It's ridiculously easy because the deck crew is effectively invulnerable to fire. In reality, even light MG fire could easily sweep a submarine deck clear of men. In SH4, only damage above a threshold does damage, and it weakens the hull. The crew remains intact til they all start dying moments before the sub is sunk.
I'd expect in RL, that almost any AAA hit on a sub should wound crew. I've tested this, and parked next to CVs so that my sub was (should have been, really) SHREDDED by AAA fire, and the crew is fine until the hull% damage sinks the sub.
While the 40mm would be quite effective against ships, it's just too easy in SH4.
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