Two of the most common handgun action operations are recoil and direct blowback operation, each of which offers a number of relative advantages and disadvantages.
That said, there is another type of blowback operation, called a roller-delayed blowback action, which, famously, has been utilized by the HK MP5 and American-made MP5 clones like the ZF-5.
If you’re looking at uppers for a build, or are interested in making your own SMG with one of these action types, you might be interested in learning more about roller-delayed vs. direct blowback.
Let this short guide answer some of your questions.
How Direct Blowback Operation Works
Direct blowback, also known as simple blowback or straight blowback, is one of the simplest of all autoloading action types.
In a direct blowback firearm, the bolt rests against the rear of the barrel but is not locked in place with any lugs. When the trigger is pulled and the propellant charge deflagrates, pressure builds in the chamber.
At this point, the bullet starts accelerating out of the barrel, and at the same time, as pressure builds, it works against the static friction of the bolt as well as against the mass of the bolt and the resistance of the recoil spring. Once the pressure builds to a certain point to overcome these forces, the bolt moves rearward and ejects the spent cartridge.
In a direct blowback action, it is necessary that these opposing forces give the bullet enough time to clear the barrel before the bolt moves to the rear and releases the spent cartridge.
Direct blowback operation is suitable for low-powered, low-pressure pistol rounds as well as some rounds that fire relatively light bullets, but encounters operability and safety issues with more powerful rounds.
A roller-delayed blowback action solves some of these potential issues, which will be explored shortly.
How Roller Delayed Blowback Operation Works
Fundamentally, the roller-delayed blowback operation is not that different from the direct blowback action. Both of them ultimately rely on the bore pressure generated during firing to cycle the action.
As in direct blowback, in a roller-delayed operation configuration, the bolt rests against the rear of the barrel, and it is also not locked in place with any lugs. However, in the roller-delayed configuration, the bolt contains special rollers that correspond to trunnions machined into the barrel.
When the bolt closes, these rollers engage the barrel trunnions, effectively sealing the bolt in place until adequate pressure has been generated. While the bolt is not strictly speaking locked in place, it is effectively held shut under a lot of pressure.
When the gun is fired, pressure builds up in the chamber and the bullet starts to travel down the bore, just as in a direct blowback firearm. However, the bolt cannot move to the rear because of the pressure holding the rollers in the barrel trunnions.
Once a certain level of bore pressure has been reached, the rollers disengage from the barrel trunnions, allowing the bolt to move rearward, ejecting the spent cartridge before continuing the cycle.
As you might expect, there are numerous advantages to roller-delayed blowback action as opposed to direct blowback action.
Advantages of Roller Delayed Blowback Actions
The main advantage of roller-delayed vs. direct blowback action is that roller delayed action is safer. Because the bolt opens at a much lower pressure, there is less risk of case rupture than there is with direct blowback action. There is also effectively no risk that the bolt will open prematurely, which can also be dangerous.
Beyond safety, there are several reasons that roller-delayed blowback is superior simply from a stance of performance. For one, the bolt can be made lighter, since the rollers secure it shut until the optimal pressure level has been reached. This reduces recoil and also means that roller-delayed blowback is a lot smoother than direct blowback. There is also a lot less port pop with roller-delayed firearms.
Roller-delayed blowback operation also responds much better to suppression and firing subsonic ammo than direct blowback actions do. In fact, roller-delayed actions are better than most gas impingement systems, too. You won’t get all that gas in your face and roller-delayed platforms are cleaner on top of that,
Ultimately, roller-delayed blowback operation is safer, cleaner, smoother shooting, produces less recoil (all else being equal) and suppresses better than direct blowback. Roller-delayed blowback actions can also work more reliably and more safely with higher pressure cartridges. At the end of the day, there is no comparison. The only big drawback, if you can even call it that, is that roller delayed firearms tend to cost more, but you get what you pay for.
Questions on Roller-Delayed vs. Direct Blowback? Get in Touch with Us!
Interested in learning more about roller-delayed vs. direct blowback operation, or about firearm platforms that utilize roller-delayed operation? Take a look through our catalog that features the ZF-5, ZF-56, and other roller-delayed platforms, or get in touch with us and we would be happy to help.