Appears to be a shortened military musket, but .90 is not normal - however it is normal for a shortened shoothbore gun for waterfowl. Francis Selous used a (or more) shortened Dutch 4 bore duck/goose gun as his elephant gun for a number of years. He wrote he wished he'd never done that as the recoil developed in him a flinch he could not eliminate.

A 6 1/2 bore would do nicely as well, I'm sure, given the normal ranges in dense bush.