whats the difference between open bolt and closed bolt?

[Cold1]

[CanadianGlockFan]

An "open bolt" submachinegun fires from the "open bolt" position, ie. when you insert a magazine and pull the cocking handle back, the bolt catches on the sear and stays there, open. Usually, these firearms have a fixed firing-pin machined onto the front of the bolt so that a cartridge is fired automatically as it's chambered, and the bolt will just keep flying back and forth, stripping cartridges out of the mag and into the chamber to fire them as long as there's ammo in the magazine. A "closed bolt" submachinegun requires an additional hammer or striker somewhere in the system, and that hammer or striker won't be released until the bolt is fully forward on a chambered round; on these designs, you insert a full magazine, pull the cocking handle rearward, and then let it go to allow the bolt to pick up a cartridge as it goes forward to the "closed" position. The "open bolt" is the original (and simpler, more trouble-free) design for the MAC, and is therefore somewhat more valuable, because the later "closed bolts" are conversions from semi-autos.

[ProGlock]

Here's is a photo description to help out...

http://www.uzitalk.com/reference/pages/bolts.htm

I'm going to relate this to belt-feds which I know slightly more about.

A belt-fed MG is designed to lay down continuous suppressive fire. Obviously such an MG, since it could be fired continuously for long periods of time, it needs to remain as cool as possible, thus the reason an open bolt is preferred for such weapons, in addition to open bolt by nature being a simpler design.

The HK21 fires in a closed bolt, like most HK weapons do. The M60 came out of the factory as open bolt...I believe the M249 does also. I would choose the open bolt anyday over the closed bolt in such a scenario, because of the reasons I mentioned previously.

However it is commonly believed that closed bolt MGs are slightly more accurate.