AR15 Firearm: Tuning the Buffer on a 12.5 in. build

AR15 Tune the Buffer

Note: You may notice a change in this article. It originally was an article about tuning the buffer on my AR15 Pistol. I have never claimed to have all the answers and in this case, the answers I thought I had were incorrect. As in many parts of my life, I learn from others and in this case, you, the readers tipped me off to my error. The error is one of legal semantics and it is unfortunate that we all have to deal with these silly rules, but we do. I have amended the post below to reflect accurately that my 12.5″ AR is, in fact, a firearm and not a pistol.

When it comes to defensive or duty firearms the most important attribute is likely reliability. When you choose a gun as a life-saving tool you need to know it will fire when you need it to: in other words, on-demand, not on command. In an AR15, the gas system is one area where reliability can be a make it or break it set of components and the most variable aspect of the gas system may very well be the buffer. It is for this reason that I paid close attention to tuning the AR15 buffer as I completed my AR pistol build.

It turned out to be a far simpler process than I expected. I feel comfortable that anyone who can assemble an AR15 can easily tune their buffer, especially with the right AR15 tools

AR15 Tune the Buffer

The contents of the H3 buffer look similar to the carbine buffer, however, the weights are tungsten and have significantly more mass than the carbon steel buffer weights.

AR Firearm: Why I decided to build now

A couple of months back I looked at the prices of AR15 components, especially lower receivers, and decided it was time to make sure I had all the components I needed to build whatever tools I wanted for the next few years. Prices were so low that I couldn’t conceive of them getting much lower and I was able to capitalize on some special deals, including a couple offered through Aero Precision ( Among those were four “blem” M4E1 Enhanced lowers,1 for my build and one for my wife and two daughters.

My AR15 build: a quick overview

AR15 Tune the Buffer

I primarily selected Aero precision components to avoid tolerance-related complications that will sometimes arise when mixing brands.

It wasn’t long before I started to collect the other parts I needed. I already had a complete bolt and bolt carrier and an ALG Defense ACT trigger, but otherwise decided to keep my build relatively “on brand” to avoid issues with tolerance stacking. Aero knows what Aero builds so I expected the parts to work together.

12.5″ AR Build Gas System

I decided on a 12.5 in. barrel and handguard mounted to the Aero M4E1 Enhanced upper receiver. The barrel has a gas port at carbine length. I added a carbine length gas tube and an Aero gas block to my cart.

Carbine length made sense for the gas system because it has about the same dwell time as a 16” mid-length gas system. Dwell time is the length from the gas port to the muzzle and is important because it is when gas is forced back into the bolt to operate the action. I have found the mid-length system to be very reliable in my JP VTac AR15 and I approached my decision from an “if it ain’t broke, don’t fix it view.”

It should also be noted that the gas block I selected is not an adjustable block. An adjustable block would provide more wiggle room, but I was building for reliability and simplicity. In addition, the gas block is easy to swap out in the future if needed.

Other Pieces, Parts, and Paraphernalia

I sourced my lower parts kit, grip, and pistol brace from a distributor taking advantage of wholesale pricing. This is also where I grabbed a Spike’s Tactical H3 buffer. 

Why the AR 15 Buffer Matters

AR15 Tune the Buffer

Disassembly of the carbine buffer. I use a special little block I drilled out and a roll of electrical tape, one of my favorite smithing tools.

The buffer has an important job. Along with the buffer spring, the buffer provides resistance to the action of the AR as it moves rearward after a shot has been fired. The buffer’s job is to help slow the BCG down so that it doesn’t damage the rifle (in this case, firearm) over time and to help the rifle/pistol/firearm feel like a soft shooter.

If the buffer is too light it can travel to quickly and batter the bolt carrier group and the buffer tube. If the buffer is too heavy, it can slow the BCG to the point where it doesn’t move enough to eject the fired round, scoop a new round from the mag, or press the bolt firmly into battery.

After a build is complete tuning the buffer is the easiest way to modify bolt speed.

Other Gas System Factors

Don’t think for a second that the buffer is the only way to tune the gas system of an AR15. It isn’t. It is the easiest way to tune the gas system after a build is complete. There are other options but each is more of a commitment and could still require the buffer to be tuned.

