are not the familiar three sided shape seen in Dardick's production firearms. They are, rather, cylindrical on one side (the side facing the cylinder axis) and nearly flat on the other (the side facing the frame). This patent defined the open-chamber gun concept, however, and quickly led to the development of the tround. Triangular cartridge theory During the 1950s U.S. military experiments with feeding devices for machine guns found that triangular cartridge cases would stack into 50 percent less space than conventional round cartridge casings. Due to problems in reliably feeding a triangular cartridge from a magazine into a triangular chamber in a gun barrel, the concept was not further pursued. Tround development Dardick's patent 2,865,126, issued in late 1958, is for a new and improved cartridge for the open-chamber gun. Unlike the previous open-chamber cartridge, this version shows the symmetrical tround shape, which would facilitate feeding from a magazine, since the trounds can enter the split in any orientation and end up seating correctly. This reduced the complexity and cost of the open-chamber firearm design, making it feasible for commercial production. The trounds were constructed of either high-strength plastic or aluminum to provide strength. While trounds were only commercially available as cartridges suitable for handguns, prototype military trounds were made in larger calibers. Tround adapters While trounds could be complete firearms cartridges, many trounds were made as chamber adapters which held a standard cartridge, such as a .38 Special. This allowed readily available ammunition to be loaded into the trounds and fired from Dardick's guns. The .22 caliber tround held standard rimfire ammunition, and the guns that could fire it were equipped with a selectable firing pin to handle centerfire trounds or trounds loaded with rimfire cartridges. SALVO trounds Dardick's patent 3,855,931, issued in 1974, expands the tround to hold multiple projectiles, firing through a set of barrels (one per projectile), making a Project SALVO–type gun. This type of tround, holding three projectiles, was used in the H & R Firearms SPIW prototype. However it was rejected due to issues with weight (due to the three barrels) and strength of the tround when used with the high-velocity flechette ammunition. Tround Terra-drill The Tround Terra-drill is described in patent 4,004,642, issued to Dardick in 1977. The Terra-drill is a drill head that combines a traditional drill bit with the SALVO tround. The tround is used to break through hard spots