Both are tracked vehicles. An assault gun is usually a turret less vehicle, mounting a (usually) high velocity gun used in a direct fire mode, used for attacking an enemy position.
A self propelled gun is usually an artillery piece (indirect fire), and is tracked to allow self propulsion.
For the purposes of automobiles, nothing
self propelled reaper
Lunokhod-1 was the first self-propelled vehicle on the moon.
An aircraft with an engine. A non-self propelled aircraft would be a balloon or a glider.
What is the difference between a self service and a self selection
How to Change the Tires on Self-Propelled lawn Mower
How do you change the blade on a Toro 21" self-propelled mower
Dyson makes several different models of self-propelled vacuums.
A rocket.
The modern definition of a torpedo is a self-propelled underwater missile. (see related question)
There were 4 wheels on the first self propelled vehicle, built by Henry Ford.
A warship is a ship for fighting. A ship is any self-propelled floating vessel.