It's all about use and needs. If you want 130mm - then your needs are primarily being able to deliver indirect fire on a far away, likely static, target. Yet you're still able to hit lightly armed ships at medium distances, if the tactical situation warrants it. And you still have some semblance of anti missile capability (with appropriate ammo) at short distances.
If you want 100mm - then you probably still want the above, though you want it on a smaller platform that can't easily house a 130mm gun, and are thus okay with weaker range/boom performance. You may enjoy somewhat better anti missile performance and perhaps more cost efficient anti-lightly-armed-ship performance.
If you want 76mm then you likely want a multipurpose gun - something that can engage both missile targets well, and still be decent and cost effective against lightly-armed-ships. Your coastal bombardment capability will suffer greatly, mostly due to very poor range that will expose you to the enemy. But it may be acceptable in some situation where threat level from the coast is extremely low.
If you want 57mm gun then you really want anti-missile capability first and foremost. While still retaining some basic capability of engaging small boats with virtually no weapons on the cheap. You can more or less forget about coastal bombardment unless the opponent is basically without proper weapons to strike back.
So... tonnage has some to do with it, but planned use has more to do with it.
If you want 100mm - then you probably still want the above, though you want it on a smaller platform that can't easily house a 130mm gun, and are thus okay with weaker range/boom performance. You may enjoy somewhat better anti missile performance and perhaps more cost efficient anti-lightly-armed-ship performance.
If you want 76mm then you likely want a multipurpose gun - something that can engage both missile targets well, and still be decent and cost effective against lightly-armed-ships. Your coastal bombardment capability will suffer greatly, mostly due to very poor range that will expose you to the enemy. But it may be acceptable in some situation where threat level from the coast is extremely low.
If you want 57mm gun then you really want anti-missile capability first and foremost. While still retaining some basic capability of engaging small boats with virtually no weapons on the cheap. You can more or less forget about coastal bombardment unless the opponent is basically without proper weapons to strike back.
So... tonnage has some to do with it, but planned use has more to do with it.
