If a turtle had a laser gun on its back, its eyes powerful enough to aim properly?

The French tried that in WW II, was called the Maginot Line...
Now we know what a turtle,a blonde and the French have in common.

A turtle would never have an ammunition shortage as he would never run out of shells.

That depends on your turtle.
I'd expect most of them to be nearsighted. Freshwater aquatic environments tend to be cloudy, so there would be no use in having good distance vision anyway.

Probably

I would suggest bunnies instead of turtles. This idea could be the end of ISIS! Get Trump on the phone and sell him your idea

Turtles actually have better eyesight than humans. So, yes.