It's my lack of knowledge and correct terminology... but I believe there are plenty of guns which are legal which have extensive rounds. Not 8, 12, 16.... but 50,75, 100, etc. Is this incorrect?
I'm thinking the Aurora shooter, and legal gun he had with 100 rounds.
A machine gun let's you pull the trigger once and hold it, and it keeps firing until you let go. That's "automatic"
A semi-automatic requires a trigger pull for each bullet.
A semi-automatic handgun (one with a magazine in the handle) fires multiple bullets much more quickly than does a revolver because the mechanics of it work a lot faster. Sometimes people get lazy and call a semi-automatic handgun an automatic, but it's not.
For semi-automatic weapons, the size of the magazine governs how many rounds you can shoot without having to stop and reload.
Some states (including MD, I think) have arbitrary limits on the max size of the magazine. Often 10.
Semi-automatics otherwise come with std magazines that range from 12 to 18, depending on the manufacturer and the calibre.
A .40 calibre and a .9mm might look the same, but the 9mm bullets are smaller, so the std magazine can fit more in the handle.
Personally, I don't see much sense in putting arbitrary limits on the size of standard magazines... but when people start having non-standard magazines that are designed *only* to let somebody shoot more bullets at people without having to stop and reload, well, that's when it starts getting demonstrably insane.
Re: what they call "assault rifles", what they really mean is "assault-looking rifles". The real military ones have a switch that toggles between semi-automatic and automatic. The legal ones are semi-automatic. You can buy one that looks like a hunting rifle, but the macho guys think it doesn't look as cool.
Some semi-automatic rifles can be converted illegally to automatic. It's not that hard to do. But bad guys tend to not do it, Because it's inconvenient. Bad guys are mostly lazy.