Vladislav Lobaev, the founder and chief engineer of Lobaev Arms is researching hypersonic ammunition and with funding could create a precision cartridge that will have a muzzle velocity of 2,000 m/s (about 6,560 fps) which is about mach 5.8.
This gun would use special chemicals to reach the higher speeds.
1930s US tested a mach 8 small gun
The .17 Remington Fireball has 4000 fps. It was created in 2007 as a response to many popular wildcat cartridges, the .17 Remington fireball factory loads drive a 20-grain bullet up to 4,000 feet per second. While its velocity is approaching that of the .17 Remington, it does so with less powder, meaning less barrel fouling and lower heat, which in turn leads to longer barrel life.