Now I am listening to Jimi Hendrix' Machine Gun, one thing I do not understand is which additional musical effects Jimi Hendrix is using.
Obviously, he was using guitar, but you are probably wondering what guitar effects he was using.
The main effect for "Machine Gun" live was a pedal called a univibe. He also used a FuzzFace-style pedal and a wah-pedal.
The univibe was invented as an attempt to imitate a Leslie Rotating speaker.
In the picture (from Woodstock) below, the pedals are (from right to left): Univibe; univibe expression pedal to control the univibe with his foot; fuzz face distortion; wah pedal.
In the video here, at the very beginning, the univibe effect is very dominant. For the length of this clip he does not appear to use or activate any other pedals, so he is probably just using univibe and fuzz.
The rest is his fingers and the tremolo bar. He gets some really wild sounds by using rapid hammer-ons and the tremolo simultaneously.