Michigan AR-15 Pistol or SBR Build Part 3 Barrel Length

Our intent for part 2 of this build was to go into the assembly process of the lower. However Keith in his rifle build article did a wonderful job. Since the assembly and process is the exact same we will not be redundant.

Instead for part 3 we are going to move into where the SBR or Pistol builds differ from their counterparts the rifle or carbine. The main divergence is in the barrel length.

Picking The Right Barrel Length

My experience has show that this decision is the one that is likely to taunt you from inception through creation of your rifle. It is very easy to second guess your decision or wish you had gone another route after completing your rifle. There is only so much advice anyone can give you related to prevent the top 2 causes for this feeling.

1. I wanted a cool SBR or Pistol. 

If you build a SBR/Pistol cause it looks cool without a purpose in mind you’re very likely to regret it later. SBR’s and Pistols are cool but like a hot girl with no brain the novelty will wear off fast without substance to back it up. Picking a purpose for the rifle and building a weapon to match that purpose will give you a reason to both train and develop a long term relationship with your new weapon.

2. I wanted X when I built it but now I want Y

While I think that people will claim this is the number one issue in likelihood most people really just built a rifle to build a rifle. However for those of you who either change your mind or end up here due to lack of planning the result is the same. You overpaid for parts your now going to ditch in favor of something else to achieve a new goal.

Be mindful of these issues. There is a reason we have published two complete articles about picking the components and getting started. Proper planning upfront will likely save you time, money, and frustration.

No part of the gun is more tied to these principles than the barrel length. It will be the deciding factor in the other parts you buy and needs to be the forefront of your buying decisions. Too long and you have a carbine or rifle not a SBR, too short and you may find you don’t like the resulting feeding issues, reliability, fireball or useability.

Barrels come in many sizes with 6.5” being the generally accepted smallest (yes there are a few smaller but this is the general stock smallest not custom) all the way up to a 14.5 (M4 length).

That sure looks like a cool pistol
That sure looks like a cool pistol


I am not a lawyer, I never have been a lawyer, I never plan to be a lawyer and I am not giving you legal advice. If you want legal advice go find a lawyer. We have a link to a very good one on our homepage. This is for academic purposes only and should not be construed as legal advice.

Federal Law vs Michigan State Law

SB610 passed, hooray!! We can own SBR’s and register them with the ATF and pay our money and shoot guns, yeah. Wouldn’t it be great if all this red tape didn’t exist? Yes it would but we don’t live in a perfect world. What is worse is that Michigan and the ATF do not even agree on how to measure a rifle. Federally the magical 26 inch number that separates SBR’s from rifles is based on the fully extended over all length or OAL. Michigan however measures based on the shortest overall length. So due to this there is actually a few pistols that can get you landed in hot water.

rjbergen on AR15.com explains it nicely.

So you’re asking about a firearm with a folding stock that has a barrel length of 16″+, an unfolded OAL greater than 26″, and a folded OAL less than 26″?
According to federal law, that firearm would be a rifle because it is measured in the fully extended configuration. Therefore, it would not be classified as an SBR.
However, MI measures in the shortest collapsed configuration and would classify that firearm as an SBR due to the folded length less than 26″. I do not believe you can own such a weapon in MI since it would be classified as an SBR under MI law, but not according to the ATF.

Gas Systems

How Direct Impingement Gas systems work.
How Direct Impingement Gas systems work.

Barrel length becomes a larger issue in Direct Impingement Gas Systems weapons than in their Piston System counterparts.  As discussed in the above paragraph. Carbine based gas systems have been proven to be reliable and even for a novice gun builder can be assembled easily.

Here is a nice chart to highlight the gas system and available barrel length.

System Barrel Length Port Distance
Pistol < 10 inches 4 inches
Carbine 10-18 inches 7 inches
Mid 14-20 inches 9 inches
Rifle 20+ inches 12 inches

Due to dwell time requiring the bullet to still be in the chamber while the weapon begins its cycle you will need to have a 10  inch barrel to use a carbine length system with a 10.3 or 10.5 increasing the reliability. As outlined in part two of this series for our build we used a Daniel Defense 10.3 carbine length gas system barrel. The down side to this is that when you add a Noveske KX3 to the end of this 10.3 barrel you’re left with effectively a 13 inch barrel. While this is perfect for me and the weapon has proved to be extremely reliable it does not meet the needs of many AR pistol owners, it is simply too long.

