Do armed merchants have limited ammo?

[Wreford-Brown]

I've been stalking a split merchant for over an hour in-game and she's been firing at me for almost all of that hour, but seems to have unlimited ammo. I'm over 4km away in rolling seas (my depth keeps moving between 4 and 11m) so the chances of me being hit are minimal (I think her guns are 20mm), but will she eventually run out of ammunition to let me get closer to put an eel in her or am I going to have to go to periscope depth to avoid her fire?