I know how fully automatic firearms work: the expanding gas released by a fired round pushes the action and chambers the next round without human assistance
But what affects how fast it does this? Is it the caliber of the bullet? Does a more forceful bang move the action faster?
Does it depend on how well the machinery of the action slides against the chamber?
In short: what factor changes the rate of fire in machine guns?
The recoil spring design make a big difference in some NFA guns. Especially the recoil operated actions.
Some slam fire from the open bolt position, some have a more complex trigger group with a hammer that must be cocked and held and released.
The path the gas must take and what it must act on to complete the job, some (gas operated of course) use pistons some act directly on the bolt.
There are so many different designs and things that act on the mechanisms, it seems to me that the open bolt designs fire faster.
Type of round is one thing…
Other factors:
Size and style of the bolt assembly and locking mechanism
Buffer spring
Size and style of the action
The Sear, hammer and trigger mechanisms
Feeding and chambering mechanism, as well as the length of the round being chambered
Gas regulation (if it’s gas operated)
Sure there’s more… that’s about what I can come up with off the top of my head.
Oh, boy! I can’t wait to see some of the responses to this one… as B & I and B_N both point out, there are many designs with many factors. I have been told that some “open-bolt guns” have a higher rate of fire because they employ an operating feature that may be described as “pre-ignition” - the firing pin (fixed on the bolt or driven by a striker) contacts the primer of the fresh round coming out of the magazine and actually starts the detonation just prior to completely entering the chamber. I think the most important factor in the ROF (rate of fire) equation for hand-held firearms is the size of the bolt. The original M11, the Skorpion, and the even the Micro-Uzi all have astonishing rates of fire. This is due to the small size of the bolt, which is possible because the round is relatively small. In an effort to slow down the ROF of these types of guns, makers have used heavier bolts, stiffer recoil springs, and innovative mechanical principles (like the inclined ramp of the Jati) - with varying degrees of success. Conversely, the ROF can be increased by lightening the bolt or using a weaker recoil spring. The problem here is that the maker approaches the safe working limits of the firearm when they begin to tinker with the machinery. BTW - it is highly advised that you not fool with the bits and pieces of any firearm, or you could be asking for physical and legal trouble.
If you want to get extremely high rates of fire you eventually have to go with multiple, rotating barrels (i.e. a gatling gun design).
I would think in a recoil full-auto (as opposed to a Gatling type), the two most significant factors governing rate of fire would be the length of the cartridge and the total mass of the parts that must move to eject the spent case and feed another into the breech. The mass of the moving parts would determine their acceleration under a given force, and the cartridge length would govern the one-way distance the parts have to move.
Sure, there are other factors, but these two seem to be the main ones.