· What exactly is controlled by gun control?

For the popular AR-15 rifle, Wikipedia says it is a fairly small portion which is marked with a serial number and legally controlled. And when this part is incompletely manufactured, requiring a little extra machining and some over-the-counter parts to become dangerous, sales of the "unfinished receivers" or "80 percent receivers" or "blanks" or "ghost guns" are not regulated.

Is this accurate, and is the situation similar for handguns?

It is accurate, yes. As explained on the ATF's website, they do not consider these to be firearms, so they're not illegal. This article from Wired talks about this too, and also notes that folks have begun experimenting with 3D-printing of parts, including the lower receiver.

As for handguns, yep.

Basically it comes down to how federal law defines what is a gun versus what isn't, and thus which parts are regulated. As you say, only the lower receiver (in the case of an AR) is required to have a serial number and be regulated, whereas a receiver-shaped block of metal is not.

I heard that sales cannot be recorded in any format that is "searchable," so many records are kept on paper. When police have to track down a firearm used in a crime, they make phone calls to dealers, who sort through files. If this is at all true, it is surprising that any firearms are ever tracked.

Pretty much true. Basically Congress said there can't be a centralized database of firearm transactions. Some states may do it, I dunno. Dealers are required to hold onto the paperwork for like 20 years (or send it to the ATF if they close shop), but that's about it.

· This probably varies by state, but I understand that the "gun show loophole" has nothing to do with gun shows. Licensed dealers are required to do background checks before a sale, but anyone else can sell firearms without background checks. Is it really legal for someone to sell a handgun to a total stranger for cash? What other criminal charges might apply in such cases?

Correct. I don't know about every state, but at least in mine you can sell a gun for cash and not incur any legal penalties. For example, here's a forum where people can sell handguns to each other (or trade them for other guns). The same site has separate fora for rifles and shotguns too. AFAIK (and the usual disclaimers about this not being legal advice apply) the only way you'd get in trouble is if you knew or reasonably should have known that the person you're selling to is not eligible to own a gun, such as due to a felony conviction or whatever.

· Forensic matching of rifling marks on bullets seems like an unreliable approach. Are TV depictions of this practice quite exaggerated? Are there other techniques that are more often helpful in investigations?

They usually are exaggerated, and not just with guns. I'm not well-versed on the subject, although there has been some snake oil out there (see for example this article about flawed hair analysis). Lawyers do worry about the so-called CSI Effect, which can affect both sides of a criminal trial. Defense lawyers worry about juries putting more faith in forensic technology than is warranted by the reliability of that tech, while prosecutors worry that people will expect magically reliable results and won't convict without it.

· There was a story somewhere about a requirement that magazines could not be ejected without a special tool. Some guy invented a magazine that could be ejected with a spent shell casing, and I think it went to court. Heard any other weird stories like this?

Not off the top of my head, simply because I live in a state that doesn't do that stuff. The so-called "bullet button" part is true, and it was found to comply with California law (which is the only state AFAIK that requires it). Ironically, the state legislator who tried to have the bullet button outlawed in California is currently in federal prison for, among other things, gun running. He bought some automatic weapons and missile launchers from an Islamic extremist group in the Philippines and tried to re-sell them to someone who turned out to be an undercover FBI agent.

· What are the rules about modifying firearms, like filing serial numbers or sawing off barrels?

Filing off a serial number is basically always illegal as far as I know (and here too, this isn't legal advice, etc.). Sawing off barrels is likely illegal as well; the Gun Control Act (the big federal law on gun regulations) frowns upon a shotgun with a barrel less than 18" and a rifle with a barrel of less than 16". I think it's more the length requirement than the modification per se...in other words, if I had a rifle with a 24" barrel and cut it down to 22" (for some reason), I'm not aware of that being illegal.

For some reason there's a subset of folks there who are super into the idea of a "short-barreled rifle", known as an SBR. I confess I'm not really sure why. But one of the things they used to do to get around this is use an "arm brace," which goes on the back where the stock would be and has a cuff-looking thing that goes around your arm, allowing you to fire it one-handed. Of course people were instead just firing it from the shoulder, which the ATF has since decided to be a "modification" and therefore make the gun fall under the National Firearms Act, or NFA.

This is the same law that governs private ownership of fully automatic weapons, as well as super-large caliber (above .50). Note that it's not actually illegal, there're just a ton of legal steps and a lot of money involved to do it.

· Silencers. Apparently very unlike James Bond depictions, and more useful to protect the shooter's hearing than for getting away with a stealthy kill. There are several scenes in "Heat" in which firearms are used inside vehicles. Permanent hearing loss, or only temporary?

First, you're right, "silencers" (which are usually called "suppressors" by the manufacturers) reduce the noise but don't make it truly silent like in the movies. It can make a gun safe to shoot without ear protection, though, with the right ammunition. But remember too that many bullets are super-sonic, and part of the sound is actually a sonic boom. So you have to get sub-sonic ammunition for this to be at its most effective.

As for hearing loss, I'd guess permanent. These OSHA standards (PDF) from the late '90s recommended less than 1 second of exposure for sounds above 130 dB. Most of the stats I've seen on firearms put them at 150 dB or above (see e.g. here and here). They're using very short-barrelled rifles in Heat (according to the Internet Movie Firearms Database, McCauley is using a Cold Model 773 which has an 11.5" barrel), which would make it worse.

· Re "Heat" "N.B. Val Kilmer's reload was so fast and smooth that this clip was used in U.S. Marine Corps training videos, to show new recruits how it should be done in combat." Citation needed?

I've heard that story too, but I have no idea if it's true or not. This behind-the-scenes bit on the shootout mentions it, but again it's not clear if it's true. I'd believe it, though.

If you've never used an AR before, what he does is hit a button on the left side of the rifle to drop the magazine. He then puts in another one. Then you see him push something on the left side of the rifle again. This second thing was the bolt release. What happens is, when the last round is fired, the bolt locks back (just like the slide locks back on a pistol when it's empty). Once the new magazine is put in there, he released the bolt which comes forward and pushes the first round into the chamber, so it's ready to go.

· Are people 3D printing guns that work better than slingshots yet?

I haven't kept up with this much, but apparently Defense Distributed's pistol, called the Liberator, can work.

· I was surprised to learn how many states have open carry laws, indeed at first I was surprised to learn that it is legal anywhere to walk around carrying a firearm. Evidence on YouTube suggests that many law enforcement officers are fuzzy on this point as well. Concealed carry often requires a special license.

Yeah, although this varies a lot by state. I personally think open carry is stupid.

· Beretta is one of the oldest companies in the world, with a sale to the Arsenal of Venice recorded in 1526. The iconic 92 series replaced the venerable M1911 which the U.S. armed forces had been using since 1911, and was the basis of the photo illustration in the Atlantic article.

Yeah, this is still pretty controversial. My understanding is that a lot of the reason was ammo standardization with NATO, since the Europeans were all using 9mm sidearms. Plus you get into things like ammo weight and capacity. The Marine Special Forces folks switched back to Colt 1911s for a couple of years, but have since gone to the Glock 19 (which is 9mm). For the record, my daily carry is a 1911, so I'm kind of a fanboy.

· Isn't that a handsome car?

Yes!