Lower Receiver Assembly (AR-9, AR-15, and AR-10/LR108)
Step L9: Receiver Extension Installation
Objective: In Step L9 of lower receiver assembly, we will install the receiver extension (including the castle nut and end plate, as applicable) onto the lower receiver.
Tools Required
1: Only required if installing a carbine/pistol receiver extension.
2: Only required if installing a rifle receiver extension.
3: Required if removing/installing a carbine/pistol receiver extension or if removing an existing rifle receiver extension.
4: Only required if installing a carbine/pistol receiver extension. When using a rifle receiver extension, there is no castle nut; therefore, you do not need to stake it.
Components Required
| Quantity | Component |
|---|---|
| 1 | Lower Receiver |
| 1 | Receiver Extension |
| 1 | Buffer Retainer Pin |
| 1 | Buffer Retainer Spring |
| 1 | Receiver Extension Castle Nut1 |
| 1 | Receiver Extension End Plate1 |
1: Only required if installing a carbine/pistol receiver extension.
Instructions:
Step L9.0: Remove Existing Receiver Extension, Castle Nut, and End Plate (replacement/upgrade only)
If you are replacing your existing receiver extension assembly (or need to remove them for any other reason), first ensure the firearm is unloaded.
Before you continue, you will need to first remove the buttstock and remove the buffer system.
For carbine/pistol receiver extensions, you will need to loosen the receiver extension castle nut using an appropriate castle nut wrench and breaker bar (DO NOT use a torque wrench to loosen the castle nut). Note that this may take some force if the receiver extension end plate/castle nut is properly staked. Thread the castle nut toward the rear of the receiver extension.
For rifle receiver extensions, you will need to loosen the receiver extension using an appropriate A2 receiver extension wrench and breaker bar (DO NOT use a torque wrench to loosen the receiver extension).
For all AR-15s and AR-9s, the pivot/takedown spring and detent are typically held in place by the buttstock (rifle) or end plate (carbine/pistol). These may fall free when the buttstock (rifle) or end plate (carbine/pistol) are removed. Use caution so as not to damage or lose these parts.
Step L9.1: Thread the Castle Nut Onto Receiver Extension (carbine/pistol only)
If installing a carbine/pistol receiver extension, fully thread (i.e. as far as it will go) the receiver extension castle nut onto the receiver extension, castle profile first (i.e. the side with the 4 large notches facing the closed end of the receiver extension).
Step L9.2: Add the End Plate to the Receiver Extension (carbine/pistol only)
If installing a carbine/pistol receiver extension, slide the receiver extension end plate onto the receiver extension. The indexing “button” on the receiver extension end plate should face forward (toward the threaded/open end of the receiver extension). If equipped, the alignment tab on the receiver extension end plate will slide into a groove on the bottom of the receiver extension.
Step L9.3: Grease the Receiver Extension Threads
WARNING: DO NOT USE THREAD LOCKER.
Apply grease to the threads of the receiver extension.
Step L9.4: Thread the Receiver Extension into the Lower Receiver
Begin to thread the receiver extension into the lower receiver by rotating clockwise. Thread 2-3 turns and stop.
Step L9.5: Install the Buffer Retaining Pin and Spring
Install the buffer retainer spring into the buffer retainer cavity in the lower receiver.
Install the buffer retainer pin into the buffer retainer cavity in the lower receiver, over the buffer retainer spring (with the open end facing down/in).
Step L9.6: Finish Threading the Receiver Extension
WARNING: DO NOT USE THREAD LOCKER.
NOTE: If you are building an AR-15 or AR-9, the takedown spring should be protruding from the rear of the lower receiver. Take care not to damage the spring.
While depressing the buffer retainer pin and spring, continue to rotate the receiver extension clockwise until 1) the buffer retainer pin is held in place by the receiver extension but can move freely up and down, and 2) the receiver extension is oriented properly (for carbine/pistol only).
Step L9.7a: Torque the Receiver Extension (rifle only)
Set your torque wrench to 35-39 ft. lbs. and install the 5/8″ A2 receiver extension wrench. If you are using an AR armorer’s tool, be sure to adjust the torque setting appropriately.
Torque the receiver extension until the torque limit is reached.
NOTE: If building an AR-15 rifle, the pivot/takedown spring for the takedown pin will not be secured at this point. Proceed to Step L10 (installation of the buttstock) immediately.
Step L9.7b: Tighten Castle Nut (carbine/pistol only)
WARNING: DO NOT USE THREAD LOCKER.
Before you torque and stake the castle nut, you should verify two things:
- There must be sufficient gap between the back of a gas key and the lower receiver, when the buffer system is fully depressed by the BCG. To verify, depress the buffer into the receiver extension using the intended BCG. With the buffer system fully depressed, the gap between the back of the gas key and the lower receiver should be at least 0.130″ (about the thickness of a quarter and dime). If the gap is less, try backing the receiver extension out by one full turn. If this backs the receiver extension out too much, you may need a spacer in your receiver extension (remove the buffer and buffer spring and drop a spacer, or quarter, into the receiver extension; reinstall the buffer spring and buffer).
- An installed upper receiver must not strike the receiver extension. Install an upper and attempt to close the two receivers together. If the upper receiver/BCG strikes the receiver extension, try backing the receiver extension out by one full turn.
Carefully slide the receiver extension end plate along the receiver extension until it is against the lower receiver. If you are building an AR-15 or AR-9, doing so will compress the pivot/takedown spring protruding from the lower receiver. Be sure that the pivot/takedown spring compresses into the lower receiver and is not pinched/kinked between the receiver extension end plate and lower receiver.
Hold the receiver extension end plate in place against the lower receiver while you turn the receiver extension castle nut clockwise, until it is seated against the receiver extension end plate. Hand tighten.
Install the castle nut wrench on the breaker bar. Exercise the castle nut 3-5 times by applying moderate force (about 20 ft. lbs) to tighten it and then loosen it.
Set your torque wrench to 40 ft. lbs. and install the castle nut wrench. If you are using an AR armorer’s tool, be sure to adjust the torque setting appropriately.
Torque the receiver extension castle nut until the torque limit is reached.
Step L9.7: Stake Castle Nut (carbine/pistol only)
Stake the castle nut. We will provide a tutorial on how we stake castle nuts soon.