Gas System Length

This is probably one of the biggest factors in how an AR works so it makes sense that changing the length of the gas system would change performance. The problem is changing the length of the gas system requires changing a barrel. The physical act of changing a barrel is easy enough if you have the right tools. The problem is the cost. Good barrels aren’t cheap so this should normally be viewed as the last resort. Choose wisely upfront.

Know that changing a barrel can also result in a larger or smaller gas port or simply a better match between the gas port and gas block. These factors are harder to control.

Gas Block

Changing to a different or an adjustable gas block is an option for tuning. This isn’t too tough, but would require the removal of the hand guard.

Changing the gas block may simply provide better alignment or a larger port in the gas block itself.

BCG Weight

Bolt carrier groups can be made out of different materials or have material removed so that their mass changes. A lighter or heavier BCG can significantly change the speed the carrier is traveling and reliability. Changing the BCG is actually the easiest mod you can make to the system, but bolt carrier groups aren’t cheap. It could get expensive quickly trying to tune the BCG.

Tuning the Buffer

AR15 Tune the Buffer

Here you can see the contents of this carbine’s AR15 buffer: 3 carbon steel weights and 3 rubber spacers.

Tuning in your AR15 buffer is pretty darn easy and it can be the most cost-effective way to increase the performance of your AR.

If you can pound in a nail you can disassemble a buffer.


What you are looking for.

AR15 Tune the Buffer

I selected a Spike’s Tactical T3 buffer as my tungsten weight donor. It was the most affordable H3 buffer I could find.

You have properly tuned your buffer when you observe the following while shooting the first few rounds of a full magazine:

  • Your AR15 cycles completely ejecting the spent casing, loading a fresh cartridge and locking fully into battery.
  • The spent brass landing between 3:00 and 5:00 if 12:00 is directly downrange.
  • Your BCG locks to the rear on empty.

What You Need

  • A carbine-length and weight buffer
  • A carbine-length H3 buffer
  • A roll pin punch
  • A hammer
  • A roll of electrical tape
  • Several magazines loaded to capacity

How to Tune your AR15 Buffer

AR15 Tune the Buffer

The contents of the H3 buffer look similar to the carbine buffer, however, the weights are tungsten and have significantly more mass than the carbon steel buffer weights.

The steps are simple. Gather the above supplies and head to the range. You’ll need a solid table to work on as there will be a bit of pounding involved.

Keep in mind that you probably want to run the heaviest buffer that works in your gun. This will help to ensure that your gun isn’t over gassed and causing long term damage to your weapon. It will also feel soft and flat compared to a lighter buffer!


1. Start HEAVY.

Break your upper and lower apart at the rear pin and place your H3 buffer in the buffer tube with your buffer spring. While on a live-fire range observing all safety protocols insert a fully loaded magazine and squeeze off a few rounds.

If the gun cycles, your spent brass clears to between 3 and 5:00 and you lock to the rear when you are at the end of the mag your job may be complete.
As a side note, this is where my tuning ended. I repeated the first shots from a full mag 10 or so times, and then a few more with some different brands of ammo and several different brands of AR mags. All worked. The gun worked. The H3 buffer was right for me.

2. Lighten UP!

AR15 Tune the Buffer

After you swap buffer weights reassemble the buffer, put it back in your gun and test it.

Let’s assume that your gun did not cycle properly or maybe the brass was just trickling out of the gun. The H3 buffer is too heavy for your system. We are going to use the carbine length carbine weight buffers to build an H2 buffer. Here is how it works.

The carbine weight buffer contains three carbon steel weights and three rubber spacers. The H3 buffer contains three tungsten weights and 3 rubber spacers. Tungsten is denser than carbon steel so the same size weight has more mass.

We can build an H2 buffer by driving the roll pin from both buffers, pulling the rubber plug and dumping the weights.

Use one of the buffer bodies and insert 2 tungsten weights and one carbon steel weight (don’t forget the rubber spacers).

Reassemble your new H2 buffer, reassemble your weapon and test it out.

3. Keep going if you need to.

You can repeat this process each time replacing a tungsten weight with a carbon steel weight. Each time the buffer gets lighter and moves faster. When you get to the combination that operates properly and feels great, load up some more mags and have fun!

Final Thoughts

AR15 Tune the Buffer

Time to test! Make sure to use magazines filled to capacity as they exert more drag on the bolt carrier. Also, make sure to test different ammunition that you may run in your gun. Don’t limit testing to a single brand or weight.

