It sounds similar to this:

THE SHARP LIGHT GAS GUN

[4.6] THE SHARP LIGHT GAS GUN
* Even as Bull was working on his Babylon supergun, John W. Hunter of the US Lawrence Livermore National Laboratory (LLNL) was developing a "light gas gun" to test concepts for space launch systems.

The light gas gun was invented in the 1950s for lab studies of hypersonic "reentry vehicles" for intercontinental ballistic missiles, and has been used as a high-speed research tool since that time. The name of the LLNL project was SHARP, for "Super HARP", and it was far bigger than any other light gas gun ever built.

Obtaining high velocities in a cannon requires a gas with a high speed of sound, exerting high pressures on the base of a projectile through a long barrel. The speed of sound squared varies inversely with the molecular weight of the gas and directly with the gas temperature, meaning that a hot gas of low molecular weight such as hydrogen or helium makes an excellent propellant for a space gun.

There are several different schemes for light gas guns, but one of the more common uses a piston to rapidly compress a "pump tube" of hot light gas. The pump tube is sealed off from the "launch tube" by a diaphragm. When the diaphragm breaks, the hot light gas expands rapidly into the launch tube, blasting the projectile out the muzzle. Hunter described the scheme as something like a "giant BB gun".

Most light gas guns built before SHARP were small benchtop devices for testing high-speed aerodynamics, but SHARP was much more ambitious. SHARP was built at Lawrence Livermore and conducted its first test shot in December 1992. It featured a pump-tube system 82 meters long and with a bore of 36 centimeters, connected at a right angle to the launch tube, which was 47 meters long and had a bore of 10 centimeters. Three large counterweights were mounted on railroad cars to absorb the recoil, which occurred in both directions along the pump tube and backwards along the launch tube.

SHARP's pump tube contained a methane-air fuel mixture on one side of a 900 kilogram piston and hydrogen on the other. Igniting the fuel mixture caused the piston to blast down the pump tube and compress the hydrogen to 2,000 atmospheres, ultimately breaking the diaphragm into the launch tube and launching the projectile. SHARP could propel a 5 kilogram projectile to a muzzle velocity of 14,500 KPH.

SHARP was laid out in a horizontal configuration for test firings. The next stage was to relocate the gun to a mountainside and use it for high-altitude shots in preparation for a practical space-launch version, but funding dried up in 1996. At last notice SHARP was still in place at Lawrence Livermore, and was used for occasional test shots.

Hunter then pursued a commercial scheme for a light gas gun, appropriately named the "Jules Verne Launcher (JVL)", for delivering small payloads to orbit. Hunter envisioned the JVL as more than three kilometers long and more than a meter in diameter. Simply scaling up SHARP wouldn't have worked, however, since that would have required a piston ten thousand times bigger and huge counterweights. JVL borrowed the idea behind the V-3 Busy Lizzie gun, gating high-pressure hydrogen from multiple compression chambers at into the launch tube at intervals along its length. It is hard to say if they would have encountered the same sorts of sequencing problems that plagued the V-3 and its brethren, since nobody with the means was seriously interested in funding it.

* Interestingly, a concept for something similar to the JVL had been proposed in 1961 by Jules Gram and Charles Smith of Babcox & Wilcox (B&W), a manufacturer of boiler systems, steam plants, nuclear power facilities, and the like. One suspects that Gram might have got some of the inspiration from his first name.

Gram and Smith envisioned digging a 3,050 meter long gun barrel 6.4 meters in diameter straight down into the top of a tall mountain. A hot-hydrogen chamber 25.9 meters in diameter and 427 meters long was dug below the bottom of the barrel, with the two connected by 70 fast-opening valves, each 1.2 meters in diameter. A spherical steam chamber 95 meters in diameter was dug below the hot-hydrogen chamber in turn, with the two also connected by 70 similar valves.

A network of tunnels and chambers was dug into the mountain to take in natural gas and water and produce hot hydrogen and steam. When the steam reached maximum pressure of about 510 atmospheres at a temperature of 540 degrees Celsius, it was vented into the hydrogen chamber through the valves.

There would be little mixing between the light hydrogen and heavy steam as the hydrogen was compressed, and so no piston was required. When the hydrogen reached maximum pressure of about 320 atmospheres at a temperature of 1,730 degrees Celsius, it was released into the barrel through the valves, blasting a rocket sitting on a sabot baseplate up the barrel and out the top of the mountain.

Foam plastic sabot segments kept the rocket upright in the barrel. The barrel was sealed at the top with a diaphragm, and partially evacuated by filling it with steam that then condensed. The rocket simply punched through the diaphragm on the way out, followed by a blazing pillar of hydrogen and then a huge mushroom cloud of steam.

Gram and Smith estimated cost for their space gun as $270 million USD in contemporary dollars. NASA didn't bite on the proposal and it never happened, either, but it certainly looked spectacular on paper.