I'll say, my example has the same effect if you do it with vec_for_screen. (It's just a bit better to edit and adjust for your game, if you use my example ^^)
And I can't give you an example for vec_for_screen because I didn't work with it before, I just know, what it does ^^
Btw. there is no really good solution for your problem, if you want, the bullet makes damage by hitting the target.
You can also calculate, if a Entity is in your eyepiece or not, but then, you wouldn't see any shots.

If you want to make an 3rd camera shooter, I don't see another way as my example. Sorry...