Building an AR15 can be an extremely rewarding process, and you don’t have to be Jim Hodge to do it. At the same time, you need to make sure that you are doing things correctly. Especially if you are building a duty or defensive tool.

Tuning your buffer won’t solve every problem. You may need to change gas blocks, swap out barrels or take other measures. Even if you have to take big steps to get your weapon running properly your last step is probably going to be tuning your buffer.

Have you ever messed with your buffer weight? Even if you didn’t build your AR pistol (or rifle, the material here obviously addresses both) you can fine-tune it over factory performance. Just be careful. A project like tuning your buffer could be your gateway drug into the world of tinkering with AR15s.

Have fun!

Wanna learn more? Check out more from our DIY series.

Read more articles by Paul Carlson.

Mag Life contributor Paul Carlson of Safety Solutions Academy LLC.

Mag Life contributor Paul Carlson of Safety Solutions Academy LLC.

Paul Carlson, owner of Safety Solutions Academy, is a Professional Defensive Shooting Instructor.  He has spent the past decade and a half studying how humans can perform more efficiently in violent confrontations and honing his skills as an instructor both in the classroom and on the range.

Through Safety Solutions Academy, Paul teaches a variety of Critical Defensive Skills courses in more than a dozen states annually.  Courses range from Concealed Carry Classes to Advanced Critical Defensive Handgun Courses and include instruction for the defensive use of handguns, rifles and shotguns.  Safety Solutions Academy regularly hosts other industry leading experts as guest instructors to make sure that SSA’s students have the opportunity for quality instruction across a broad range of Critical Defensive disciplines.

  • Vic Diaz

    Correct me if I’m wrong, but last time I checked once you start passing 12 inches you are in SBR territory?

    • Blumpkin

      It’s an article about tuning an AR pistol not an AR rifle. Generally, a barrel less than 16″ on a rifle = SBR.

      • Vic Diaz

        From what I’m told once the barrel passes 12” it’s considered an SBR even if the gun was registered as a pistol, it’s no longer considered a pistol after 12 inches. I’m just telling you what the law told me. I’m pretty sure they know more than both of us.

        • Blumpkin

          Cool story bro. Did the policeman let you wear his hat too? Did the policeman let you turn the flashing lights on in his cruiser?

          Read the nfa for yourself. 12 inch barrels refers to “any other weapon” which is a different thing all together
          U.S.C. 5845; 27 CFR 479.11
          “Any other weapon.
          Any weapon or device capable of being concealed on the person from which a shot can be discharged through the energy of an explosive, a pistol or revolver having a barrel with a smooth bore designed or redesigned to fire a fixed shotgun shell, weapons with combination shotgun and rifle barrels 12 inches or more, less than 18 inches in length, from which only a single discharge can be made from either barrel without manual reloading, and shall include any such weapon which may be readily restored to fire. Such term shall not include a pistol or a revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.”

          • Vic Diaz

            Lost all respect for you now that I see that you just are a keyboard warrior.

            While there are two different arguments here, I’d rather obey state police rules here because that is what is going to keep me out of jail not the NFA. I’m not going to tell my local police officer “this is what the NFA says“ because There are different laws here that overrule the NFA to a certain extent. in my state you’re able to own an SBR AR-15 labeled as “other multical” let me move to a different state like Texas or Ohio and tell my local police my firearm is labeled as other in an SBR form, and I could Karen T guarantee you I will go to jail. so as much as I would like to wear his hat or turn on his flashing lights on his cruiser I will obey what my state police says because people like you don’t understand that some laws like the ones we have in this state where I live overrule some of the NFA rules such as “other AR-15” in an SBR format, and if you have an AR pistol over 12 inches in my state, you are going to jail regardless what the NFA says. So much for NFA rules Right?

          • Blumpkin

            Your original statement made no mention of state specific laws. You made a sweeping generalization. Anyone reading that would have assumed you were talking about NFA laws.
            What specific state are we talking about?

            I will also reiterate the point Karl made above. Being a police officer does not make you a legal expert. This is particularly evident when you are in the weeds of convoluted firearms rules. This is why there are judges and lawyers.
            Why would you blindly take the word of an officer without verification of the statement later? They write laws down for a reason.

