Today’s invention is a new form of non-lethal baton round (or rubber bullet).
Rounds are each supplied in a cannister with a near vacuum ahead of the bullet. The weapon makes a hole in the rear face of the cannister, so that air rushes in and propels the projectile towards the front of the cannister, piercing it on exit towards the target.
My calculations show that even if some energy is lost in breaking out of the cannister and due to subsequent air friction, a standard sized baton round, fired from a 1m long gun, can deliver bullet energy of about 100J.
This is about 1/3 of the energy of a standard round (making it much safer) but requiring no chemical propellant.
In order to ensure that the energy from the air pressure is not all wasted in breaking the cannister open, I can imagine a way to make it super resistant to external pressure, whilst easily broken from inside.