Yes, the MK 19 is a fully automatic 40 mm grenade launcher. It will launch about 6 rounds per second. It's pretty cool. And I am sure that is sucks to be on the receiving end.
I am only interested in an interpretation of the 2nd amendment that could reasonably be upheld in a court of law. When I read the plain clear language of the 2nd, it seems to imply no limit whatsoever. But I am not a lawyer, and smart people like Scalia have clearly stated that the states can impose limits on the RKBA. If he says so I believe him... His opinion counts for a lot, mine doesn't count for squat.
And as I have said several times above, I don't think courts would ever find that a full auto weapon is protected in the same way as a handgun is. I think that an argument could be successfully made that a semi-auto military style firearm (AR-15, AK-47, etc) with 30 round mag IS a protected weapon. I think the courts might accept that. I hope they do.
So when someone asks "should machine guns be allowed under the 2nd amendment", the question is meaningless. Should or should not is irrelevant. Machine guns are NOT currently protected, and that is unlikely to change. And we are wasting energy talking about it. We need to keep our focus on AR-15s and 30 round mags. That is the battle.