I’m not sure. The SR71 got real hot after flying Mach 3+ for long durations. Not sure how quick friction would work its magic on something this size (I’m a software engineer, not an aero or thermodynamics guy). The round wouldn’t be in transit that long, and the friction effect would depend on velocity, surface area, air density, etc. Anyway, I’m betting the slug isn’t a run of the mill material.
Depleted Uranium I bet.