I believe the LQMT projectile has been tested to withstand Mach 7.
In 2010, the United States Navy tested a BAE Systems-designed compact-sized railgun for ship emplacement that accelerated a 3.2 kg (7 pound) projectile to hypersonic velocities of approximately 2.4 kilometres per second (8,600 km/h), or about Mach 7, with 33-megajoules of kinetic energy.