Lower Parts Kit Install
Published by CMMG Team on
Jul 20, 2021 2:31:30 PM
Before we begin this adventure, it should be noted that, while this can certainly be done with common tools, it will make life easier if you have tools that are specific to building ARs, such as a punch set, an armorer’s wrench, or a bench block (used for building smaller AR-component assemblies and building a lower receiver group). Real Avid offers a complete tool set dedicated to building AR15s.
If you do not have any of the aforementioned tools handy, but would like to proceed anyway, we would also advise you to do installations of small springs and detents with the components and your hands inside something such as a large clear plastic bag so you don’t lose the parts when trying to install them. The clear plastic bag will create a barrier that keeps the little “flyers” contained within a set perimeter, speaking from experience. Also, keep in mind that we are installing these on CMMG lower receivers (AR15 and our .308 platform, the Mk3™), so there may be some differences between manufacturers.
With that being said… let’s begin!
The Clear/White Bag
The front Pivot Pin is a more involved process than installing the rear Takedown Pin, so we’ll start with that. It will be much easier to do if you have a Pivot Pin installation kit, but it can be done without it. We also recommend keeping the lower as vertical as possible to do this.
For the AR15, you want to install the spring in the hole. Lining up the detent can be a challenge, without the kit, but, you want to either use your index finger and thumb, or a pair of tweezers, to keep the pin lined up with the spring. Use the Pivot Pin to push the detent down into place and push the Pivot Pin through the guide hole. This has to be a very quick and precise movement, so don’t be discouraged if it takes a few tries.
A .308 Pivot Pin is a little easier, as the hole for the spring and detent are actually inside the Pivot Pin guide channel. Drop the spring into the hole at the top, then drop the detent directly on top of it. The hole at the top should hold the detent in place for you. Now, using a paper clip, push the detent down. Insert the Pivot Pin until it stops against the paper clip. Brace yourself, as this is where you need to be quick and precise again. Push the pin forward, while also removing the paperclip. If you did it right, the pin has captured the detent and you are good to go.
The Takedown Pins are much easier to install, but are done at a different step in the process than the pivot pin, as they are reliant on other components to stay in place. We will include installation steps for the Takedown Pins when we reach the steps they correspond with. For an AR15, that would be when we install a Receiver Extension Tube (more commonly called a Buffer Tube). For our .308 lower, it would be when you install the pistol grip.
The Yellow Bag
This is a step that we highly recommend at least having a set of punches handy. A good set comes with the tool kit we mentioned a the beginning of the article, but if you’d rather save a bit of money they also have just the punch set available, and it works really well. We recommend punches solely for the reason of keeping the lower from getting damaged, as you will be using a hammer for installing the bolt catch on an AR15. The .308 bolt catch is a little easier to install and only requires a 1/16 allen wrench.
If you do not have a punch set, you’ll at least need a set of tweezers or needle-nose pliers to hold the Bolt Catch Roll pin in place for your AR15 lower. Once the roll pin is in place and the lower receiver is braced so it doesn’t move on you (it might be best to have the lower receiver in a vertical position), proceed to tap the roll pin in, just to get it started. Once you’ve done that, install the spring to the bolt catch plunger, then install them both in the lower receiver, in the designated hole. Once this is done, install the bolt catch, making sure the holes are lined up for the roll pin. Once you have that, carefully tap the roll pin into place until you can see the roll pin is flush with the other side, going through the bolt catch. Again, this is best done using a punch, for the sake of precision and safety.
If you’re building a .308 lower, install the spring on the plunger and put them into place. Install the bolt catch in the designated area and make sure the holes are all lined up. Once that is established, use the 1/16 allen wrench we mentioned to thread the bolt catch retention pin into place. If you tighten it all the way, you may notice the catch doesn’t function properly. Slowly back the threads off until you feel the catch functioning properly.
The Buffer Retainer and Buffer Retainer Spring will be installed at a different step of the process, as it is held in place by the Receiver Extension Tube.
The Blue Bag
Install the Magazine Release Lever Assembly into the designated slotted area. On the other side, install the spring. Line up the magazine release button, using the head of the threaded area on the lever, and proceed to push the button in. this will make the lever on the opposite side pop back out. Use the lever to get the threads started with the button. Once you feel the threads grab the button, you need something sturdy but with a soft tip to push the button even further into the lower and hold it there. This will make the lever on the opposite side pop out even more. At this point, you will thread the release lever onto the button, using approximately seven full rotations until it is lined up with the relief area for the lever. The end of the threads should be lined up flush, or close to it, with the raised ribbed areas on the magazine release button on the opposite side. Once you get to that seventh turn and it’s lined up with the designated area, release the lever and check to see if the button and tip of the lever are lined up.
* This step may be done BEFORE you install the bolt catch, as it may be easier to do without the bolt catch in the way.
** Not all .308 lowers have the same design. As such, some manufacturers will use either an extended magazine release lever, or an extended magazine release button. Since there is no real set standard for .308, there is some freedom to be had to the overall design.
