Basically the upper theoretical limit for a gun, i.e. something which accelerates a projectile through gas pressure, is limited to the speed of sound in the pushing gas.
Hypervelocity guns achieve very high velocities by using a two stages system were stage one is a classical gunpowder gun which fires a large massive projectile in a large bore barrel which is filled with hydrogen or helium, then using the energy of that projectile to compress that second-stage gas and pushing it through a smaller bore barrel for the real projectile.
As the speed of sound in those gases are >3 times that of regular air, it's possible to get velocities far faster that the speed of sound in air.
Reductionist and proud of it.
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