            Lastly, I think you are confused about what a keyboard warrior is.
            Your original post stated “correct me if I’m wrong”, so I took the time to clarify. Then you condescendingly disagreed, stating “I’m pretty sure they know more than both of us.”. So if you already knew the answer, why ask in the first place? Furthermore, you have no idea who I am, what I do, or what I know.

          • Vic Diaz

            Correct, I indeed said all that in my original post and that’s the reason why I said the police here know more than both of us so if that’s going to keep me out of court rooms then that is what I will have to do for now. People like you who think that you are a tough guy can go ahead go to jail get your 2A stripped away permanently and go through a whole bunch of unnecessary court Proceedings, but hey some people love mischief.

            There are lots of laws that I disagree with but if you’re told something is illegal and you go ahead and do something else then you’re playing with fire.

            Like I said I am told after 12 inches it’s considered an SBR so regardless what you say I have to follow the law if I want to keep my Guns

          • Blumpkin

            Which state are we talking about?

            Again. I would reiterate, just because someone is police does not mean they know the laws. You should read them for yourself.

            Choosing to comply or not is a personal decision. As is being a an intellectually lazy person, such as yourself, and not investigating things on your own.

            Furthermore, I never told you to break the law. I merely suggested you figure out what the law actually is and not just take someone’s word for it.

            As an aside… since you brought it up. In this country, it was once illegal to use the bathroom that didn’t match the designated skin tone. It is currently perfectly legal to marry a 13 year old in New Hampshire. Don’t conflate legalities with fundamentals and ethics.
            Non-compliance and protest are as American as it gets.

          • Jeff Kermath

            State laws vary from state to state, and as the author points out, you need to check local laws. We aren’t discussing local laws here because there too many localities to deal with in a single thread. Under federal law it needs a stock intended to be shouldered to be considered an SBR, or just a rifle for that manner. No stock = no rifle, hence no SBR. Even past the 26″ overall length it continues to be a pistol unless you modify it further, like adding a vertical foregrip, at which it becomes a “firearm”. Under 26″ if you add a vertical fore grip it becomes an NFA pistol. 26″ is the magical number where a pistol stops being easily concealable according to federal law, which is why you can add a vertical fore grip and change your pistol to a firearm without it becoming a NFA pistol. Franklin Armory sells a gun just as this, pistol brace, 12.5″ barrel, vertical fore grip, LOA just over 26″. If you dig around on their website you can find the determination letter from the BATF that it is in fact a firearm and not an SBR or AOW that requires registration and a tax stamp.

      • Karl Kimball

        Police officers generally DON’T know NFA laws (or any other law nowadays for that matter) better then those that actually read and understand the law themselves, that is why it’s in your best interest to know the law!

        Please do tell us where in the NFA does it specifically state a barrel between 12.5′ & 16′ is considered ONLY an SBR.

      • Paul Carlson

        So, I did a bunch of diggin on this. As I wanted to make sure I had solid information. That being said, I’m not an attorney. I’m not providing legal advice. I’m simply sharing my personal opinion on these issues. You need to do your own research and if you are at all uncertain you should consult an attorney that specializes in NFA matters.

        This is my understanding and I’m sure you could poke some holes in the words I’m using, but I believe it sums up the different classifications an AR-15 could fall into.
        An AR-15 pistol has a barrel less than 16″ and an overall length of more than 26″ and a brace not intended to be shouldered.
        An AR-15 rifle has a barrel of 16″ or greater, an overall length greater than 26″ and a stock intended to be shouldered.
        An AR-15 Short Barreled Rifle (SBR) has a barrel less than 16″ and a stock intended to be shouldered.
        An AR-15 FIREARM has a barrel less than 16″ an overall length more than 26″ and a brace not intended to be shouldered.

        Of these different groups, the only one that is regulated under the NFA is the SBR. A pistol a rifle or a firearm are all legal to own federally. Your state and local laws may not allow one or more of the above categories. Remember, it is your responsibility to research this yourself.

        I hope this helps and I hope we can be kind.

        • Blumpkin

          Thanks, Paul.

          If you don’t mind, I had a question about your gas tube length. You mentioned, “Carbine length made sense for the gas system because it has the same dwell time as a 16” mid-length gas system.”. I was under the impression that the idea behind a mid-length system is that it increases dwell time. How is it that a carbine length gas tube on a 12.5″ barrel is the same as a mid-length gas tube on a 16″ barrel w.r.t. dwell time? Thanks again.

