Choosing a river gunboat's armament

What determined a mid-twentieth-century river gunboat's armament, and why choose artillery over automatic weapons?

The mission determined the weapons: artillery supplied high explosive and ranged support, while river combat and amphibious operations could require a broader armament.

A river gunboat's intended work determines its armament. An artillery piece provides high-explosive effect and a ranged attack. Mobile artillery support, fighting other vessels on a river, and supporting amphibious operations place different demands on the same hull. A vessel expected to perform more of these tasks needs a wider selection of weapons. A large gun can also contribute to coercion without being fired.