Lets start with the Maschinengewehr 34
MG-34 early 800-900 RPM
Standard MG-34 600-1000 RPM selectable on pistol grip
MG-34"S" 1500 RPM
MG-34/41 1200 RPM
So the way it works for the MG-34 is add a MG-34 progression tree after unlocking the standard MG-34.
Now for the Maschinengewehr 42:
Depending on the bolt it varied from 900-1550 RPM
So add unlockable bolts for the MG-42
Now for the Maschinengewehr 45:
1350-1800 RPM! Make it happen darkflow! Add unlockable bolts!