AR15 Rebuild Build: Updating the Musket teams up with Brownell’s to update an old AR-15

The following article chronicles our build and is for informational purposes only.  

This AR handles well and shoots great. It cleaned the CSAT standards from 50 yards and in the first time out.
Updating the musket

Reflecting on the round count for the carbine I am currently shooting, I stop counting at 10,000 rounds.  Consider this, the barrel was fairly heavily used when I purchased it a few years ago- now we are well past the point of getting my money’s worth out of it.  The trigger pins had begun wearing away at the receiver until I installed KNS pins three years ago.  Notably, most major AR15 manufacturers are represented in this rifle; the Colt upper and SOCOM profile barrel; Daniel Defense Omega Rail; Larue BUIS, Aimpoint mount and bolt carrier group; DPMS gas tube, buffer and ejection port cover; Magpul grip and MOE stock; Geiselle SSA trigger; Vltor butt stock extension, Bushmaster lower and VTAC sling.   The hodge podge of parts are covered in multiples layers of chipped paint highlighted with various witness marks and arrows drawn on with an assortment of paint markers.  This is a “Frankenrifle” in every sense.

Time to build a new rifle.  Looking through my safe, I found my old Colt 6601, a pre 1994 20″ rifle with a 1/7 20″ government profile barrel.  This rifle, equipped with A2 sights and an A2 stock is similar in appearance to the M16A2 I was first issued in the Marines.  While there is a sentimental value to the configuration, I rarely shoot the rifle, and speaking to Marines currently in service, I was horrified to learn the 20″ guns are now referred to as “muskets”.

The "Musket"

Before beginning the update, it is important to make the rifle safe and empty.

Begin by ensuring that the rifle is pointed in a safe direction, with the selector on “safe”.  If a magazine were present, it would be removed.  Then the bolt is locked to the rear and the chamber and magazine well are inspected TWICE to ensure that no ammunition is present.

Rifle is pointed in a safe direction and on safe.
Bolt locked to rear. Check the chamber and magazine well twice to ensure it is empty

Rifle is clear.  Lets begin!

The bolt is released forward and the front and rear pivot pins removed.  The upper and lower receiver are now separated.

Upper and lower receivers are separated.

Since the the entire upper receiver is going to be replaced, it will be stowed for later, sentimental use.  The butt stock can now be disassembled.

Begin by removing the top screw located at the stock in the butt plate. I prefer Magnatip screwdriver bits; with the proper set, you can rest assured you will not damage the screws
With the screw removed, slide the stock to the rear and remove it from the rifle. Note the small detent spring that is exposed. This spring engages the detent that keeps the rear detent spring in place. Remove the spring and turn the lower on its end, buffer tube towards the bench and tap it gently to remove the detent. You can now remove the rear take down spring. Secure these parts for re assembly.
Depress the buffer retainer and remove the buffer and recoil spring
Push down on the buffer retainer and unscrew the buffer tube. I placed a lower receiver bench block in my lower and secured it in a vise. Use an armorers tool or wrench to unscrew the tube.
This shows the lower receiver bench block in the vise. The lower is pushed down onto the block until the magazine catch engages a cut out in the block.
Use a Magnatip screwdriver to remove the screw securing the pistol grip.
Once the screw securing the pistol grip to the lower receiver is removed, carefully remove the pistol grip. Secure the safety detent spring and safety detent. The detent may be stuck in the hole, turn the receiver right side up and move the safety, the detent should drop out.
Once the detent is removed, you can pull the safety out of the right side of the receiver. Use a punch to drift the the trigger and hammer pins and remove the trigger assembly.

The lower receiver is now stripped.

Upper Receiver:

I started this upper receiver build by selecting the barrel I wanted to use.  The choices in barrels may seem overwhelming at first, but the basics of selection are fairly easy.  I prefer direct impingement guns to piston driven guns, so direct impingement it is.  Barrel length, well that is easy, 16″.  Sight radius is determined by the length of the gas system, rifle (longest) mid length or carbine (shortest).  I selected the carbine gas system.  Why?  I have spare parts.   Daniel Defense’s 16″ M4 profile barrel (100-005-853) fit the bill.  Daniel Defense hammer forges, then chrome lines their barrels.  This yields an incredibly durable product that last for thousands of rounds.

