Here’s what HWS Vol III says in Chapter 6, including Gene Scranton’s drawing:
"On 13 January 1960, the Ordnance Technical Committee assigned the designation XM115 to a 7.62mm ball cartridge having a pressure-sustaining propelling system to reduce noise and weapon firing signature. This round was part of Project Whisper and was being developed as a companion cartridge to the Cal. .30 XM76 covered in Chapter 5. The XM115 had a total cartridge weight of 335 grs. and propelled a 110-gr. steel bullet to a velocity of 800 fps. Like the XM76, the development program for the XM115 was originally classified Secret. Unfortunately, the cartridge assembly drawing (FB 53140 dated Jan. 28, 1960) was destroyed when Frankford Arsenal closed, but the component drawings survived and were used to draw Fig. 255.
FIG. 255. 7.62mm Ball Cartridge XM115 (reconstructed from Frankford Arsenal Component Dwgs. FB 53141, 53148, 53201 and 53214). Length, 2.726"
The development of the XM115 cartridge was terminated in August 1960 and it is possible that only component metal parts were actually fabricated, since there is no record of test firings."
Progress on the book continues steadily, if slowly. I’ll have draft copies of the first few chapters at SLICS.