          • Paul Carlson

            It’s a great question Blumpkin.
            I’ll explain what I know of course with the qualification that I’m not a gunsmith, blah blah… Also, I’m going to explain simply not because I think you don’t understand, but for others reading who may not. And hey, why make things complicated.

            I think the place to start is with what dwell time. This is the time that gas is contained and being sent to the bolt. It is the time where the system is pressurized if you will.

            Think of the bullet itself as a plug. It seals the barrel and keeps gasses from passing it.

            So you pull the trigger and all the cool stuff happens in the chamber and the bullet starts to travel down the barrel. The bolt doesn’t start moving yet because no gasses are able to move through the gas port yet. The bullet is blocking them.

            Once the slug moves past the gas port the gasses can now exit through the gas port into the gas tube, the gas key and then the bolt. This is when the bolt unlocks and begin to move to the rear. It is also when the dwell time begins.

            This pressure keeps up until the bullet exits the barrel. At this time the gases have an easier path to expand and they take it. The pressure drops in the gas system and the pressure exits out the muzzle. This is also when dwell time ends.

            The thing most people miss is that dwell time is created by the distance after the gas port to the muzzle. The length of the gas system has an impact on this as it determines how much distance there is between the gas port and the muzzle, but it’s the length from the gas port to the muzzle that matters not the actual length of the gas tube.

            So next, why is a mid-length gas system good for a 16″ gun? The mil uses carbine length gas systems on 14.5″ barrels. If we put a carbine length system on a 16″ gun there is MORE dwell time and as a result there is gas supplied to the bolt for a longer time. Most 16″ carbine length guns are OVER gassed. The solution to this was to make the gas system LONGER this makes the distance from the gas port to the muzzle SHORTER. This in turn makes the dwell time SHORTER resulting in LESS gas to the bolt.

            Mid-length systems are used on 16″ guns to reduce the amount of gas to the bolt. They run more reliably, last longer and tend to stay a bit cleaner.

            Now let’s apply this to the 12.5″. If I were to choose the same mid-length system as was on the 16″ it would be a longer gas system, but there would be 3.5″ LESS distance from the gas port to the muzzle. As a result there would be LESS dwell time and LESS gas than a mid 16″

            To keep the dwell time the same with the shorter barrel I had to go with a shorter gas system. That made the carbine a decent choice. Now it isn’t exactly the same dwell time. There is about an 1.5″ less dwell time on the 12.5 compared to the 16, but it is close enough that the gun is running well.

            This is why the carbine length gas system is typically chosen for 12.5 Inch guns that will be shot unsuppressed, because the distance from the gas port to the barrel is about the same.

            Now, beyond your question here is what has been on my mind: After building this gun I’m wondering if I could have gotten away with a mid length system. I’m running the heaviest buffer I can and I still have plenty of gas. If I made the gas system longer I would reduce the gas. I in turn reduce the weight of the buffer for the gun to run…

            When I put a suppressor on this 12.5″ gun, that is going to increase the dwell time, and the gas. I can’t make the buffer any heavier, so that means more gas than I need… And probably an adjustable gas block… Or maybe I just need to build another upper…. 😉

            I hope all this helped and answered your question. I think this is another blog post…

          • Blumpkin

            I’m much more clear now. That’s an excellent explanation.
            It’ll be fun to tinker with the tuning of my 11.5″ BCM (carbine gas system) once my can gets out of ATF jail (first rifle silencer).

          • Paul Carlson

            I’m glad that helped! I’ll be looking forward to hearing about that 11.5”!

            A Question for you now…is that a rifle suppressor or a pistol suppressor? 😉

          • Blumpkin

            Well geez… I guess I’d say it’s a 5.56 rifle rated can, so hopefully it’ll still handle the 11.5″ pistol chambered in 5.56.
            hmm… So if I shoot the pistol until the suppressor is sufficiently carbon locked… Is it then permanently attached?


    • Karl Kimball

      Police officers generally DON’T know NFA laws (or any other law nowadays for that matter) better then those that actually read and understand the law themselves, that is why it’s in your best interest to know the law!

      Please do tell us where in the NFA does it specifically state a barrel between 12″ & 16″ is considered ONLY an SBR.

  • Jackson_Johnson

    Or you can just buy a couple weight buffers and swap them out

    • Karl Kimball

      Save money if you can punch a pin out and do it yourself you don’t need to buy and store extra unneeded parts.