Confirmation of the theorists' contention that the number of neutrons emitted per fission was the same for both fast and slow neutrons took longer, and was not completed until the autumn of 1944. John von Neumann visited the Los Alamos Laboratory in September 1943 and participated in discussions of the damage that an atomic bomb would do. He explained that while the damage done by a small explosion was proportional to the impulse (the average pressure of the explosion times its duration), the damage from large explosions such as an atomic bomb would be determined by the peak pressure, which depends on the cube root of its energy. Bethe then calculated that a explosion would result in an overpressure of at , and therefore result in severe damage within that radius. Von Neumann also suggested that, because pressure increases when shock waves bounce off solid objects, the damage could be increased if the bomb was detonated at an altitude comparable to the damage radius, approximately . Parsons was appointed the head of Ordnance and Engineering Division in June 1943 on the recommendation of Bush and Conant. To staff the division, Tolman, who acted as a coordinator of the gun development effort, brought in John Streib, Charles Critchfield and Seth Neddermeyer from the National Bureau of Standards. The division was initially organized into five groups, with original group leaders being McMillan of the E-1 (Proving Ground) Group, Kenneth Bainbridge of the E-2 (Instrumentation) Group, Robert Brode of the E-3 (Fuse Development) Group, Critchfield of the E-4 (Projectile, Target, and Source) Group and Neddermeyer of the E-5 (Implosion) Group. Two more groups were added in the autumn of 1943, the E-7 (Delivery) Group under Norman Ramsey and the E-8 (Interior Ballistics) Group under Joseph O. Hirschfelder. A proving ground was established at the Anchor Ranch. The gun would be an unusual one, and it had to be designed in the absence of crucial data about the critical mass. The design criteria were that the gun would have a muzzle velocity of ; that the tube would weigh only instead of the conventional for a tube with that energy; that, as a consequence it would be made of alloyed steel; that it should have a maximum breech pressure of ; and that it should have three independent primers. Because it would need to be fired only once, the barrel could be made lighter than the conventional gun.