a powerful spring or something

10⁵ cm/sec is over 2000 mph; that's a powerful spring.

Richard Rhodes described a number of improvements to the gun design used in the atomic bomb. A 21-foot barrel was already too long to fit in a B-29, and facing smaller guns at each other raised difficult timing challenges.

High muzzle velocity was essential: "Typically the chain reaction takes less than 1 μs (100 shakes), during which time the bullet travels only 0.3 mm."

In April 1943 Seth Neddermeyer was attending ordinance discussions at Los Alamos when he hit on the spherical implosion design. This would prove an exquisitely difficult engineering challenge, but the war provided urgency. Another engineer had a more prosaic insight: the five-ton Army gun under consideration was sturdy enough to withstand multiple firings; a bomb gun would be vaporized on first use and could be flimsier and lighter. But it was still very long, hence the design was nicknamed Thin Man.

Later that year Emilio Segrè made the final essential contribution to a portable gun design, measuring rates of spontaneous uranium fission at the secluded Pond Cabin, "one of the most picturesque settings one could dream of." He found that the rate of spontaneous fission in U235 was higher at elevated Los Alamos than at sea-level Berkeley. Cosmic rays caused the higher rate of fission, threatening to detonate the critical assembly early, resulting in a fizzle.

Cosmic ray shielding reduced the minimum muzzle velocity, allowing use of a smaller gun, and Little Boy was born.