The barrel will be installed into a flat top upper receiver (100-005-853), that was included as part of a kit I received from Brownells.  This kit includes a upper receiver, charging handle, hammer and complete bolt carrier group coated in Fail Zero’s proprietary EXO coating system which provides a permanent dry lubricity to the treated parts.

Fail Zero EXO Coated Kit

To address the small parts I will need to finish this build, I ordered a Colt upper receiver completion kit (080-000-607).  The kit contains all of the small parts you need to turn your barrel assembly, hand guards, upper receiver, charging handle and bolt carrier group into a complete upper.  The Colt kit costs slightly more then some of those by other vendors, but you get what you pay for.  In this case, parts from the company who has been building this rifle for over 40 years.

I start by installing the forward assist into the upper receiver.

Forward assist parts (right). Roll pin holder and punch (left).

I like to start with the pin that retains the forward assist first.  Use a roll pin holder to guide the pin and gently tap it in enough to start it.  Be careful not to drive it in to the point where it will impede installation of the forward assist.  I placed the upper on a rubber mat for this operation.

Roll pin holder starts the pin which retains the forward assist.

The spring slides over the forward assist’s body, then insert it into the upper.  If you have an AR with a forward assist, you can reference the inside in order to properly align the forward assist pawl.

Forward assist being inserted into the stripped upper.

Push the forward assist in the remainder of the way and use a roll pin punch to drive the pin flush.  The use of the proper punch is critical with roll pins, convention punches can damage the pins and slip off, possibly damaging the parts.  The roll pin punch has a small guide in its center that prevents this from occurring.

The forward assist retaining pin is driven flush.

Next step, install the ejection port cover.  Here are the parts:

Ejection port cover parts. Notice the small "c" clip in the photo below the ejection port cover spring.

Begin by placing the “c” clip into the ejection port cover pin.

The ejection port cover pin has a small recess milled into one end. Press the "c" clip into this recess. Be careful not to drop the clip, it is small and hard to find on the floor.

The remaining installation is simple.  I start by inserting the ejection port cover pin into the ejection port cover.  Once the pin is started, align the spring as shown below and finish pushing the pin through until the “c” clip hits the receiver.  Close the cover and flip it open when you are done.  The cover should be driven open by the spring, if the cover is not under tension to stay open, you installed the spring backwards.

Ejection port cover fully assembled.

Time to prep the barrel for installation.  Begin by removing the front sight base.  This can be a tricky process if your front sight base is installed with mil spec taper pins, this is the case with the Daniel Defense barrel.  To aid in the process, Brownells makes an AR15 front sight bench block  (080-000-252) specifically designed for the task.  I have tried this with standard bench blocks and hockey pucks, this is another case where the right tool helps get the job done.  Once the barrel is secured in the block, I start the pins with a short replaceable pin punch and then finish driving them out with a standard punch.  Slide the front sight assembly off of the barrel and secure the pins.  I typically keep a supply of small bags on hand to secure parts as I progress along.

Drifting taper pins with the barrel secured in the AR-15 front sight bench block.
Once the pins are removed, simply slide the front sight assembly off of the front of the barrel.

This upper receiver will use a Daniel Defense FSP Rail Interface System II (RIS) in flat dark earth (100-005-809).  Weighing in at 16.2 ounces, the RIS II rail system is constructed of aircraft grade aluminum and coated in military specification hard coat type III anodizing.  It has 12.57″ of rail surface and a cut out to allow a standard front sight to protrude through the rail.  Begin by placing the bolt up plate and the barrel nut onto the barrel.

Bolt up plate (left) and RIS II barrel nut (right)
Note orientation of barrel nut and bolt up plate.

Insert the barrel extension into the upper receiver.  I typically put a light  coat of oil on the barrel extension to help it slip into place.  In this case I am using a light coat of Clenzoil Field and Range.

