The lesson of Project SALVO was that nothing works quite as well as a conventional rifle, chambered for a conventional cartridge, firing one bullet at a time. But the lesson was not learned so quickly because following the SALVO II trials in December 1957, Operations Research Office (OCO) recommended the development of a standard length duplex cartridge based on the 7.62mm NATO case.
The cartridge showing the most promise was the T314, loaded with two copper-plated steel bullets in tandem. But, a U.S.Army Infantry Board report released in 1959 concluded that the T314 was not suitable for Army use and did not offer a substantial combat advantage over the standard Ball cartridge (7.62mm NATO M59). Unconvinced, the Army continued development of the cartridge through at least two more iterations culminating in the T314E3. In 1964 it was standardized as 7.62MM BALL-DUPLEX M198.
The M198 entered full scale production with the USMC receiving 4 million rounds for additional field testing. But, as the previous Infantry Board trials had concluded, the cartridge proved to be a disappointment. It did not improve single shot hit probability to the extent hoped for, which after all, was the goal of the duplex concept. There have been reports that some units in Viet Nam were equipped with M198 but, for the most part, production was halted and the entire program abandoned in the early 1970s. Reclassified as Obsolete (OBS), no cartridges remain in military stockpiles.
Loose T314 and M198 cartridges can still be found by collectors, but full boxes are becoming very scarce, sometimes fetching more than $100. So, get them while you can.