I think I've found a small omission. On page 300 (Core rulebook), the Antipersonnel Weapon entry states
An antipersonnel weapon must be mounted near the boarding ramp of a Medium or smaller starship.
But on page 296, the discussion of Thrusters says "Large and smaller Starships generally have little difficulty landing on and taking off from a planet with low gravity or standard gravity."
So, I'm inclined to assume Large starships that land on planets also use boarding ramps, and thus they too are entitled to have Antipersonnel Weapons for security. (Or did I miss the part that Large starships are too fancy to use boarding ramps and use tractor beams or large elevator pads that just don't allow undesirables to enter, making Antipersonnel Weapons an unnecessary option. ;-)
Considering that the only tractor beam available is the Graviton Cannon (6d6 ship damage), that makes for a very rough boarding procedure :P
I was hoping that if you had excellent gunnery skills, you could maybe "suck up" cattle without harming them. Umm, asking for a friend, not that I'd ever do something like that.
My guess is, its an oversight. The intent is that all vehicles which can land, can mount an anti-personnel weapon on their boarding ramp. A better question is "Which boarding ramp?" After all, why should even a medium vessel only have one way to get on board?
It seemed reasonable to me to assume the same thing.
The rules don't specify a limit on how many of these anti-personnel weapons can be installed, so one per boarding ramp makes sense--unless some of the ramps have other defenses.