I think all talk of Mk19 rounds is all theortical, since I don't know where you could possibly find rounds for it.
The Mk19 out there is basically a massive Penis-Enlarger that you would be unable to shoot.
But I have often wondered if there is any affluent people out there who created their own 40mm HE rounds (either legally as a DD Manufacturer or payed a $200 tax for one BOOM) but I'm sure some Bubba made something for it (and possibly lived to tell about it).