Insert barrel extension into upper receiver


The barrel extension is fully seated. Slide the bolt up plate back into place and hand tighten the barrel nut
Barrel nut is hand tight

Now it’s time to torque the barrel onto the upper receiver.  Begin by securing the upper in an upper receiver bench block.  These blocks are typically equipped with a polymer insert to prevent receiver breakage.

Place the upper receiver block insert into the upper. The ejection port cover will open when you do this. Once the insert is fully seated, close the ejection port cover.
Place the upper receiver into the block and place in a vise.

Once the receiver is in a vise, use the Daniel Defense Lite Rail wrench to tighten the barrel nut.  This wrench is not included.  Do not use a single point steel spanner, you will end up gouging the aluminum barrel nut.  Daniel Defense recommends between 50-75 foot pounds of torque be applied.

The barrel is now attached to the upper receiver.  To proceed, we will need to attach the gas tube to the front sight base.  This requires the carbine length gas tube and gas tube pin, both of which are included in the upper receiver completion kit.

Insert the gas tube into the front sight base assembly. Note: the gas port is facing down, towards the barrel.


Secure the front sight base assembly in the front sight bench block and set the roll pin with a roll pin holder. Finish driving the roll pin flush with a roll pin punch.

With the gas tube attached to the front sight base assembly, the rail and front sight can be slid onto the barrel.

Place the front sight base assembly into the slot in the top of the rail and slide both into place.


Lift the rail up slightly and insert the taper pins into the holes. Drive them home with a punch.


Insert the two 3/4" screws into the bottom of the rail and hand tighten.
Now insert the four 7/8" screws into the front of the plate and hand tighten.

Once the four screws are installed,  verify alignment of the rail and begin to tighten the six screws to between 29-32 inch pounds of torque.  Daniel Defense recommends that you torque the center screws first, then the upper screws and finally the lower screws.

Now you can slide in the bottom part of the rail.

Tighten the four screws that secure the bottom rail to the top.

Installation of the rail is complete.


The Primary Weapons FSC 5.56 is an outstanding muzzle brake.  Even though a 2.23 rifle doesn’t kick much, the reduction in muzzle flip is immediately noticeable.

FSC 5.56 with shim kit

I’ve installed these brakes in the past and indexing them can be tricky.  Normally, alignment is accomplished through a crush washer or a peel washer.  Primary Weapons now includes a “Muzzle Device Alignment Set”, this set includes three different thicknesses of washers, each offering a its own correction of the orientation of the muzzle.

Note the recess towards the back of the brake. This hides the indexing washers shown in the last photo

Since the FSP RIS II rail is so long on this upper, the barrel cannot be secured in the vise using a barrel block.  In this case we will use some rubber lined vise jaws.

Barrel secured in rubber vise pads
Indexing the brake: adding and changing the shims from the kit allow the installer to index the brake in the proper orientation


Install the charging handle and the bolt carrier group and the upper receiver is complete.  Time to work on the lower.

Lower Receiver

Since this lower receiver uses nonstandard .169″ trigger pins (most AR-15s  use .154″ pins), trigger choice is somewhat restricted.   Geiselle makes excellent triggers for the AR platform, and their service rifle trigger is available with .169″ trigger pins (100-003-615).  The Geiselle Hi-Speed Service Trigger is an adjustable two stage trigger with a first stage from 3.2 to 5 pounds and a second stage from 1/2 to 1 1/2 pounds.  More information on this trigger is available here, check out the trigger profile on the second page.

Since the trigger installation is quite complex, it will not be covered in detail here.  If you would like complete instructions, Geiselle has a them posted on their website (installation instructions).

I used my trigger weight set (678-017-000) to verify the trigger could pick up a 4.5 pound weight (this is the lightest I like my triggers).

Note:  Colt used to install hardened steel sear blocks in their lower receivers.  These were not required by law, but installed by the company.  If you plan on using this trigger in a lower with a sear block like I did, you will need to remove the sear block.  The block was very difficult to remove and is designed to destroy the receiver.  I Google’d “Colt sear block removal” and found a few links that offered advice.  Basically you cut the block lengthwise with a Dremel and the side pins fall out.  Then you pull up on the block itself.

