I post these in this section only because the Navy called them "grenades". During early 1970, the Navy began an in-house program to replace the 40mm grenade on their small patrol boats. They worked on a scaled up version of the 40mm round. The Military Arms Division of Colt also submitted multiple cartridge designs and four weapons candidates to the Navy. All of the ammunition was 70mm in diameter. There was a caseless hi-lo pressure system, a caseless mono-pressure system and a cased hi-lo pressure system. One of the cased versions is the upper grenade in photo 1. It is 10.5" long with a 2.5" long tapered case. The mono-pressure caseless system was truely a unique design. The propellant chamber was attached to the base of the projectile and crimped shut. When fired the crimped chamber opened. The length of this version was 7.25" and it weighed 4.3 pounds. It carried 4 pounds of explosives. An example is the lower grenade in photo 1. Photo 2 shows the crimped base of the grenade. The program died a quiet death and no prodcution was undertaken on any of the designs.