Using modern day technology, how could I create a gun (regardless of firing mechanism and ignore space constraints) that could fire a radioactive bullet (maximum size capped at a cubic meters) which would produce a mushroom cloud (must be visible to naked eyes from an aircraft at 15,000m altitudes) upon impact? In short I need the smallest tactical nuclear sniper rifle in the world! (Battery not included but must support remote control function)

This question is inspired by a man who worked on Manhattan project, Louis Stolin. He discovered how to stabilize a plutonium bomb with a screwdriver, he adjusted the reflectors so that the plutonium never exceed subcritical mass. Unfortunately his hand slipped and the rest is history... let's continue his legacy with the smallest nuclear weapon ever existed.