Plot hole: The huge guns are set high up on a cliff face facing out to sea and it is obvious that they cannot be depressed to fire at a downward angle - the massive gun carriages set on rails would prevent that happening. They cannot be elevated to fire at an upward angle, too, because they fit pretty snugly in the hole cut into the cliff face to accommodate them. This means that their maximum and minimum ranges would be quite close together, covering a strip of maybe a few hundred metres either side. Given that the sea is completely open on the side of the island they are protecting, why don't the ships targeted by the guns while passing the island simply sail inside or outside of the narrow stretch of sea the guns can hit?
Suggested correction: The guns are firing across a strait. A strait is a "narrow passage." Since the targets must appear at a limited range, the guns only need a limited elevation angle.
Noman ★
Watch the film again. The guns are facing the open sea. There is no land visible anywhere behind the ships. If that's a strait, it's a very, very wide one.
The mission given says the guns are guarding a strait. The last shot of the six destroyers shows land behind them on the opposite side of the strait.
Noman ★
Other summaries explain that the strait is only deep enough for the ships at the place which matches the guns' range. So ships could not take advantage of further away or closer in.
Then what are the dark shapes rising out of the sea on the far side of the ships. If they are not islands, what are they?
Noman ★
Suggested correction: That the gun carriages are supposedly set on rails is not correct. In the novel template, as well as in the film, it is shown that the guns were installed on turntables. And as for their variable angles of fire - it could be due to (fictional) modifications.
Daniel4646