When working with a pistol length sub 10 inch barrel, like the very popular 7.5 inch barrel, you encounter a series of issues that need to be overcome.

  1. Heat: The closer the gas port is to the barrel the hotter the gas is that is being pushed back into your receiver.
  2. Unburnt Powder: The closer the gas port is to the barrel the more powder remains unburnt causing  a dirtier chamber and gas tube
  3. Fireballs: Some people really like fireballs and build guns to specifically generate them. However if using a weapon as a PDW inside a home you may want to consider ways to mitigate the excessive fireballs often associated with shorter barrels.
  4. Cycling Issues: With the extra force of the pressurized gas on shorter weapons there is an increase in cycling issues. Having enough dwell time (period bullet remains in chamber after passing the gas port) is critical to ensuring a proper cycling of the weapon.
  5. Recoil: With a proper dwell time that ensures a reliable cycling force in pistol length systems is greatly increased. This will lead to additional force rearward causing additional recoil. Linear comps that increase back pressure to reduce fireballs will only add to this recoil.
  6. Wear and tear: With more hot gas more debris and more force being pushed into the barrel the shorter the gas system is the more wear and tear on your rifle. This is why we highly recommend using Anderson Rifles RF85 treated uppers when building a SBR or Pistol. The RF85 treatment is designed to reduce wear on components and handle the high temperatures that come with these types of weapon systems.

Mitigating Gas System Flaws

While shorter gas systems such as the carbine and pistol length systems commonly used on SBR or Pistol builds have disadvantages those obstacles can be overcome with proper planning. Choosing the right parts to compliment a shorter barrel will allow you to build a quality and reliable product that is fun to shoot.

Heavy Buffers

Using heavier buffers like the Spikes T3 will help reduce the reward force common in short gas systems. This is beneficial in reducing the felt recoil and can have a marginal effect on wear and tear.


Buffer Springs

Increased buffer springs will work like the buffer to reduce the felt recoil. With both options you need to be aware of dwell time and ensure that you are not putting to much forward pressure on the bolt carrier. This will cause failure to feed problems and may result in reduced reliability.

Linear Comps

To combat fireballs and dwell time issues in short pistol builds linear comps can be applied to the end of the barrel. These comps will reduce some noise (not enough to be considered a suppressor but certainly better than nothing at all) as well as increase back pressure. While this may be counter intuitive there is several reasons to force pressure reward instead of to the side as many muzzle breaks do.

Typically a pistol or SBR is designated for CQB such as in a house. When firing a weapon in close quarters using a traditional muzzle break you will find that noise is directed at you instead of away from you. Also debris and pressure will often come back at your face. Using certain linear comps you can force sound forward instead of to your sides and back at you. Further you direct the force rearward instead of to the sides which affect your teammates. On a 223 or 556 round the slight increase in recoil is worth the gains in sound and force management in CQB.


Piston Systems

There are some great companies making Piston system upgrades or complete uppers. While Piston systems will add weight to the weapon they will reduce the amount of hot gas and debris blown back into the upper receiver. Some argue this does not make a difference on a 16 inch rifle running a carbine length gas system or longer, but as the gas system gets shorter, the case for a piston based system gains some serious traction.

My general rule of thumb is if you’re going to go shorter than a carbine length gas system go piston. This rule was taught to me by Keith our resident building expert and further supported by hours of research on the topic. While you can tune a pistol length gas system to be reliable the opportunity for failure and the requirement for more advanced knowledge makes shorter gas systems less advantageous for the average shooter.


Ultimately you will need to decide for yourself based on your needs. While maneuverable is important you need to take all the deciding factors into play. We would love to see you comment below with an image of your SBR or Pistol or share them on our Facebook page.

Charles is the editor for 248 Shooter a midwest based gun news and gear review site as well as Online Content Director for On Target Magazine. He is an avid student taking classes from top tier trainers around the country. Charles shares his love for training as well as experience and opinions on some of the most talked about gear and products used by competitive shooters, military, leo and civilians.