DIY AR-15 Build: Buttstock Installation - ITS Tactical

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DIY AR-15 Build: Buttstock Installation

By The ITS Crew

8 of 8 in the series DIY AR-15 Build

Today in part eight of our DIY AR-15 Build we’ll be showing you how to install the final part onto your Lower Receiver, the Buttstock.

While we say Buttstock, this step is comprised of quite a few different parts that aren’t in the standard Lower Parts Kit, like the one we recommend from Stag Arms.

This isn’t the place to skimp on quality parts and doing so will cause unnecessary problems that could have been avoided in the first place, namely the Buffer and Action Spring. Believe it or not these are some of the most important pieces of the entire Lower Receiver and are the cause of many puzzling AR-15 issues.

Continue reading for our step-by-step instructions for installing the Buttstock with detailed photos and video.

Buttstock Installation

Before we get into the actual steps of installation, let’s talk a bit about the selections we’ve made on parts here and why we recommend them.

Mil-Spec Receiver Extension (buffer tube)

Commonly referred to as the buffer tube, Receiver Extensions are available in a Mil-Spec or Commercial version. The differences are typically that the Commercial tube is made from extruded aluminum rather than forged aluminum like the stronger Mil-Spec tube. The two differ in size as well and you’ll find more aftermarket stocks built for the Mil-Spec tube.

DIY AR-15 Build: Buttstock Installation

As a rule of thumb here, inquire about the manufacturing process of your Receiver Extension, as there are instances of extruded Mil-Spec diameter extensions being sold out there.

Why Stake?

DIY AR-15 Build: Buttstock Installation

Something we’ll get into with the installation video is staking the Receiver End Plate and Castle Nut. This is done to ensure the Castle Nut doesn’t work free and allow your Receiver Extension to turn. If allowed to turn, the buffer retainer could release causing a headache and leaving you with a non-functioning firearm.

Staking is the process of pushing the metal from the Receiver Endplate (Sling Plate) into the staking slots on the Castle Nut. Four of them are available and optimally you should at least try to stake in two spots if possible. With many of the newer sling plates and their flared orientation, you may not be able to get more than one spot staked. That was the case with our IKICKHIPPIES SLAP Plate as well as the Magpul ASAP we’ve got on another build.

If you plan on switching out your endplates and experimenting to find the one you like best, you can utilize blue Loctite (non-permanent) on your Castle Nut after tightening or torquing it down (specs below). The Castle Nut is also a great place to add a witness mark, particularly if you leave it unstaked.

Buffer Differences

DIY AR-15 Build: Buttstock Installation

With this build we’re using a Bravo Company H (Heavy) Carbine Buffer, which is weighted with 1 Tungsten and 2 Steel weights. As your gas tube is shortened (From Rifle Length to Mid-Length to Carbine Length) it becomes more important to utilize a heavier buffer to regulate the added pressure pushed into the Bolt Carrier Group by a Direct Impingement system like we’re building here.

The heavier buffer absorbs the added pressure, as well as partially delaying the “unlocking” phase in the cycle of operations. This delay can help with early/hard extraction problems which occur when the bullet hasn’t departed the barrel and there’s still built up pressure in the cartridge case.

Piston guns also tend to run better with an H buffer to mitigate their inherent hard extraction due to the close proximity of the piston to the gas port. Heavier buffers can also help to control bolt bounce, which is the bounce that occurs when the Bolt Carrier Group returns after making contact with the Receiver Extension.

Utilizing an H Buffer in this build is also a test for us, as you’re never going to truly know how everything in a build functions until you get out and shoot it. One last thing to mention here is that underpowered ammunition in combination with a heavier buffer may cause malfunctions. We’ll be testing that too though.

Action Springs

Commonly known as a Buffer Spring, the Action Spring is an often neglected and tremendously important part in maintaining a properly cycling AR-15.

A Carbine Action Spring, like we’re using with this build should be between 10 1/16 in. and 11 1/4 in. If it falls outside of these parameters it should be replaced, this is it’s go/no-go measurement. On a rifle length spring the go/no-go changes to 11 3/4 in. to 13 1/2 in.

Parts Needed

DIY AR-15 Build: Buttstock Installation

Tools Needed

Assembly Instructions

DIY AR-15 Build: Buttstock Installation

  • Thread the Castle Nut onto the Receiver Extension Tube with the Cuts facing aft.
  • Place the Receiver End Plate, in this case a SLAP plate, on the Receiver Extension with the raised area facing forward.
  • Thread the Receiver Extension Tube through the back of the Lower Receiver, stop just before the Buffer Retainer hole.
  • Insert the Buffer Retainer Spring and Buffer Retainer into the hole and depress the corner of the retainer with your index finger.
  • Hold it down while continuing to screw in the Receiver Extension Tube until the Buffer Retainer is trapped down by the tube.
  • Insert the Rear Takedown Pin halfway in the hole from the starboard side. Ensure that the cut in the pin is facing aft.
  • Place the Takedown Pin Detent into the hole in the back of the Lower Receiver, followed by the spring.
  • Move the Receiver End Plate into position to set just on top of the Takedown Pin Spring. Press it against the spring, being careful not to bend the spring.
  • Screw down the Castle Nut to lock the spring in place and tighten with your buttstock tool to the proper torque of 40 ft. lbs. or just make it tight!
  • Alternatively you can use Blue Loctite (not Red!) to ensure your Castle Nut doesn’t come free or stake it.
  • To stake the Castle Nut, take your Center Punch and drive some of the material from the Receiver End Plate into the pre-cut areas in the Castle Nut. Optimally you want at least two areas staked, but when using some end plates you may have just the top area to stake. This was the case with the SLAP Plate and also the Magpul ASAP we’ve used in the past.
  • It’s also a good idea to make a Witness Mark so you visually know if the Castle Nut is coming loose on you.
  • Insert the Action Spring and Buffer into the Receiver Extension Tube and finish the Lower up by installing the Magpul MOE Buttstock on to the Receiver Extension Tube.
  • This is done by pulling the release lever pin straight down to get it all the way on. It’s a pain in the butt on the Magpul MOE, as most collapsible buttstocks simply require you to pull town on the release lever.

