Building your first lower receiver can be an intimidating task. Unlike your upper receiver, there are several more involved systems of small parts that are not as straightforward to install. Luckily if you chose an M4E1 as the base of your lower receiver build, you will save some headaches on some critical steps.
In this guide, we will be going over how to build your M4E1 step-by-step.
The items needed for lower receiver builds are generally very short. Many of the parts are included in kits making shopping to finish your lower receiver a painless task in most cases. The parts needed are:
* You can use an AR15 Lower Parts Kit with your M4E1, but you will be leftover with some extra componentry. “Minus” lower parts kits are available without triggers or grips if you intend on sourcing these specific upgrades separately.
Before we can get into building, you will first want to ensure you have all the correct tools. Below is a tool list of all the specific tools you will need to build your M4E1 Lower Receiver:
- AR15 mag block or Geiselle Reaction Block.
- 1/2” Drive Torque Wrench
- AR15 Armorers Wrench
- A long 3/16” Hex Driver (it needs to be long enough to reach into the pistol grip).
- .050” Hex Driver
- 1/16” Hex Driver
- Pivot Pin Install Tool
- Staking Punch
- Wire Cutters
- Aeroshell 64/Aeroshell 33ms anti-seize
Pivot Pin Installation
A pivot pin installation tool is highly recommended for the following steps.
Insert the pivot pin installation tool from the left side of the receiver. The hole in the tool should align with the pivot pin detent and spring channel in the lower receiver. Insert the spring and detent into the channel. Using a properly sized pin punch, depress the detent and spring into the lower receiver, and rotate the installation tool 90 degrees. Hold the pivot pin up against the installation tool and in one fluid motion push the pivot pin into its channel while maintaining pressure on the installation tool. Test this assembly to ensure proper function.
Trigger Group Installation
Install your trigger spring onto the trigger. Once the trigger spring is installed insert the flared (wide) end into the circular channel at the top of your trigger. Once the disconnector spring is installed lay the disconnector over the top of the trigger to both line up the pin holes and capture the disconnector spring within the slot on the disconnector. Set the components into the trigger pocket ensuring the trigger shoe is protruding into the trigger guard area. Using your thumb carefully align the pin holes in your lower, trigger, and disconnector and push the trigger pin into place.
Next place the hammer spring onto the hammer taking care to orient it in the correct direction with the “flat” end facing the magazine well/front of the lower receiver. Lay the arms of the hammer spring over the trigger pin and “wind” it into place, once the pin holes are aligned press your hammer pin into place.
Driving your trigger and hammer pins into place shouldn’t require much force and does not require the use of any tools such as a hammer. Using a hammer or similar tool to force the trigger pin into place may result in damage to your fire control group or lower receiver.
Magazine Catch Installation
Place the magazine catch into its slot in the lower receiver. While holding it in place, insert the spring over the magazine catch from the slot on the right side of the lower receiver. Place the magazine catch button over the magazine release and depress it. Rotate the magazine catch clockwise to begin threading the magazine catch into the magazine catch button (a non-marring tool may be required to fully depress the magazine catch button). Tighten the magazine catch until it is fully installed into the magazine catch button (flush).
Bolt Catch Installation
Insert the bolt catch plunger and spring into the slot in your lower receiver. Insert the bolt catch and push the pin through the pinhole until it bottoms out on the threads. Using a 1/16” hex driver, tighten the bolt catch pin until snug.
Safety/Pistol Grip Installation
With the hammer cocked, fully insert the safety into the lower receiver ensuring that the lever is on the LEFT side for non-ambidextrous safety selectors. If you are using an ambidextrous safety selector ensure the detent channel on the barrel of your safety is aligned with the safety detent hole on your lower receiver. Then insert the safety detent into its channel pointed end first and follow it with the spring. Insert pistol grip onto the lower receiver ensuring that the safety spring channel properly captures the safety spring. Install the pistol grip screw using a long 5/32” hex wrench.
Test the safety for function in both the SAFE and FIRE positions.
Receiver Extension Installation
Start by securely mounting your upper receiver into a vice. Fully thread the castle nut onto the receiver extension and insert the end plate with the raised side of the dimple facing the open end of the receiver extension. Insert the buffer retainer and buffer retainer spring into your upper receiver and depress them. Thread the receiver extension into place as far as you can while still allowing the buffer retainer to freely travel up and down.
Seat the receiver extension end plate against the upper receiver and tighten your castle nut to 40 ft-lbs. Using an appropriate staking punch, stake the castle nut into place.
Re-install your stock onto the receiver extension (this will look different for various buttstock models. For most Magpul stocks, simply pull down on the pins at the bottom of the buttstock while pushing the stock onto the receiver extension).
Lastly, you will want to install your stock or arm brace of choice. This process will be different depending on the stock or brace you use so you should consult the instructions from the manufacturer of your accessory.
With this, you have officially completed your M4E1 Lower Receiver! If it was your first, congratulations! If it was your 10th, thank you for building along with us!