A correspondent recently emailed me links to a couple of YouTube videos from a firm that trains people in the use of modern weaponry, with the suggestion that it might be more relevant to the discussion of how a stateless society could defend against states than the example of paintball that I offer in a chapter in the new edition of The Machinery of Freedom. The first video included a discussion of a technical point new to me, the distinction between open bolt and closed bolt firearms.
After the video finished, I went downstairs to tell my younger son about it. Bill is unlikely to ever have any use for an open bolt, or even a closed bolt, firearm. He is, however, a would-be novelist, I think likely to be a good one, and almost any information is potentially useful in that profession—which is the point of the title of this post.
For instance ...
Most modern firearms are closed bolt. When the gun is ready to be fired the bolt is closed, holding the cartridge firmly in place. Pulling the trigger releases the firing pin, which strikes the primer, which fires the gun.
In the less common open bolt design, the firing pin is a fixed part of the bolt, not a piece free to move within it. When the gun is ready to fire, the bolt is open, pulled back. Pulling the trigger releases the bolt, which slides forward, closing the breach and driving the firing pin into the primer.
That works fine, provided you are standing still. But with the bolt open, there is nothing holding the cartridge in place. If you are running around with the weapon ready to fire—not a problem with the closed bolt weapons you are used to—the cartridge can come loose, either falling out entirely or jamming the mechanism that is supposed to fire it.
That is what happens in the video to a trainee using a weapon new to him. It could happen easily enough to a fictional character, providing both one more way in which the author can complicate the action and a nice technical detail to help the author pretend to know what he's talking about.
Which is why I thought my son would be interested.
6 comments:
The Uzi (https://en.wikipedia.org/wiki/Uzi) and Ingram MAC 10 (https://en.wikipedia.org/wiki/MAC-10) are examples of very successful open bolt sub-machine gun designs. In these weapons, it's the magazine that is responsible for securely holding the round waiting to be chambered so you're free to maneuver, even strenuously, without much risk of putting the weapon out of action. That PKM design featured in the video depends on a belt to feed the ammunition during firing. It's the return action of the bolt that strips a round from the belt to be fed into the chamber and fired while the trigger is depressed. If you mount a belt, then pull the charging handle to cock the gun in preparation to fire, a round is pulled from the belt and dropped in to the feeding area in front of the chamber where it waits for the bolt to travel forward to fire. The instructor was recommending that you mount the belt and then wait to charge the gun until just before you need to fire it to avoid that issue. I've never shot the PKM, but I imagine that it is also challenging to operate at odd angles without misfeeds. The American M60 machine gun (https://en.wikipedia.org/wiki/M60_machine_gun) is also an open bolt design and does not suffer from the same issue. I have shot that one.
So what are the advantages of open bolt weapons (sorry if it is mentioned in the video, I didn't watch it)?
There are many jobs besides author where it helps if you have a lot of random knowledge, some of it rather deep. One such job, for example, is filmmaker. Another is customer- or user-experience designer. Reference librarians, I imagine, need this kind of knowledge, and ones who have it will still do much better than Google.
As Attemptingtobeskeptical says, pretty much all submachine guns and machine guns operate from an open bolt, and don't have this problem. In a magazine fed submachine gun the round should be held securely in place by the magazine lips, and in most machine guns the round is held in the belt until it is pushed forward out of the belt and into the chamber just before firing.
The PKM fires a rimmed cartridge, where the rim at the base of the round that the extractor grabs on to protrudes wider than the diameter of the rest of the round. This is an old design, most modern military ammunition has a narrower area just in front of the rim so that the rim can be the same diameter as the rest of the case.
This rim is why the PKM has to pull the cartridge back before pushing it forward, making this malfunction possible.
What's the advantage of the open bolt design, aside from making the firing pin mechanism simpler?
The primary advantages are rate of fire and robustness. The MAC 10 for example, would easily reach 1,100 rounds per minute of cyclic rate. A common modification would be to increase the strength of the main recoil spring to lower the cyclic to around 600 RPM. Open bolt designs have fewer parts and don't rely in tight tolerances to contain the expanding gases from firing the cartridge.
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