"What is the 42 caliber to which they refer?"
That had me wondering, too.
The cases for the grenade launcher aren't brass, they're aluminum, so I don't think that's it.
The only other 40mm I know of would be the 40mm Bofors, and I think that they're all out of service now.
At one time the military collected every piece of spent brass and sent it back to Frankford Arsenal for reloading for practice. Made tight military budgets go farther.