Rotating Bolt, Tilting Bolt, and Roller-Delayed Blowback




Introduction:

Modern semi-automatic rifles typically operate through one of three locking systems, with only a few exceptions. These systems are rotating bolt, tilting bolt, and roller-delayed lockups. The lockup of the rifle is largely dependent on the design goals of the rifle. Aspects of certain systems may be preferred when they provide distinct advantages over others. I'll do my best to give an overview of how each locking system works, as well as go over some of the pros and cons of each one. I currently only have access to rotating bolt and tilting bolt rifles, so those two may be more picture-heavy than the roller delayed. Enjoy, and thanks for reading.

Details:

Rotating Bolt:

The first locking system we're going to go over is probably the most common one used in modern day rifles. This is, of course, the rotating bolt. Although there are variations between semi-automatic, rotating bolt rifles, the lockup generally functions the same. 

AKM-Type Front Trunnion

Modern rotating bolt firearms typically function via the bolt face locking into a front trunnion or barrel extension. The bolt slides forward and is rotated into a locked position in the trunnion, typically by a bolt carrier. The bolt is held in place throughout firing via locking lugs. These lugs on the bolt rotate into locking recesses, creating to a solid lockup wherein the bolt cannot move rearward.

Bolt in the locked position in the front trunnion

This interfacing of the bolt face and front trunnion is critical to the function and safety of the firearm. The lockup between the bolt head and the trunnion is what contains all of the pressure from the cartridge being fired. If either part is poorly manufactured, it can lead to a catastrophic failure, potentially injuring or killing the shooter. 

Bolt in the unlocked position

Upon firing, the bolt is rotated to the unlocked position as the bolt carrier moves rearward. This is accomplished via some sort of cam track in the bolt carrier interfacing with a lug on the bolt in AK-type rifles, or a cam pin in AR's. From the unlocked position, the bolt is pulled backward, extracting and ejecting the spent round. It then picks up a fresh round from the magazine as it moves forward before locking into place again.

There are some serious advantages of this system that make it the most popular way to make a semi-automatic rifle. First off, it's more accurate than other systems. Particularly in AR systems, the presence of an 8-lug bolt allows for a very consistent lockup with the barrel extension. The system is also very strong. Multiple lugs locking into multiple recesses allows for a lot of contact and a lot of metal to contain the pressure of firing even over-loaded ammunition. 

Tilting Bolt:

Tilting bolt firearms are not as common in new firearms in production. However, two of some of the most common rifles in the world use tilting-bolt systems: the Russian SKS and the Belgian FAL. Tilting bolt rifles function via the bolt actually pivoting into a recess in the receiver of the firearm. Typically the bolt shift downward into a locking shoulder, but there are cases such as the Czech ZH-29, where the bolt actually locked into the left side of the receiver.

The locking shoulder on an SKS
In the case of the SKS, as the bolt carrier moves forward the last quarter inch, the bolt is pushed downward until the rear of the bolt rests against the locking shoulder. The bolt is locked in position within the receiver so long as the bolt carrier is fully forward.

SKS bolt in the locked position
Upon firing, the bolt carrier moves rearward after being acted on by the gas system. This lifts the bolt up and out of the locking shoulder, allowing the bolt and carrier to travel rearward in order to extract the spent round.

SKS bolt in the unlocked position
The primary advantage of tilting bolt designs is the overall ruggedness and simplicity of the design. The bolt on an SKS is must more simple than that of an AK rifle. There is no need for it to rotate into anything, it just shifts slightly downward. The disadvantages of the design come in the form of a lack of accuracy. Because there is less contact between the bolt and the locking surfaces, there is a less consistent lockup. In short, the tilting bolt design trades accuracy for ruggedness.

Roller Delayed Blowback:

Roller-delayed blowback rifles, often called roller-locked, and few and far between. The most popular rifle of this design is the G3/CETME style of rifle and their derivatives. Essentially, the system functions via rollers in the bolt head pressing out from the sides of the bolt, securing the bolt into the front trunnion in the receiver. 

These rollers don't actually "lock" the bolt in place in a traditional sense. All they do is delay the bolt moving rearward until the chamber pressure has dropped to a point where the cartridge can safely extract and eject. The timing for the bolt opening relies on a locking wedge within the bolt that pushes the rollers into place. The angle on this wedge is critical to making sure the bolt opens with enough force to extract and eject, while also keeping everything safe.

Because the system operation via the chamber pressure created by the round firing, this is a blowback design that utilizes rollers to make the system safe. This creates some inherit advantages and disadvantages. First off, it makes for an extremely rugged design. The G3 is a very reliable rifle that can function in some very adverse conditions. The high chamber pressure allows the rifle to bust through dirt and grime without much issue. It also allows rifles to be fairly cheap to manufacture. The critical components like the front trunnion can simply be spot welded into a stamped sheet metal receiver. This is the case with the G3. The disadvantages are just as apparent though. The high chamber pressure makes for a rifle with more recoil than other rifles in the same caliber. The high pressures also typically require a fluted chamber to aid in extraction. Without this, it's possible that the bolt would just rip off the case heads of spent cartridges.

Conclusion:

Whether rotating bolt, tilting bolt, or roller delayed, all three systems provide the basis for a solid rifle. All three systems have been implemented in firearms that have been battle-proven in the past century. In my opinion, the success of the rotating-bolt system can be largely attributed to its versatility. Both the AR and AK implement the same thing in different ways. The design of the AK favors pure ruggedness over precision, whereas the AR favors tight tolerances and accuracy. Both do their jobs well and are used the world over.

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