Circular connectors falling under the Mil-Spec MIL-DTL-38999 Series 3 are:

Threaded Coupling
A method for coupling a mating connector pair by connecting the interior threads of a plug with the exterior threads of a receptacle.

This design is intended to eliminate the need for lock-wiring the coupling nut in place with tie wire and is achieved via a spring-loaded, corrugated washer.

Triple Start
A coupling mechanism that protects against vibration and makes pairing and unpairing the connectors faster.

A long shell that prevents damage to the exposed contact pins during coupling, making it impossible to damage the pins or electrically short the contacts.

These high-density electrical connectors are provided as environment-resistant connectors with crimp contacts or hermetically sealed connectors with fixed, nonremovable contacts. Operating in temperature ranges of-65°C to +200°C, they provide EMI Shielding, signal/power control, and optical needs in harsh environments.