4500 mph? That’s only Mach 6. I once worked on missiles that were faster than that. I think someone got the wrong information.
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That is the “un-classified” speed. Research papers say 7000 MPH.
Yes at 125 miles the slug will be less speed than it left but not much. It will go through a “Lot” of concrete and kill a lot of people inside a concrete building. It will tear right through most armor because of the speed which is a force multiplier. There is no armored vehicle that can be protected from that kind of speed.
The speed is something like a hundred miles a minute. I would not want to be on the receiving end.
Good Old 1/2 MV^2 to the rescue. That squaring of the velocity is a real “killer”.