Sunday, August 29, 2010

Actions: Blowback Action: Recap

In this post, we will recap all the stuff we've studied on blowback systems:
  1. In blowback systems, the bolt is not locked when the gun is fired. This is the defining characteristic of all blowback systems.
  2. All blowback systems use some of the gas generated when the cartridge is fired, to operate their mechanisms.
  3. All blowback systems use the empty cartridge case as a sort of piston to operate their mechanisms.
  4. The main problem to overcome on blowback systems is how to prevent the breech from opening the instant the gun is fired. The breech should only begin to open after the bullet has left the front of the gun and gas pressure in the chamber has dropped to a safe level. All blowback systems have different ways of overcoming this problem. Some use inertia, others use the principles of levers, still others use friction, gas etc.
So what are the advantages and disadvantages of such systems?

  1. Blowback systems have fewer moving parts than other systems.
  2. In most cases, there is no gas tube hanging off of one side of the barrel and affecting the natural harmonics of the barrel. This allows the barrel to be free-floated.
  3. Generally reliable provided they are used with the ammo that they are designed for.
  4. Relatively cheaper to manufacture.
  5. Some blowback systems offer recoil reduction and less muzzle climb when firing rapidly.
  1. Cannot be used with powerful cartridges. These are generally suited to low-powered or medium-powered weapons at most. This is why most blowback weapons are pistols or submachine guns, not high powered rifles.
  2. Some actions (such as straight blowback) make the gun heavier by the nature of how they work.
  3. Some blowback actions are extremely sensitive to the type of cartridge used. While they are reliable when used with the cartridge that they're designed for, slight variations (such as the weight or the material of the case) can cause problems. Unlike a gas operated system which has a user-tunable gas regulator, these cannot be tuned to different cartridges.
  4. Some blowback systems quickly get dirty with use.
Blowback systems are generally most prevalent in smaller pistols and submachine guns, because they cannot reliably handle higher powered cartridges.


  1. I just found this blog, and I must say I'm impressed. It looks like quite a bit of work went into these write ups.

    A few nit-picks:

    -The delay effect of the API fixed firing pin in SMGs is very small. I've seen experts debate whether it exists or not. The reason for this is that the timing between detonation of the primer and the bolt coming to rest on the receiver is very short, and since most open bolt SMGs are designed to be cheap it's anyone's guess whether any particular one enjoys much delay given the tolerance stacking.

    This is not to say API isn't important, but that its most important application was not in the field of small arms, but rather in aircraft cannon. The Oerlikon blowback family was used by both sides in WWII. The British Hispano Suiza, Japanese Type 99 and German MK 108 were all API blowback type weapons.

    All these designs used cases with a rebated rim, that is, the case head had a smaller diameter than the case body. This means that the extractors could be the same diameter as the case body. The firing chamber could then be cut extra long. This has two benefits, the first is that the case has a much greater distance to retreat before the case head is unsupported by the chamber, so the bolt mass can be made drastically lower. The second benefit is that the case is inside the chamber for a longer period of time, making it much easier to time the ignition to take advantage of the API effect. This is better explained, if by a somewhat circuitous route here.

    -There was actually another design that used hesitation locking besides the Remington 51 and 53, the SIG MKMS. If you read the description you'll note that it's using the same principle as the Pedersen design. I have corresponded with Max Popenker, and he has confirmed that the SIG MKMS also used hesitation locking.

    -While we're on the subject of obscure blowback designs, there's one you missed; barrel-chase blowback as used in the P90 (and a few others). If you live in the United States, check out a PS90 in a gun store some time. There's a little bit of play between the barrel and the receiver, and that's intentional. The barrel is allowed to float backwards a small amount to reduce case friction with the firing chamber. The barrel is not locked to the bolt, however, so this is not a recoil operated system.

    -Where did you find the citation that Pedersen invented lever blowback? I hadn't heard that one before.

  2. Hi Neutrino Cannon,
    First, thank you very much for your kind comments and all the helpful feedback. It is much appreciated.

    As to your question about Pedersen and lever delayed blowback, a quick search on google for "lever delay Pedersen" returns some citations about John Pedersen being the first to patent a lever delay blowback design, as well as references to patent #1,410,270 from March 21st 1922, registered to John D. Pedersen of Jackson, WY.

  3. Most interesting.

    The early Kiraly SMG designs were substantially similar in outline to the SIG MKMS, and Kiraly had worked for SIG before. Those designs would have used lever and hesitation blowback respectively, both invented by Pedersen.

    I wonder if Pedersen ever corresponded with the SIG engineers or worked as a consultant for them.