Wait, were you talking about AGS or something else entirely?
Anyway the AGS with its 150km range LRLAP shelll is designed for fire support (fire support as in bombarding shore or inland targets), not long range and ship duties. The AGS uses a conventional, "short range" shell for normal anti ship duty.
Ah... my bad. I am referring to the 155mm canons with the 100km shell that was brought up first by Lion.
Anyway... even if that large naval canon was for bombardment of shore or inland targets... I fail to actually see the effectiveness of that weapon, unless there are quite a number of ships that actually could form an artillery battery by themselves.
Do note that at 150km... or lets take the Chinese 100km 155mm round for example, the CEP is at 40m. Firing 1 shot at a time, it would take plenty of shots to peppered a reasonable sized beach or inland targets. The round is not precise enough to do much harm.
As such... I would think a cruise missile would make more sense than a large naval canon.