Well apparently it is supposed to be based on frame rate. I used to just hold the fire key after the first shot and I would always get the stunlock every time.......that is until a few days ago. As far as I know there is no foolproof way to get the stunlock. I just use one of the older strats which involves timing your second shot just as the ripples in the water disappear.

Shunpuk has a good video guide on this strategy here:

Cheers and good luck!

Out of maybe 30 attempts, doing the animation skip then holding the action button has worked all but twice. Then its just a matter of doing the animation skip again after 3-4 shots and you should be golden. Some people have a lot more trouble though just depending on their computer, it's pretty dumb.

Well you shouldn't be holding the buttons through the cutscene when you can use the animation skip like Big_Jim said. I usually do the skip and then hold the buttons and I've had a 90% success rate.
Honestly, I'm pretty sure its not RNG but you might as well call it that x3