Geiselle trigger
Installing the trigger that Geiselle provides


Last up is furniture and pistol grip installation.  The pistol grip is the Tango down BG-17  (100-006-310).  The BG-17 has a larger size then the standard AR-15 and fills most shooters hands a little better.

Once the safety is in place, insert the safety detent and spring and attach the pistol grip.

Brownells offers AR-15 stock mounting kits from various manufactures.  These kits, available for A2 and collapsible style stocks, include all parts necessary to install a collapsible stock.  In this case, a receiver extension, end plate, castle nut, buffer and recoil spring.  Since this is a Colt receiver, a Colt kit is being used (080-000-620).

Original Colt parts. A Colt buffer tube extension, end plate and castle nut
Thread the castle nut on first with the four notches facing outboard. Slide the end plate into position as shown.
Place the rear take down pin into the lower receiver. Insert the push pin detent and spring.
Begin turning the receiver extension into the lower. Make sure you stop when the hole for the buffer retainer is still unobstructed.
Place the buffer retainer and spring into the hole and finish turning the receiver extension in place. Notice the receiver extension retains the retainer in place and prevents it from popping out.
Slide the end plate forward. Make sure you guide the detent spring into the hole and then hand tighten the castle nut.
Place the lower in the vise block and use a castle nut spanner and torque wrench to tighten to spec.

Finishing touches

I like to keep a light on all my carbines.  The primary advantage of the Daniel Defense FSP RIS II is the extended surface in front of the factory front sight base.  This is an excellent location for a light.

The Surefire X300 (152-000-041) simply slides onto the rail and is quickly removable by the user.  The light has a rugged LED lamp and a 170-lumen output with a 2.4 hour run time and includes adapters for various handgun rails as well as the standard weaver rails found on many long guns.

Surefire X300
The switch on the back of the light housing can be easily manipulated by the shooter's thumb without moving the support hand.

Sling and Mount

Readers of this site will not be surprised that we selected the VTAC sling (100-005-404) for this carbine.  Since the VTAC sling is a two point design, requiring a mounting interface on the front and back of the rifle, an attachment point is needed on the rail. The Daniel Defense RIS II does not include a QD loop mounting hole on its surface like some.  While there are a few ways to address this, VTAC has once again answered the called with a novel sling mount that works with most systems.

The VTAC L.U.S.A. (100-004-331), attached to a rifles rail system via an integral clamp, provides the shooter the ability to attach the sling via a QD sling swivel, traditional loop, hook attachment or by simply running the sling through the slot on the mount- one adapter, many options.

VTAC L.U.S.A. with QD Sling Loop
VTAC L.U.S.A. with sling in integral loop
VTAC L.U.S.A. with traditional sling mount




Reader of this site are well aware of how much we like the Aimpoint T1.  The sight, while small and compact does have two slight disadvantages; an uncommon battery size and a small tube.  For this build we selected the Aimpoint Comp M4S.  This site is equipped with a 2 MOA dot, half the size of the T1, which should aid in shooting past 100 yards.   The 4MOA dot on a T1 subtends 8″ at 200, which is quite a bit larger, this will only subtend 4″ at that distance.  The M4S has a larger tube and the dot is easier to pick up for the new shooter, additionally the optic uses standard AA batteries.

AImpoint Comp M4S
Close up of the factory Aimpoint mount. If you look where the knurled knob meets the base you will notice a ratcheting mechanism that provides consistent torque each time the shooter mounts the optic. Simply turn the knob until it slips and the torque is the same.
All ready to go!
Completed rifle, left side


Upon completion of the build, this old musket headed to the range.  The system worked perfectly, there is something to be said for using quality parts to build a carbine.  This project truly is greater then the sum of its parts.


For the AR rebuild 
Fail zero upper kit 100-005-446
barrel 100-005-853
rail 100-005-809
upper completion kit 080-000-607
Magpul CTR stock 100-002-946
Colt  stock mounting kit (080-000-620)
(2x) QD swivels 100-004-880
Vtac sling 100-005-404
Aimpoint Comp 2.0 MOA M4S 100-003-734
Trigger 100-003-615
Trigger scale 678-017-000
Tangodown BG-17  (100-006-310)
AR15 fron sight bench block 080-000-252