Well, I was talking about the NFA definition of a machinegun--that is, any weapon capable of firing more than one round of fixed ammuntion with one pull of the trigger. I agree, it is not a heavy or even a light machine gun by military definition--it is what the true definition of an assault rifle is--a submachinegun capable of select fire of a smaller cartridge. There's a lot of definitions to keep up with in terms of the legislation.
The AR-15 wasn't a predecessor--but the problem is that some of the AR-15s made in the 1970s were select fire--thus making them the exact same as an M-16. Many national guard rifles used in Vietnam were that way. The M-16 has always been select fire. The AR-15 is now, and usually was then, a semi-auto only. However, some of the ARs produced for the national guard were select fire. As I understand it, it was a different model number, and there were some small differences in parts, such as a moly versus a chromium barrel coating, and a heavier barrel. I'm not for sure on this, though.
Thanks for the comments on the guns. The mauser is a gas to shoot. It was made in 1944 in Germany. Pretty amazing piece of history. The really amazing thing about the gun is the shape it's in--something hard to tell from the crappy picture I have of it. The whole thing is nearly packed in grease new--the bolt is incredibly tight and there is no sloppiness at all. The gun is devastatingly accurate, too.
Yeah, if you include birdshot, then there's a lot under .22 caliber. The .17 Remington and .17 HMR are the most common uner .22 in terms of rifles, though. I saw the world's smallest gun at a gun show once, though--a 1.7mm cartridge. Total crackup.
The .50 BMG is just an incredible round. Think of this comparison: the muzzle energy from a .44 magnum is around 1400 ft.-lbs. with a really hot load. The .50 BMG has 14,000 ft.-lbs. Depending on the type of bullet, it may very well have gone through an engine block, especially if it were made from aluminum and from a small car. It's not going to to through a diesel engine or anything, but it is still an incredibly powerful round. Somewhere on the internet someone posted a video of someone shooting a running lawnmower with a .50 BMG--the lawnmower lurched, and then it had a hole in it the size of a basketball. Kind of amusing, really.