Pneumatic guns for shipboard use, or at fixed coastal fortifications, could rely on a steam-driven gas compressor as an air source. For use by troops in the field, this was impractical. The Dudley-Sims dynamite gun used a smokeless powder charge to compress the air. Beneath the gun barrel was a cylinder into which the powder charge was loaded. When fired, the expanding gas from the smokeless powder compressed the air in the cylinder, which was then fed into the gun barrel, accelerating the explosive projectile. The US Army bought sixteen of these guns.