Open Bolt vs. Closed Bolt: What's the Difference?

The terms open bolt and closed bolt frequently appear in discussions of submachine guns, pistol-caliber carbines (PCCs), and historic military firearms. But what do they actually mean? How do these operating systems work, and why has the closed-bolt design become the standard for most modern firearms? Let's take a closer look at the mechanics behind both systems, along with their strengths and limitations.

You may have come across references to firearms that fire from an "open bolt" or from the "rear position". For decades, open-bolt operation was the preferred solution for automatic weapons, particularly machine guns and submachine guns. While it remains common in many machine gun designs, it has all but disappeared from modern submachine guns and PCCs. To understand why, it's worth examining how each system functions and the practical advantages and trade-offs they offer.

Modern PCCs use a variety of operating systems, ranging from simple blowback to delayed-blowback and fully locked-breech designs. Photo: Rigad

A Brief History

As mentioned earlier, firing from an open bolt was once the standard for submachine guns. Just look at the earliest designs adopted by military forces toward the end of the First World War. Germany's MP 18 established the layout and operating principles that would define submachine guns for decades to come, while the iconic Thompson hardly needs an introduction.

During the interwar years, submachine guns steadily gained popularity, although many conservative military leaders remained unconvinced of their battlefield value. It took the Second World War to prove their effectiveness. Nearly every major combatant fielded its own design: Britain's STEN, the Soviet PPSh-41 and PPS-43, Germany's MP 40, the American Thompson in its various versions, and later the M3 Grease Gun.

Now guess which operating system these firearms shared. Almost all of them fired from an open bolt. There were only a handful of exceptions, such as the American Reising M50, but it never achieved widespread adoption.

The post-war years saw little change in this regard until the introduction of the Heckler & Koch MP5. The MP5 demonstrated that the benefits of closed-bolt operation were not limited to assault rifles. They could also significantly improve the performance of submachine guns.

From that point on, the balance gradually shifted in favor of closed-bolt designs. The transition was far from immediate, but today virtually every modern submachine gun is built around a closed-bolt operating system. Why did the industry move in that direction? Let's take a closer look.

Tip: Curious how the Czechoslovak Škorpion vz. 61 evolved into the modern CZ Scorpion EVO 3? Read the story behind one of the country's most influential firearm designs.

Virtually all modern PCCs and submachine guns operate from a closed bolt, making closed-bolt operation the dominant standard today. Photo: Rigad

Open-Bolt Operation

We've covered the historical background of open-bolt submachine guns, but one important question remains: why was this operating system so widely adopted in the first place?

The answer lies in the practical requirements of military and law enforcement users. Open-bolt operation wasn't chosen by chance—it addressed several challenges that were especially important for automatic firearms of the time.

Before we explore those advantages, let's first look at how the system actually works.

How Does an Open-Bolt System Work?

Most open-bolt submachine guns rely on nothing more than bolt mass and recoil spring tension to operate. In most designs, the firing pin is fixed to the bolt. Before firing, the bolt is pulled fully to the rear, where it is held in place by the sear—hence the term open bolt.

When the trigger is pulled, the bolt is released and driven forward by the recoil spring. As it travels, it strips a cartridge from the magazine and chambers it. The moment the bolt reaches its forward-most position, the fixed firing pin strikes the primer and the round is fired.

Gas pressure generated by the shot immediately drives the bolt rearward. On its way back, the bolt extracts and ejects the spent casing before being caught by the sear again. In a fully automatic firearm, as long as the trigger remains depressed, the cycle repeats continuously.

Advantages and Disadvantages of Open-Bolt Operation

✅ Simple Design and Low Manufacturing Costs

Firearms that operate from an open bolt are generally very simple mechanically. In most cases, the action consists of little more than a heavy bolt with no locking mechanism. Fewer moving parts mean lower production costs, easier maintenance, and straightforward mass production. Simplicity also contributes to one of the system's greatest strengths—reliability.

✅ Excellent Reliability in Harsh Conditions

The old saying "what isn't there can't break" applies especially well to firearms. Military weapons are expected to function in mud, sand, snow, and other harsh environments, often with minimal maintenance.

That said, an open-bolt action also leaves the internal mechanism more exposed whenever the weapon is cocked. Dirt, sand, mud, or snow can easily enter through the ejection port. Manufacturers addressed this challenge in different ways.

The French MAT-49, for example, used a dust cover to protect the action, while the Czechoslovak Sa 23, Sa 24, Sa 25, and Sa 26 series featured an ejection port arrangement that remained closed until the weapon was ready to fire. Other designs offered little protection at all—yet still earned a reputation for exceptional reliability.

✅ Reduced Felt Recoil

The forward movement of the bolt helps counteract some of the rearward recoil impulse. If you've ever fired an open-bolt submachine gun, you've probably noticed this effect. The same principle also applies to modern civilian semiautomatic variants, including some pistol-caliber carbines (PCCs).

However, this advantage comes with a trade-off. Because the shot is fired only after the relatively heavy bolt has traveled forward and come to a stop, the firearm experiences a slight disturbance just before ignition. Try dry-firing an open-bolt firearm and you'll see the sights dip as the bolt slams into battery.

In fully automatic fire, this has little practical impact. At typical engagement distances, ultimate precision is far less important than sustained fire. In semiautomatic shooting, however, especially with PCCs used for target shooting or competition, the movement becomes much more noticeable and can affect practical accuracy.

