Significance of open bolt firing?

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wacki

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Future weapons featured this rifle:

http://www.defensereview.com/modules.php?name=News&file=article&sid=885

The shows IAR was basically a 5.56 m4 with a slightly heavier barrel and a selector switch that switched the rifle from closed bolt firing (semi) to open bolt firing (full auto). He unloaded 2 clips in in the M4 and then the IAR. He poured water on the m4's barrel and made a big deal about the steam. He then closed the bolt of the IAR and touched the bolt with his finger bragging about how cool it was to the touch. Not exactly the same test and a little deceptive but whatever. How much of a difference does open bolt firing make? I'm guessing the theory is that it allows more airflow through the barrel allowing for greater cooling. But does anyone have any solid numbers displaying the differences? For instance a rifle in closed bolt can fire 700 rounds in 2 minutes while an open bolt can fire 1000 rounds before over heating.

Just interesting in know what the real differences are.
 
stops uncontrolled firing

In prolonged firing with a closed bolt the chamber can become hot enough to cook off a round without pulling the trigger, since the open bolt does not chamber and fire a round until the trigger is pulled you can't cook off a round. However the open bolt is more susceptible to getting dirt into the weapon. Open or closed does not make weapons fire any faster. The manufacturers typically have to slow the rate of fire down to help control muzzle rise and very rapid emptying of magazine.
 
I think the idea behind the bolt being cooler wasn't related to the open bolt but rather to the fact that the gas tube doesn't drive hot gases directly back into the bolt carrier. If I remember correctly the rifle has a gas piston and thus it keeps the heat from being dumped back into the bolt/bolt carrier.

Just my .02

Regards,
Dave
 
An open bolt gives you better cooling on a weapon designed for sustained automatic fire, but at the cost of accuracy, as each shot begins with the bolt flying forward and chambering a round. This is not usually considered a liability in a machinegun, however, where you actually want some inaccuracy/dispersion built into the weapon and its ammunition to give you a better beaten zone on the receiving end (in a submachinegun may be a different issue, though that's off topic).
 
Most submachine-guns fire from an open bolt. Many machine-guns, that is belt fed, do as well. In the case of subguns, it's often ease of design, in the case of belt-feds, it's to prevent cook-off.

An open bolt M16 variant is nothing new, the M231 "port firing weapon" (designed to be fired through an armored fighting vehicle port) was an open bolt M16.

In addition to the accuracy comments, another reason you don't see open-bolt semis is that they tend to be fairly easy to convert to full-auto. This was the case of the open-bolt semi MACs.
 
Well so what. Open bolt firing just jars the heck out of the weapon. Sure it avoids cookoff, but it does not reduce the amount of heat that a cartridge produces when it goes off. And that heat has to go somewhere, and most of it goes into the metal parts, barrel, receiver, etc. Metal conducts heat far faster than air, at some point the metal just keeps on getting hotter because air cannot wick away the heat fast enough.

Thermal management issues do not go away just because someone creates an open bolt system.

As far as basic operating principals, everything patentable was patented before 1900.
 
For what it is worth, you can stick your thumb on the bolt carrier of an M16 that uses the LWRC gas piston system with a closed bolt as well. The cooler bolt is because the gas piston doesn't vent hot gas back onto the bolt and bolt carrier, so only the actual bolt face gets heated up.

Open bolt does deal with heat better; but the test he demonstrated had more to do with the gas system and less to do with the open bolt/closed bolt issue.
 
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