A volley gun is a gun with several barrels for firing a number of shots, either simultaneously or in sequence. They differ from modern machine guns in that they lack automatic loading and automatic fire and are limited by the number of barrels bundled together. In practice the large ones were not particularly more useful than a cannon firing canister shot or grapeshot. Since they were still mounted on a carriage, they could be as hard to aim and move around as a cannon, and the many barrels took as long or longer to reload.
They also tended to be relatively expensive since they were more complex than a cannon, due to all the barrels and ignition fuses, and each barrel had to be individually maintained and cleaned.