On an AR15 lower receiver, the ball detent end on the trigger guard assembly will go on the end that is closet to the magazine well. For the roll pin, we strongly suggest having some support under the lower, particularly on the ear portion that holds the trigger guard into place. Without properly supporting that, you risk hitting it too hard when you tap in the roll pin and breaking that ear off. Once you have the lower in a good, supported position, line up the holes from the trigger guard and the lower receiver and tap the roll pin into place using a pair of tweezers, or, if you have an AR15 punch set, the punch that holds roll pins in place. Once you’ve tapped that in as far as you can with the tweezers or the punch, finish it off with another punch or whatever you have handy to finish the job.
The Red Bag and the Hammer Assembly and Trigger
These components compliment each other, and must be done at the same time, regardless of when, during the process, you choose to do this. Align the trigger spring and install it on the trigger. The “arms” of the trigger should have the bended portion facing upward and outward from the curve in the trigger. The disconnect spring and disconnect will install into the trigger, in the hollow are designed to hold it.
Carefully install the trigger assembly you now have into the lower receiver. You will need to push down on the disconnect, to put tension on the spring, and line up all of the holes on the disconnect, the trigger, and the lower receiver. Once you have that in place, push one of the hammer/trigger pins into place. You may need to tap this in with a hammer. If everything is aligned properly, it should tap right in. The pin should be flush (or close to it) on both sides with the lower receiver.
Align the hammer spring with the hammer assembly. The arm should be on the outside of the assembly when installed and moving away from the hooked area on the hammer assembly. When installing in the lower receiver, the Hammer Spring’s arms need to rest ON TOP of the trigger, and they will be UNDER the rest of the Hammer Spring. Push the Hammer Assembly into place, making sure the holes on the Hammer Assembly are aligned with holes on the lower receiver. Once you have the holes lined up, install the second Hammer/Trigger Pin the same way you did the first one.
*There is a step in between installing the trigger and hammer. You will simply insert the Safety Selector Switch in place, once you have the trigger pinned in. Afterward, you will proceed with installing the hammer.
The Green Bag and the Pistol Grip
As mentioned above, the Safety Selector Switch should already be in place on the lower receiver. There is a hole on the bottom side of the lower receiver, where the pistol grip would be installed. That hole is where the detent for the Safety Selector Switch goes. The spring for the Safety Selector Switch goes in the pistol grip. It would be a small hole that will line up with the hole on the bottom of the lower receiver.
**This is also when you would install the detent and spring for the Takedown Pin on our MK3 lower receiver, but, both pieces would install inside the lower receiver, as there are no extra holes on the pistol grip. Otherwise, the following pertains to both lower receiver platforms.
This next step is best approached at almost a horizontal positioning. You may notice the open area on the pistol grip appears to be a dovetail design. This is done intentionally for the sake of a tighter fit. Line the BACK SIDE of the pistol grip, as the opening is wider on that end, up with the wedge area for the pistol grip on the lower receiver. Carefully install the pistol grip, making sure to keep all of the springs as perfectly straight as possible. It would be best to install the pistol grip in a manner that would allow you to finish the installation by pushing it directly toward the bottom of the lower receiver, as this would keep the springs from getting damaged.
With the pistol grip now in place and ready to be tightened down, install the lock washer on the pistol grip screw, and insert the pistol grip screw into the opening on the pistol grip and install the screw. There is no torques setting for this, just make sure it’s good and tight.
Installing a Receiver Extension Tube
While this is not part of the Lower Parts Kit, it is necessary to cover this step, as it is necessary to house the spring and detent for your Takedown Pin on your Ar15 lower receiver.
With the threaded hole for the Receiver Extension facing upward, make sure your Takedown Pin is installed properly and the channel on the bottom is lined up with the small hole you can see that is on the back side (currently at the top) of the lower receiver. If you are installing a carbine Receiver Extension Tube, make sure you have the castle nut and receiver end plate pre-installed and as far to the rear of the tube as possible. Begin threading the tube onto the lower receiver, keeping the castle nut and receiver end plate as far to the top/back of the tube as you can, while continuing to thread the tube onto the lower. You’ll get to a point where you will have to be careful that the receiver end plate doesn’t bend the spring sticking out of the receiver. Before you do that, though, you will also need to install the Buffer Retainer Pin and Buffer Retainer Spring. Being mindful of this AND the Takedown Pin Spring, proceed to thread the Receiver Extension Tube onto the lower receiver until the protruding bottom lip of the tube can hold the outer edge of the Buffer Retainer Pin down. Once you have that and you are certain the spring for the Takedown Pin is straight, try to push the Receiver End Plate directly forward, all at once, in order to keep the spring straight. Once this is done properly, and you are holding the plate in place, begin threading the castle nut toward the Receiver End Plate (essentially, as if you are now unthreading it from the Receiver Extension Tube, itself). Once you have that hand tight, use a castle nut wrench to tighten to about 35lb.-ft.
If you are installing a rifle Receiver Extension Tube and fixed stock, you’ll simply thread the tube on, making sure it secures the Buffer Retainer Pin. The spring for the Takedown Pin will be held in place by the fixed stock. All you need to do is install that, getting it close to the spring. Once you’re close enough, carefully proceed forward, being careful not to bend the spring. Once that is done, screw the butt stock into place!
To see all of this take place, feel free to take a look at the video we’ve included, showing how each part gets installed.
If you have any further questions, comments, or concerns, please feel free to email techsupport@cmmg.com, and one of our Tech Support staff members will be more than happy to assist you. And remember, all of our Products are backed by our Lifetime Product Quality Guarantee!