If the United States Ordnance Department learned anything from Project SALVO it was that nothing works quite as well as a conventional rifle, chambered for a conventional cartridge, shooting one bullet at a time. Of all the SALVO cartridges tested, the only ones that proved to be even marginally successful can be counted on one finger.
Following the SALVO II trials in December 1957, Operations Research Office (ORO) recommended the development of a standard length Duplex cartridge based on the 7.62x51 NATO case. By 1958 Frankford Arsenal had assembled a prototype, the FAT314E2. It was loaded with two 77 grain bullets and had an OAL of 2.500". Development continued and in 1963 the FAT314E3 was adopted as the standard, and designated the Cartridge, 7.62MM, NATO, Duplex Ball M198. It was loaded with two 80 grain copper plated steel bullets with an OAL of 2.790".
A cartridge that appears to be an intermediate step in the FAT314 development is known to exist. One from my collection is pictured below. It is loaded with two 88 grain copper plated steel bullets to an overall length of 2.650". Anyone with information on this cartridge is encouraged to share it (the information, that is).
The M198 proved to be a disappointment. In real-life it did not deliver the desired hit probability and production was stopped. By the early 1970s further attempts to increase performance were halted and the concept was abandonded - for a while. It was resurrected a few years later as part of the Advanced Combat Rifle (ACR) program with the same disappointing results. There is no doubt in my mind that the concept is still being considered today. Maybe nothing was learned from Project SALVO after all.