Closed bolt vs. open bolt

Open bolt, the bolt is open till you pull the trigger. Then, it slams forward, strips a round off the mag into the chamber and fires. Open bolts usually don't have a firing pin, just a 'nub' on the front of the bolt face.
 
Open bolt is an extremely simple system as there is no separate firing pin or spring, nor locking system for the bolt.
A dimple is machined on the face of the bolt which acts as a firing pin, so as the bolt slams home on a cartridge it hits the primer. Few, if any new open bolt guns are allowed to be manufactured in the US for civilian sales in recent years because there is very little difference between a semi only gun and select fire. Simply moving the disconnector out of the way provides full auto fire.
 
i don't think that all open bolts fire with an unlocked bolt, i believe that the RPD and the 1919 both are open bolt and lock upon closing and firing. most open bolt sub-machineguns fire from an unlocked bolt due to the lower amount of blowback energy.
 
Here is an image of a Sten (which uses advanced primer ignition); it is an open bolt firearm.
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The round is chambered only upon the trigger (and sear) releasing the bolt. The cartridge is discharged just before the bolt is fully forward.
 
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Open bolt designs also enhance the breathability of the action so it can cool quicker as well as preventing the weapon from cooking off a round in the chamber.
 
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