Luckily, most don't know how to effectively use the 40mm Bofors when engaging other PT boats, and to a lesser extent, airplanes.
I consider it the only effetive gun onboard. The AAMG's just don't have the firepower to stop a plane before it easily sinks the fragile Elco.
In a PT vs PT engagement, the 40mm can be used to land shots with relative impunity. As the opposing PT comes into view, do a full 180, placing the stern of the boat toward the enemy, while still keeping underway. Using the first ring inside the 40mm sight, the gun will hit out to 3k easily.
The beauty of it is, they cannot use their own 40mm as you are still moving, and they have to follow to stay in range. The 37mm has no sight, and little ammo so is usually easy to avoid it's fire. The other AAMG's won't reach as far either.
Engage planes using the same method. Turn the stern towards them, and start firing at 3k adjusting aim as needed. Usually they will fly straight towards the PT stern, making them easy to down before they get within 1500 yards.