Where do you draw the line between what is and isn't a firearm?
Hmm, tough one. I tend to go with the line that if it isn't a personal weapon (as opposed to crew-served), it's not "arms" per the 2nd. In addition, I'd compare it to the standard infantry weapon of the time (which would leave out rocket launchers (till we start issuing rocket launchers as our standard infantry weapon, at least)). Note an exception for a Gyrojet, which is a rocket launcher, but shoots a bullet-sized rocket out of a pistol-shaped rocket launcher.
Does the 2nd Amendment allow (in your mind at least) a citizen to have a rocket launcher or a laser gun?
Soon as the Army starts issuing them as standard service rifles, sure. Well, realistically, they probably won't be available on the civilian market for 10-20 years after the Army starts issuing them, but if every grunt gets one, no reason I shouldn't be able to buy one (assuming I have the funds).
Note, by the by, that I don't expect the Army to EVER issue either as a standard service rifle. A rocket launcher is a wonderful way to say "here I am!!" to everyone in line of sight, so a very bad idea. A laser rifle, if it were appreciably better than a modern rifle would have an ammo pack that looked a lot like a nuclear reactor, or be a single-digit-shot weapon, neither of which the military has all that much use for.
What are you going to do when the technology of simple side arms develops to the point where you an take out a room full of people by pressing a trigger and letting you gun do all the aiming etc..?
Quit shooting for sport. There's no skill involved in pressing a button and letting the gun do the work.
Will it bother me that someone might commit a massacre with such a gun? Not especially - a soldier with a REAL assault rifle (as opposed to the EVVVIIIIIILLL "assault weapon") can do pretty much that now - that's what full-auto is all about.
Note, by the by, that you can do this with a bomb today (and have been able to do so with a bomb for about a century and a half (percussion cap meant no more slow fuses on your bombs)), so it's not like the world is going to be terribly more dangerous then than now.
Would genuinely like to hear from a pro gun NRA type.
Okay, you've heard from an NRA member.
Now, tell me how you'll feel about all those possibilities being true, but only the government gets to use them....