The ones that I have taken apart and/or worked on appeared to be a mixture of:
MIM.
Milled steel.
Stamped steel.
Investment casting.
If you include the barrels and frames, you can add die-cast.
The 'standard' (small) model, in particular, had more MIM or cast parts (pretty much everything but the pins).
All of the above were of recent production (2014 or newer). I have no idea how the older versions compare.