Right as Frag Machine says, its dependent on the BSP structure q1bsp uses in regard to entity collision on BSP. Entity on entity collisions seems that it can be done by checkind the mins and maxs of each entity to see if they overlap each others bounds, but Im not sure if thats exactly how the engine does it.
I always pondered that it ought to be possible to do collisions model to model without "hitboxes" if you mapped each pixel out with a center reference says its origin point with regard to the model, then you should be able to calculate collision by comparing each specific coordinate to detect a collision, and map out each frame of the moel the same way - thus overcoming the concept of a hitbox completely. Would be time consuming and resource heavy yea, but on the newer technology cpu's out there might not be as demanding if its coded right.
As with the way it is now, the issue becomes more complex with moving entities and I suppose would be very demanding when detecting if a point sized entity collided with another point size entity. In actuality you ought to be able to see rocket to rocket mid air collisions in vanilla Quake but in all the years I have been playing have not see any and by now ought to have seen at least one
