How boarders could get through a defended gunport

Would boarding through narrow gunports cause very high casualties?

The answer argues that surprise and a preceding broadside could disrupt the defenders long enough for small armed boarders and then larger men to enter.

The objection is that forcing men through a narrow gunport should expose them to heavy casualties. The reply depends on timing: a broadside immediately before the ships close can leave defenders stunned or injured, while the guns take minutes to reload. Small ship's boys, described here as seven or eight years old, could go through first with pistols to cover the men following. Several aligned gunports could admit boarders at once. Larger men could cross with Marines and use weapons such as halberds to clear and hold the deck.