While it's true the cyclic rate of fire is as specified, say 600RPM for say, an M16, and that will empty a 30 round mag in 3 seconds (exactly).  The real rate of fire is limited to (a) how fast one can change magazines (don't throw them away - put the empty back in the pouch!) and (b) over heating.  The practical sustained rate of fire for most assault rifles is ~90 RPM.  Oddly enough it the same on full auto or single.  Again the heating is the limiting factor. 

Real machine guns have a higher sustained rate due to belt-reed and quick change barrels.  An MG240 will have somthing more like 200 RPM sustained rate.

TenX out

Actually, the M16 does indeed empty the magazine in bout 2.5 secs. Been there, done that. Just because Hollywood does depict it doesn't mean its not true. Of course, if Rambo had to reload every 2~3 seconds he wouldn't look quite as cool.

However, even with weapons not equipped with the burst fire rachet assembly, the operator will typically squeeze off 2~5 rounds at a time in auto. This is because that is the only way to shoot the weapon with some measure of accuracy. That is, you depress the trigger for 1/4~1/2 a second at a time.