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    Building Up M4/AR15 Lower Receiver


    Before you can start assembling your lower receiver, you have to make a few choices on what direction you want to go. There are countless configurations of grips, build groups, bolt carriers, and other parts out there. This lower is built to host large bore SBR uppers so we opted for a military style trigger and robust build group from Daniel Defense. You can get this kit from Midway, but with free shipping you can sometimes find it cheaper here: Daniel Defense DD.21007 Lower Kit. The only substitution I made was to the grip. I have the CAA UPG-16 on all my builds. With all the potential combinations it should suit just about anyone. The best price at the time of this article is Midway if you are already ordering, or from here due to free shipping: CAA UPG-16 Grip.


    Daniel Defense Build Kit And CAA UPG16
    Click Image For Larger View
    The night before you plan to start the assembly, place all the metal build parts on a baking sheet covered in foil and lightly coat them with Militec-1 or something similar. Then heat them to 160 degrees F for approximately 20 minutes. This treatment gives you a good starting point for maintenance as well as protecting the parts from skin oils during assembly. Once the parts have cooled off, wipe off any excess oil and set them aside. Through all of these steps you are working with springs under pressure. For that reason, safety glasses should always be worn. With all of the prep out of the way, the first thing to install should be the magazine release. This will allow you to use a magazine vice block in the later steps should you need one. To begin, install the magazine catch through the left side of the weapon as shown below.

    Magazine catch install
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    Now flip the weapon over while holding the magazine catch in place. Install the the magazine catch spring over the the threaded portion of the catch as seen below. While holding the catch flush with the receiver, press the magazine release button down onto the spring and get a few threads started. Once you have a few threads of engagement, you can let go of the magazine catch. Now position the button into its recess and press it as far into the receiver as you can. While holding the button in, you can screw the catch in the rest of the way from the opposite side. Use care not to scratch the receiver. Using a space magazine, now preform a function test.

    screw on magazine catch button
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    While the next step is not difficult, it is where the most people damage the finish on their receiver. The bolt catch can be tricky, you just need to be patient. With the receiver resting on its right side, drop in the bolt catch spring followed by the plunger as shown below. If you have a threaded receiver, you can hold the bolt catch in place and screw in the set pin. If you have a traditional receiver, you need to insert your magazine vice block and place the weapon face down in a vice. Get your roll pin started before placing the bolt catch. Once the pin is started, you can either hammer it directly, or use a 12" 1/8th punch to drive it into place. The punch is the better method as it will help prevent scratches. This is one of the reasons I prefer to convert to screw in pins over roll pins for the bolt catch.

    bolt release
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    The following pivot pin installation procedure is hands down my least favorite step. While straight forward, it is tedious with new springs. Start by placing the detent spring in its recess as shown below. From here you will need to press the detent into place and hold it flush with a razor blade. With the detent held in place with the razor, install the pivot pin from right to left. Once the pin is started you can remove the razor. I usually perform this step someplace where I can easily find the spring, should it go flying.

    pivot pin
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    Because of the numerous trigger combinations and install methods, I am going to skim over this step covering the key points. If you have a drop in trigger, it just sits in place while you press in the trigger and hammer pins. The pins usually will press in by hand. If you are using a military style trigger, insure the disconnector spring is installed with the wider side down. If you are using the Daniel Defense kit, the disconnector spring is the green spring. With the trigger and disconnector set up, you can simply press them into place and install the trigger pin. Follow this with the hammer and hammer spring, which is also secured with a pin.

    trigger spring
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    Now cock the hammer back. With the hammer in the back position, you can insert the fire selector in the fire position. With the selector in place, turn it to safe. While holding the hammer pull the trigger. The selector should prevent the hammer from falling. If it does not, review the previous steps to identify the problem. Now set the selector to fire. While holding the hammer pull the trigger. The trigger should try to fall. While still holding the trigger in lower the hammer and recock it. The disconnector should catch the hammer and prevent it from falling a second time on a semi-auto weapon.

    insert fire selector
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    After you have confirmed a properly functioning fire selector, you can finish installing it. This is done by flipping the weapon over as shown below. Now drop in the safety selector detent. On the grip locate the small hole on the top right side. Drop the safety spring into this hole. Set the grip aside. Before installing the grip you need to confirm if your take down pin is installed from the bottom, or the traditional way in front of the buffer tube. In the example below you can see a tiny set screw just behind the install brass safety detent. If you have this style, insert the take down pin, followed by a detent, then reinstall the tiny set screw. If you have the traditional style the take down pin procedure will be addressed in part two of this review. Now install the lock washer over the grip screw. Even though the screw has a washer, I still like to use a small amount of Guntite on the screw. With the screw and washer ready, press the grip down onto the spring and get the screw started as shown below.

    safety spring and detent
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    Slowly tighten the grip screw while confirming that the spring is being pressed into the recess and not kinking over on itself. Once the grip is installed, go back and confirm that the fire selector switch is functioning correctly. Take care not to drop the hammer, as it will strike the bolt release with no upper in place.

    Install grip
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    At this point you will have a few extra pieces. You should have the trigger guard, split pin, set screw, two springs, buffer retainer, and a detent left. The trigger guard was not used in this build, but should you need to install it, it is just snapped into place on one side then secured with the set screw on the other. If your take down pin was the traditional style, you cannot install it till the second part of this guide when the buffer tube and stock get installed. The same goes for the buffer spring and detent.


    Here is a short movie to help you with the first few steps:

    In the next guide we will cover the completion of the lower, and the final inspection and function test. While these guides seem simple enough, you should consult a licensed gunsmith if you have any questions or doubts about your work. Even if you put everything together and it seems to work fine, there is no harm in having someone double check your work.

    Edited by VS762

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