But both have tracks and require a driver right?
I think Griffith is pretty much correct. I’ll try to elaborate further. Think of ammunition categories, e.g. mortars, rockets, grenades, recoilless, projectiles, etc. These are also broken down further into subcategories, e.g. HE (high explosive), HEAT (high explosive anti-tank, meaning it has a shaped charge), Carrier (meaning smoke, illumination, cluster bombs, etc.).
There are important differences regarding how these munitions reach their target. For instance, grenades may be hand-thrown or propelled/projected from a launcher. Mortars go down a mortar barrel and have a percussion cap, spigot holes and charges filled with black powder which dictate how far it travels depending on the amount of charges you attach.
Rockets use a venturi exhaust system to create a constriction of the gases being burned as the powder is ignited, which gives a very long-range capability. Projectiles (shells) can have their propellant inside the casing or use separate, bagged propellant charges, and are usually fired out of a rifled barrel. When we talk of propellant inside the ammunition we are talking about low-explosives like black powder/flash powder that propel it to its target once it is ignited.
With regard to a 'self-propelled gun', we are talking about how the weapons system itself moves to its location (tank tracks/tyres). Typically, mortars (81 mm or 82 mm) are carried to location and then bedded in to provide indirect fire support. Larger mortars and field guns which fire shells or rockets need to be towed to location and then also remain there.
If, however, they are built into self-propelled guns, they remain mobile, easy to maneuver, and can provide indirect fire-support using a variety of different ammunition types (mortars/rockets/projectiles). It’s quite a broad term, as opposed to a tank. A tank is typically more heavily armored, has a turret, fires projectiles, is heavier, and is used in a direct fire capability. Self-propelled guns may often look like tanks but they differ in the ways I’ve tried to articulate above.
