MIL STD- Human Engineering Design Criteria for Military Systems, Equipment and Facilities (1992) notice of change

5.4.3 .1.1.1 Use
Push buttons should be used when a controller or an array of controls is needed for momentary contact or for actuating a locking circuit, particularly in high frequency-of-use situations. Push buttons should not be used for discrete control where the functions status is determined exclusively by position of the switch. (i.e., An on-off-push button that is pressed in and retained to turn a circuit on and pressed again to release the push button and turn the circuit off.)

5.4.3 .1.1.2 Shape.
The push button surface should normally be concave (indented) to fit the finger. When this is impractical, the surface shall provide a high degree of frictional resistance to prevent slipping.

5.4.3 .1.1.3 Positive Indication.
A positive indication of control actuation shall be provided (e.g., snap feel, audible click or integral light).

5.4.3.1.1.4 Chanel or cover guard.
A channel or cover guard shall be provided when it is imperative to prevent accidental actuation of the controls. When a cover guard is in the open position, it shall not interfere with operation of the protected device or adjacent controls.