Asshole, the money is going to pay my salary. You don't get to know how I spend it.

It pays to be specific.

According to studies from Florida State University, there are on average 2.5 million defensive gun uses annually. Surely at least one of those people is a "good guy". Or are you looking for individual cases?

Well, when you have gun control advocates in Congress trying to pass laws regulating things that they know absolutely nothing about... you get the current state of affairs.

How Carolyn McCarthy thinks a heat shroud is a defining characteristic of an "assault weapon", and then describes it was "a shoulder thing that goes up".. yeah. Enjoy your laws.

Barrels are not serialized.

Bullets can be matched up to a point. What you're describing are known an individual characteristics, which are the unique properties of each barrel. Those properties change through regular wear and tear, or by deliberate action such as running a file or other abrasive down the barrel. My hunting rifle is a tack driver, but over time it will start getting worse and worse due to wear. In my case, it will happen quickly as the rifle is a magnum. If I were a suspect, the cops would need to get hold of my rifle quickly before I go for target practice because the properties of my barrel would change. In my case, I'd go to the range and fire off a munch rounds in quick succession, heating up the barrel so it wears faster. My rifle is uncommon enough that just based on the class characteristics, it may be enough to stand up in court.

On certain rifles, such as anything from Savage or an AR, it's pretty easy to change out a barrel, or even change the caliber entirely. I can change out a Savage barrel in about 20 minutes.

Of course, this is assuming that you have a suspect and a rifle to test.

9mm > 5.56mm

None of the rifles you linked to are assault rifles, and the definition of what an assault rifle is is most definitely settled. This vendor has absolutely no idea of what an assault rifle is (in one case, he's selling an "assalut" rifle), or is attempting to capitalize on idiots who think they're buying a rifle capable of select fire.

This is so phenomenally stupid, it has to be satire.

Tomatoe, tomahto

I'll take you up on that challenge.

The guy who did that is an experienced manufacturer of AK-47 rifles, with access to all the right tools. The AK-47 is not a "precision" firearm to begin with, the fit and finish are pretty loose compared to other rifles, leading to it's legendary reliability.

Is my dog always going to be awake, uninjured and in the same room as me?

Well, there's the difference between Oz and the US. In the US, we have these things called 'rights', which are inherent in every citizen. Our Constitution and Bill of Rights are a collection of rules that protect these rights from the government- this is one reason why the 18th Amendment failed so spectacularly. One of these rights says that we get to keep things that go bang. That's why we don't have to justify owning a gun. A citizen shouldn't have to justify anything to his government.

Are other sorts of violent deaths acceptable to you?

Yes, there are societies out in the world where murder is socially accepted. In some cases, it's expected.