Staking Your AR-15 Castle Nut
Why is staking a castle nut so important? Why can’t I use blue thread lock?” That is something people often ask me about building an AR-15. I have found that a large number of people with whom I have spoken do not realize the importance of staking the castle nut on their AR-15. I am a big proponent for staking a castle nut and will always do it on any AR-15 variant that I build for myself or others.
What is a Castle Nut
But let’s back up a minute, what is the “castle nut” and what does it do? A castle nut, also referred to as a buffer tube nut, is a circular, notched nut that screws onto the AR-15 lower receiver extension (buffer tube) and keeps the receiver extension and lower receiver end plate secured in place.
On a standard mil-spec castle nut, one side will have large notches that look like little squares have been cut out of it (hence the nickname because it resembles the top of a castle tower). On the other side, there are smaller notches or indents, which should be facing the receiver end plate when correctly installed. These smaller notches are there and have been designed explicitly for staking purposes.
Why Stake Your Castle Nut
Not staking the castle nut on your AR-15 is taking quite a considerable risk. Many people, myself included, own an AR-15 for self-defense purposes because of its proven reliability and accuracy. By not staking your castle nut, you reduce the overall reliability of your AR-15 and, in turn, introduce a higher probability of a catastrophic failure.
Take a moment and consider the following scenario. Say you chose not to stake the castle nut on your AR-15, and after some range time, the castle nut becomes loose. That causes the endplate to become loose. Once the end plate is loose, the takedown pin spring and detent can fall out. Now, the receiver extension can move, which means it will no longer hold the buffer retainer in place, so your buffer and buffer spring can’t stay in place. All of this can happen in just a second or after few shots without you even noticing. That is, until either your AR-15 explodes or the receiver extension breaks apart—both of which would render your AR-15 completely inoperable.
“My AR-15 could explode? Ha, that’s doubtful.” Not necessarily. All it takes is for the buffer retainer to become trapped somewhere in the upper receiver to prevent your bolt from fully seating, which can make a round go off inside the receiver but not in the chamber. That is especially true with AR-15 style bolt carriers instead of M-16 bolt carriers because of the typical lightning cut.
Many have told me that they “have used blue thread lock on the castle nut and have never had a problem in all their years of shooting AR-15s.” My response to them is to remind them that something catastrophic will likely not have convenient timing. Why take the chance and allow the dangerous possibility of that catastrophic failure? You don’t want your AR-15’s stock to fall off.
How to Stake Your Castle Nut
You can avoid all of this trouble by taking roughly 60 seconds to stake your castle nut properly. Follow these simple steps:
- Make sure that you have correctly installed your castle nut at 40-foot pounds of torque on the receiver extension with the small notches facing the receiver end plate.
- Using either a spring-loaded center punch or a standard center punch and a hammer, place the tip of the punch onto the lower receiver end plate that is in line with a small staking notch on the castle nut.
- Using force, manipulate enough material from the endplate into one (or two) of the small notches on the castle nut. That will render the castle nut properly staked and semi-permanent.
Check out our list of the best buffers for your AR-15.
This comment is in response to ZACH, and to anybody who may see his comment –
Everything that you’ve said is wrong, and anybody who takes your advice will be in for a very bad day. Maybe the first, second, or third time out to shoot, but sooner or later that buffer tube is going to break loose.
Before I say anything more, I am a >professional mechanical engineer< with over 10 years in the aerospace industry as both a designer and assembly engineer. What I am about to say isn't speculation, it is citable fact.
First off, Zach – car wheel lug nuts should be torqued to 80 – 100 ft-lbs. If you're stopping at 40-ft-lbs, I can only hope those suckers get rusted on or else one of these days your wheels are coming loose.
Second off – 40 INCH-lbs = 2.5 FOOT-lbs. No. Just NO. That is 1/16 of the specified torque.
The purpose of torquing is to elastically stretch the threads to a defined length/length, so that the bolt acts like a spring pulling against the piece being fastened. Even if that piece were to become slightly thinner (lets say its really cold that day), the bolt would still be pulling with enough force to take up the slack and prevent it from vibrating off. THIS IS WHAT YOU WANT. If you do NOT torque it high enough, the threads will NOT act like a spring and the castle nut WILL vibrate loose.
Do not EVER disregard the torque spec just because "i dunno it seems kinda high." Its a torque spec for a reason – some very smart people spent a lot of time in a lab figuring out exactly what this value should be so that everything functions as intended under all conditions.
40 ft-lbs isn't even a high value. At all. Its a tiny fraction of what it would take to shear the threads off even the lowest-end buffer tube and/or lower receiver. (even the junkiest aliexpress parts) Seriously, that is a ridiculous, and you almost certainly didn't verify it before posting. If you did, you wouldn't have posted it.
