Your cartridge is in fact an MBA .30-caliber Gyrojet rocket, recovered after being fired. It was one of a series of “Delay-Fuse Gyrojets” designed to be dropped in large groups from aircraft flying over the jungles of Vietnam. As they were being ejected in canisters from the aircraft, a sheet igniter lit long pyrotechnic delay fuses on the back of the rockets as they fell to the ground, where the delay fuses continued to burn toward the rocket’s base with exposed nozzle ports, through which the rocket propellant was ignited after hours or even days. Then the rocket fired with a bang in a more or less horizontal trajectory, depending on how it was laying on the ground. The idea was to surprise and kill enemy troops long after the aircraft was gone. When a group of these fired, it sounded like a fire fight. The middle hole in the base was not for a primer: It held a small nail which secured the delay fuse to the rocket. They actually worked, sort of, and passed the Army’s test. But they were not deployed operationally. All the variations of these are covered in my Gyrojet book in Chapter 9. Here are two pics of your rocket before it was fired, one without the delay fuse and the other with it, protected by a fiberglass covering.