Although the problem of producing a gun weapon seemed relatively simple as the laboratory entered the summer of 1943, William Parsons and Charles Critchfield soon recognized that the guns needed to assemble uranium and plutonium would be very different from the ordinary ones that shot high-explosive projectiles at targets. The Ordnance and Engineering Division (E) did not know enough about the critical masses of plutonium and uranium, their metallurgical properties, or the speed required by nuclear constraints to assemble an effective plutonium weapon. Three points were clear: the guns would have to be designed from scratch – standard ordnance cannon could not be adapted to this use; extensive testing of both the plutonium and uranium guns and their mockup target and projectile components would be required; and targets and projectiles of active material would have to be designed. No one had yet tried to create an explosion using two or more pieces of fissionable material. Work on the gun gadget proceeded along three paths – interior ballistics research, experimental testing of designs, and target-projectile-initiator development. Each proceeded more or less independently until the reorganization of the laboratory in August 1944 consolidated all gun work.

Between April 1943 and August 1944, Parsons, Critchfield, Edwin McMillan, and Joseph Hirschfelder designed, created, and tested all the principal components of the gun gadget, including the gun mechanism, target and projectile geometries, and initiators.