Pissed old hack baffled by (not very) new technology
Mach 7.5 sounds a lot more impressive than 2400 metres per second, which is what this is.
It's worth noting that modern tank guns firing APFSHS-DU penetrators routinely achieve 1600 metres per second, that tweaked APFSHS-DU has tested out to 1900 metres per second, and that it's generally agreed that it should be possible to push conventional explosive-driven projectiles to 2000 metres per second in the next generation of guns. So what they've got here is maybe a 15-20% improvement over where the state of the art is with explosive-driven guns at the same level of development.
Meanwhile, did you notice the sparks flying from the underside of the railgun? Serious arcing -- always a problem when you're throwing millions of joules around in under a millisecond -- tends to wreck railguns. And you get arcing when you mix that kind of current with air. Especially damp, salt-laden air (hello, paging the Navy: you are aware that your ships sail on top of seawater which is (a) conductive and (b) tends to splash everywhere? There's a reason naval guns on real warships come with protective caps which are only removed just before firing ...)
Railgun technology isn't new; it's been a hangar queen ever since the Nazis scratched their heads over it in the 1940s. Now DARPA have got it to work about as well as the conventional alternative, in a demo. Nothing to see here, move along ...