That completes the Lower Receiver portion of this DIY AR-15 Build. Join us back here soon for our first installment in the Upper Receiver Assembly!

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  • Christian Nadeau

    Great series guys! After the first article for this build came out I started acquiring the parts and tools to start my own build. I was wondering, though, how you guys store your tools. Do you use a regular tool box, keep everything in your range bag, or use some other storage method? Also, which tools do you consider to be the bare minimum to have out at the range without taking a whole tool box?

    Thanks guys. Keep the articles coming and stay safe.


    • Hey Christian, while at the range we usually have most of what we’d need to fix firearms in the field contained in this We’ve added and subtracted to it since I put together this review, but this goes with us if we’re doing anything at the range. Used to be my personal box, but it’s become the go to kit that we keep up at ITS HQ.

      Thanks for the comment and support!

  • Brandon

    What was the issue with the ASAP? I’m torn between it and the SLAP for use with the MS2 sling.

    Thank you for the articles and videos. Keep up the great work!

    • Brian C.

      Hey, I’m with Bramdom. What’s the deal with ASAP vs the Slap Plate? Did you ever do a review or determine if it works better for you?

      Let us know and keep up the great work and articles.

  • WOLF45

    Bryan, I have found on carbine length AR’s that weak or bad action springs (buffer springs) tend to be one of the biggest causes of malfunctions. I highly recomend Tactical Springs extra power reliability kit. The kit has a extra power (heavier) action spring and a extra power extractor spring. These kits are first rate an solve a lot of the cycling an ejection problems encountered with carbine length AR’s. Also I highly recomend anyone looking for parts kits to checkout Del-Ton Inc. There kits are assemble with all first rate parts an there carbine kits come with H-buffers. They are also very good to deal with.

  • Allen Vernon

    Thanks guys for the great series! I did notice that the castle nut was not listed in the original parts list. Are there any parts not listed for the upper receiver? Allen

  • Larry

    Do you guys have a date in mind for continuing your AR build articles with the upper eceiver?

    Nice job on the series so far! Thanks.


  • Hugh

    Thought you’d be interested in this little tip I found in the premier issue of Recoil magazine.
    If your going to be changing your buttock out from time to time, & don’t want to repeatedly chase down the reap take down detent spring, disassemble the lower to the point or if your building, tap the detent spring hole with a 4-40 tap, the blow out the chips from the front of the hole, use a black alum dye to coat the threads, cut the spring 1/8″ (to compensate for setscrew length) replace take-down pin, insert detent & shortened spring install 4-40 x 1/8″ setscrew til flush with the rear face of receiver & no more “Jesus” spring moments

  • Billy Watkins

    Ok. I have a question. Everytime i pull the trigger on mine. The little spring that holds the buffer spring in the stock keeps coming out and getting stuck in the upper reciever. Is something not adjusted right?

    • Brian C.


      Is your receeiver extension tube screwed in far enough? It should capture the edge of the detent and keep it depressed. The bolt carrier should be in contact with the buffer. The buffer should only contact the pin when the upper is removed from the lower.

  • joey garcia

    hey guys great series… my question has to do with the blue locktight. you mentioned to use it on the castle nut and i was just wondering where exactly i should put it?

  • Robin Haffer

    Hi Guys,

    great article so far. I am almost done with my lower. The only question I have is: Can I still stick to the H buffer and Caribine spring and have a longer barrel. I don’t know if I want to go with a shorter barrel. Would be nice to get some insides since I didn’t really understand how this all works together. Thanks!

  • ChrisOneArm

    Amazing series guys. I cant wait to start my first build!! I do have a question though.. I will be doing a LBR, and I want to ensure that it is 5.56 capable. Are there any special requirements for the lower for this, or is that all determined by the upper? thanks guys. You can also email responses to Chris at [email protected]. Thanks again, and I will now be watching your “Upper” series for more info.

  • KenNungesser

    Awesome. This made my first build go smooth as silk. Thanks for the time it took to do this. Now…on to the upper!!

  • Stephen Buckley

    Just for anyone interested, Strike Industries has a buffer tube now that has a small c-clip sort of extension that captures the detent pin, so that the tube cant turn beyond a couple millimeters either way, so staking isnt required. There are also some end plate/castle nut pairs on the market that have some non-turn measures that also negate the staking

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