Quantum Tunneling Neutron Cluster Railgun capable of assassinating a target from the opposite side of the earth at slightly less than the speed of light.
Main problem is nobody made optics that can see to the opposite side of the planet. Personally, I think we should just take the direct route and fire it through the planet but the egghead sciency types keep complaining about "destabilizing the earths crust". Whatever that means.
That's actually not a bad downside, it still pierces literally any armor if you can guess the distance and point it at the target by sight. I'll take 5,000 for $6.4 Million each, send the invoice to Congress.
Imagine taking shots in the dark at the Kremlin and exploding random furniture and windows. Putin: "That's the third time this week, we have to move offices around again."
Aliens already have that technology, and they're determined to prevent any other species from developing it. They zap your prototype and research facility from orbit.