I looks like this is indeed from the Bushmaster program (or more accurately era). I tracked down the ECCC article by R. Gerber, which was referenced in the ECDV entry for the rimless version of this round. Here’s a summary:
During the development of the VRFWS/Bushmaster gun, there were concerns that the 25-30mm cartridges under consideration wouldn’t have sufficient range and terminal effectiveness. As a result, this 40mm cartridge was created in-house at Picatinny Arsenal as a potential successor.
The intent was to use a rocket-assisted projectile. Solid aluminum projectiles, matching combat projectiles in weight and configuration, were loaded for testing. Other projectiles were designed but apparently never manufactured.
The cartridge was eventually deemed to large and bulky for the intended use, and it was abandoned. It was never given an official model number.
Apparently there were three case types created during the program: rimless (as shown in ECDV), rimmed (same dimensions otherwise, as shown above), and a shorter bottle necked case.