A 40MM grenade launcher
(yes folks that object exiting the barrel is not a bullet of incredible girth but indeed is an actual grenade, on it’s way to ruin a pirate’s day.)
My favorite feature of the MK19 was not so much the size of the death pill that came out of the muzzle, but the rapidity with which it could empty a 48-round ammo can ;-)
So how fast is it going? Guestimating from the photo, maybe 2cm/.001sec or 20 m/sec ? That’s not very fast. Well, could be 3cm/.0005sec or 60 m/sec. Maybe more like it.