Why choose Canadian and British guns already in service?

Why prefer a 57 mm gun for a Canadian patrol vessel and a 40 mm gun for a British one instead of adopting a different weapon such as the 76 mm?

Because those choices match systems already used or being bought by each navy, reducing separate training and logistics burdens.

Canada already uses the 57 mm gun on its Halifax-class frigates, which makes it a practical alternative to the suggested OTO Melara 76 mm. Britain can apply the same logic by fitting River-class patrol vessels with the 40 mm weapon being bought for Type 31 frigates. Patrol ships should avoid custom systems that require separate training, spares, and maintenance arrangements. Fleet supportability matters more here than choosing the most powerful available mount.