For the engine yes. The guns and ammo were considered part of the payload so no there is no rule of thumb for that.
Rules of thumb would Break the gross weight into engine/power plant, structure (sometimes even wing and fuselage), sometimes landing gear (around 5%) and so on.
But since you can swap guns/ammo for fuel or increased electronics (night fighters) that made no difference to the basic aircraft size/structure (within reason).
It also helps to keep aircraft in certain classifications, obviously a fighter will use a higher proportion of it's weight for propulsion that a single engine transport will even though their landing gear weights might be close (assuming similar gross weights). Likewise a plane expected to pull 6-9 Gs on a regular basis will need heavier structure than a plane limited to under 4 Gs.
For a fighter the propulsion system would be in the high 20% to low/mid 30% of the gross weight.