A particle canon that can accelerate a gas with good directional control (that can be aimed and focused) with sufficient energy to cause damage to an object 5m away will probably be better applied as deep space propulsion than as a weapon.

It's a bit like a space invasion - if you plan on invading another planet, just accelerate for the first part of the trip, point the engine to the destination and fire it for the rest of the trip. When you arrive, there will probably be no armies, of lifeforms, to resist you.

OTOH, you may need to wait a couple million years before the planet is cool, or inhabitable again.