Lastly – and this is VERY important – staking does NOT replace torque. Its a secondary failsafe in case vibrations are extreme enough that the torque preload can't compensate. it is NOT your primary means of keeping the nut in place. Its equivalent to using Loctite or a locking helical insert. Preload compensates for expansion and contraction due to temperature changes. Staking does not.
I hope this reaches people before anybody hurts themselves with bad information.
Be careful torqueing on the castle nut, 40 foot-pounds is a lot of torque. Most car wheel lug nuts only get about 60 foot-pounds. A low-end extruded buffer tube could conceivably be stripped right out of a lower receiver at that torque level. A torque of 40 INCH-pounds is probably safer if you don’t know for absolute certain that your buffer tube is forged. If you’re properly staking your castle nut the torque shouldn’t matter much (as long as it’s solidly assembled) since you’re relying on the staking to prevent the castle nut from backing out and not the friction between the threads.
Latecomer to this party, but let me say the post is greatly appreciated! While my new SSK12 Minuteman from Midwest Industries came with a staked castle nut, my other entry-level ARs did not. Sooo, I made a trip to Lowe’s and got a General automatic punch and am taking care of business! Thanks again!
Had one come loose over the weekend. Where’s my center punch….?
cwoods3 Window punch, center punch, whatever…as long as it moves a little bit of metal into the staking slot (what… you thought those notches on the Castle Nut were decorative?) does it really matter what type of punch he used?
Yeah..you are that guy…
🙂
Stake it! Stake it, before dusk falls and its power grows to great to control!
…I may need to cut back on re-reading Dracula. >.>
I can tell that this topic is really important to you and I am happy that you were able to learn something from my article. 🙂
Nate Schultz
Apparently, that is a newer version of TM that I have. However, I still don’t think that it is necassary. And I also believe that it is irresponsible to tell people that their rifle is going to explode if they don’t do it.
It wans’t covered in any of my armorer schools, and no one that I spoke with about it thinks it has to be done.
Nate Schultz
I understand that many manufactureres are staking their castle nuts at the factory now. Thats great, it keeps people from screwing with them. In looking at Army FM’s and TM’s that I have, I can’t find where it mandates staking the castle nut. What do you mean by “Mil Spec”? Being a veteran, to me, that only means that a given item will function properly “most of the time”. Also, in consulting the other armorers that I work with, staking can be done, but it is a field expediant fix if the castle nut is worn to the point it won’t hold any torque. What manual are you referencing?
When my Bushmaster had the castle nut problem I ended up switching out buttstocks. Our department armorer put the new one on and staked it. I’ve since sold that weapon and bought a 6920 with a Rogers stock. The castle nut on that one came staked straight from Colt.
Staking the castle nut twice is per the manual and is per mil-spec. I am not sure what you are referencing to claim that it doesn’t need to be staked. Please enlighten me.
Okay, I’ll be that guy. I have been shooting AR platforms for over 18 years (Military/LE/ civilian), not once have I ever had a problem with a castle nut coming loose. If people who do not have training or experience with working on these guns would just leave things alone, there would not be a reason to “fix” it. All of these weapons come from the factory tuned and tightened to the proper specs. Did you really use a window punch to demonstrate this? Can’t emphasize this enough-proper training and tools. Some things (gas keys) need to be staked, others (castle nuts) don’t.
Ha, well I am glad I was able to be informational as well as entertaining. 😛
Congrats on your new AR! If you ever have any questions, please feel free to contact me 🙂
I believe you are correct. In fact, I think there is some type of press involved with some manufacturers. I have always known BCM as having the best stake jobs on keys.
I’ve heard they use a special peening hammer on gas keys. My Colt came staked pretty tight so I’ve never done a key job.
Do you have a good technique for Gas key staking? My last BCG has somewhat anemic staking. I attempted to push a little more material, but found the key was nearly as hard as my punch, and I didn’t accomplish much. I am sure my technique is to blame.
Thanks for this info! I am new to the AR scene, and recently added a sling plate to my AR. I didn’t know the staking was this important. Looks like I’ve got some staking to do…
It happened to me on my first Bushmaster. My buffer spring sheared off my detent and I didn’t even know. This was due to too much play in the buffer tube. My Colt is staked beautifully and is good to go.
It happened to me on my first Bushmaster. My buffer spring sheared off my detent and I didn’t even know. This was due to too much play in the buffer tube. My Colt is staked beautifully and is good to go.
Interesting and informative-I didn’t know about that procedure before. Plus it’s just funny to hear someone say castle nut. Kind of like urinal cake. Just sounds funny.