That's one of the main reasons why modern PCCs—and virtually all contemporary submachine guns—operate from a closed bolt. Let's take a look at how that system works.

Compared to traditional open-bolt designs, closed-bolt operation offers greater practical accuracy and smoother handling characteristics. Photo: Rigad

Closed-Bolt Operation

Today, virtually every modern submachine gun and pistol-caliber carbine (PCC) operates from a closed bolt. But how does this system work, and what makes it different from an open-bolt design?

How Does a Closed-Bolt System Work?

Unlike an open-bolt firearm, a closed-bolt design chambers a round before the trigger is pulled. Once the action is cycled, the bolt moves forward, strips a cartridge from the magazine, chambers it, and remains fully in battery until the shot is fired.

In principle, the system operates much like a modern semiautomatic rifle or pistol. Pulling the trigger releases the firing mechanism rather than the bolt itself, allowing the shot to break without any large moving mass traveling forward immediately beforehand.

Simple Blowback, Delayed Blowback, or Locked Breech?

Not all closed-bolt PCCs use the same operating mechanism. Broadly speaking, they can be divided into two categories.

The first includes simple blowback firearms. These rely solely on bolt mass and recoil spring tension to keep the action closed until chamber pressure drops to a safe level. Examples include the CZ Scorpion EVO 3, Heckler & Koch UMP, Ruger PC Carbine, and many AR-pattern PCCs such as the ADC AR-9 and Schmeisser AR15-9.

The second category consists of firearms that also operate from a closed bolt but incorporate a delayed or fully locked action. In these designs, bolt movement is controlled by more than just its weight and the recoil spring.

The Heckler & Koch MP5 is the classic example, using the company's well-known roller-delayed blowback system. The SIG MPX takes a different approach with a short-stroke gas piston and rotating bolt, closely resembling the operating system of a modern assault rifle.

One firearm that deserves its own category is the KRISS Vector. As its unconventional shape suggests, it uses a unique operating principle known as the KRISS Super V System. Although it is still fundamentally a blowback design, the system redirects the bolt's rearward movement downward through a specially designed mechanism, reducing muzzle rise and changing the way recoil is transmitted to the shooter.

Tip: Want to know why PCCs have become so popular with competitive and recreational shooters? Read our guide to their key advantages and most common applications.

Advantages and Disadvantages of Closed-Bolt Operation

✅ Greater Accuracy

One of the primary advantages of a closed-bolt firearm is improved practical accuracy. Because the bolt is already fully in battery before the shot breaks, there is no heavy mass moving forward immediately before ignition, as there is with an open-bolt design. This allows the shooter to maintain a steadier sight picture and minimizes disturbance just before the shot is fired.

✅ Better Trigger Characteristics

Closed-bolt firearms also tend to offer a cleaner, lighter trigger pull. Instead of releasing the entire bolt assembly against the force of the recoil spring, the trigger typically releases only the hammer or striker. This makes it easier to shoot accurately, particularly during slow, deliberate fire.

✅ Better Sealed Action and Improved Reliability

Another advantage is that the action remains closed while the firearm is ready to fire, making it less susceptible to dirt, sand, and other debris entering the mechanism.

Reliability also benefits from the fact that a cartridge is already chambered before the trigger is pulled. As a result, there is less chance of a feeding malfunction on the first shot. Open-bolt submachine guns could be more susceptible to this type of stoppage, particularly if the magazine was not fully seated or had become damaged or contaminated. The failed assassination attempt on Reinhard Heydrich by Jozef Gabčík is perhaps the best-known historical example of how such a malfunction can have dramatic consequences.

❌ More Complex Design

The main drawback of closed-bolt firearms is their increased mechanical complexity. Even simple blowback designs generally require more components than comparable open-bolt firearms, while delayed-blowback and locked-breech systems are considerably more sophisticated.

As a result, reliability depends much more heavily on sound engineering, manufacturing quality, and proper tuning. When executed well, these systems are exceptionally dependable, but they place greater demands on both the designer and the manufacturer than the inherently simple open-bolt concept.

Why Has Closed-Bolt Operation Become the Standard?

In short, while all of the operating systems discussed above can still be found in various firearms, virtually every modern submachine gun and pistol-caliber carbine is built around a closed-bolt design.

The reasons are straightforward. Closed-bolt operation offers better practical accuracy, improved trigger characteristics, and a more enclosed action that is less susceptible to contamination. These advantages make it particularly well suited to today's military, law enforcement, and civilian applications.

Another factor is legislation in the United States. Since the early 1980s, newly manufactured semiautomatic firearms that operate from an open bolt have faced significant regulatory obstacles because they are generally considered easier to convert to fully automatic fire. As a result, firearms originally designed as open-bolt weapons—such as civilian versions of the UZI—were redesigned to operate from a closed bolt for the commercial market.

That doesn't mean the open-bolt concept is obsolete. Its simplicity, durability, and excellent heat management still make it an effective choice for certain types of firearms, particularly machine guns and other weapons intended for sustained automatic fire.

Whether you're interested in historic submachine guns, modern PCCs, or military small arms, understanding the differences between open-bolt and closed-bolt operation provides valuable insight into why these firearms are designed the way they are—and how those design choices influence reliability, handling, and shooting performance.