That depends, doesn't it? I mean, a purpose-built drone is going to be fairly successful at hitting a human sized target from a respectable distance. Of course, the drone is using guided munitions (which would be reflected in the weapon quality) and is travelling relatively slowly.
The main gun on a main battle tank, on the other hand, is designed to hit other tanks and would be completely unwieldy against a person, unless you were just trying to annihilate a building and everyone in it. That same tank might have a machine gun mounted on it that is designed for infantry types, and would be much more successful at hitting individuals.
Battleships (like the now decommissioned U.S.S. Missouri and U.S.S. Iowa) used huge guns to barrage an area, but when they needed a specific target destroyed, they could fire cruise missiles. Carriers have point-defense weapons for use against anti-ship missiles, but they're going to farm out all of their "attack" jobs to fighters, which may or may not be better equipped to attack a person sized target depending on their loadout and the type of aircraft.
All that to say: I doubt a Star Destroyer (the Battleship-Carriers of Star Wars) is going to drop into the upper atmosphere and use its turbolasers to bump off a single individual. The Star Destroyer is purpose built for orbital bombardment, which is going to destroy everything in a huge area and/or ship-to-ship combat. The quad-lasers on a YT-1300 (or similar) are probably more analogous to a couple of .50 cals mounted on the side of a helicopter or in the turrets of a B-29.
Imo opinion you are confusing what is a gun on a weapon system designed to destroy with can it hit a target. The 120 mm smoothbore is designed to destroy other tanks. It is fully capable of hitting a man sized target. Sitting on Bradley ranges with the 30mm chain guns. They are capable of easily hitting man sized targets. The principle difference between a weapon in a tank turret as opposed to a crew served weapon is the turreted weapon is a vastly more stable firing platform with a vastly better gun sight.
You don't miss with those weapons and if someone is approaching a perimeter with them you stay out of the arc of fire.
In reagrds to a shell from an old 16" battleship gun, they have a ridiculous kill radius invluding blast and frgamentary effects of up to several hundred meters. No need for too much accuracy....