place hold your rifle.  The Barrel gets pretty hot, so you don’t want to be stabilizing your rifle holding that!  Hand Guards come in all shapes and sizes, made of polymers and aluminum, etc etc.  The most common type I’ve seen is what they call a 4-Rail system.  Meaning is has 4 sets of Picatinny accessory rails on the top, bottom, left and right.  The correct length hand guard you purchase should correspond with the length of barrel you’re utilizing.  Typically, Carbine Length (16″ Barrel), or Rifle Length (20+” Barrel).  There are some Mid-Length hand-Guards that are designed to cover the Gas Block.  More on that below in the Gas Block section.

Free-Float Hand-Guards – There is a variation of the Hand-Guard referred to as Free-Float.  What this means is that the Hand-Guard only attaches to the upper at the rear position delta ring / barrel nut, and NOT again at the front gas block area.  The idea here is that by not letting the hand-guard touch the Barrel at all, any pressure exerted on the Hand-Guard doesn’t flex the barrel.  Even though this ‘flex’ of the barrel would be minimal, it is still a deviation in barrel angle and the effects are exacerbated by shooting distance.  These Hand-Guard designs are a touch more expensive as I’ve noticed, but increase rifle accuracy.  I’m sold.

Gas Block / Gas Tube

The Gas Block attaches to the Barrel at the point where the ‘Gas Port’ is cut into the barrel.  Much like Hand-Guards, there are tons of different styles of these.  Some with Picatinny Rail systems, some without.  Above I mentioned that Mid-Length Hang-Guards were designed to cover the Gas Block… in that case, you need a ‘Low Profile’ Gas Block so there’s room for the Hand-Guard.  I think it’s safe to say that your Gas Block should be chosen to work as a pair with your choice of Hard-Guard.

The Gas Tube’s role is to transport the high pressure gas that was fielded by the Gas Block, and channel it back to the Upper Receiver and into the Gas Tube receiving end of the bolt.  The Barrel Nut is made so that once you get it tightened down, there’s only a small adjustment needed to feed the Gas Tube into the Upper Receiver.  WTF is all this needed for?  Check out the Gas System post for details on why this is necessary.

Muzzle Break / Flash Hider

This screws onto the threaded muzzle of the Barrel and comes in a multitude of designs.  The main purpose is to diffuse the muzzle flash from the rifle to make it less observable in battle.  It also typically has no cuts on the bottom so when firing in the prone position, no dust is kicked up into the air.

See all of this stuff in action! Check out this cool old school US Army training video form 1966: