The close-in defence case for a 40 mm wall of fire

Would 40 mm be a useful minimum calibre for a close-in anti-missile gun system?

He thinks a fragmenting 40 mm system could make a larger interception pattern than 20 or 30 mm, but notes the associated weight, ammunition, and development costs.

Close-in defence requires a dense wall of fire far enough from the ship to keep debris and momentum from carrying through, but close enough that the incoming weapon cannot evade it. Fragmenting 40 mm rounds could create a larger interception pattern than 20 or 30 mm ammunition, at the cost of more weight, storage, and expense. A much larger 152 mm weapon would struggle to train and fire enough rounds in the short engagement window. A proposed swarm of small defensive drones also appears unattractive at the 2020 cost ratio discussed in the stream. These are design arguments about a possible system, rather than claims about a weapon already in service.