DTIC and NTIS have the following reports on file for the Caliber .30 M76 cartridge, along with the Silent Weapon System - Alpha mentioned in Issue 406.
SILENT WEAPON SYSTEM CARTRIDGE DESIGN AND DEVELOPMENT. PHASE I: DESIGN (This one can actually be downloaded as a .pdf.)
The same article in Issue 406, along with another in Issue 409, mentions the underwater silent cartridges designed by AAI’s Irwin Barr. These were actually fielded by US Navy SEALs according to SEAL historian Kevin Dockery. The design of the Underwater Defense Gun Mk 1 Mod 0 is not that of the patent shown in Issue 409, but is covered by another patent. The piston cartridge was reportedly named Projectile, Underwater Gun Mk 59 Mod 0.