I will try to make this a layman's presentation.

First, we have to look at the definition of "assault weapon." 

There is a military definition of "Assault rifle," which is a rifle of intermediate caliber, firing from a closed bolt, in select fire (both self-loading, and automatic or burst), intended for engagements primarily under 200 yards.  Assault rifles are strictly regulated by the National Fireams Act of 1934 (It predates assault rifles by a decade), requiring a background check, law enforcement approval, a $200 tax stamp, and are illegal in 12 states at the state level.  No new ones can be manufactured for civilian sale since 1986.  Current market prices are around $12,000 and up.  My research shows none have ever been used to commit a crime of violence.

There are two cases of registered full autos that were not assault rifles used for violence.  Both perpetrators were law enforcement officers.

Using the AR15 as an example, the civilian version has 7 different components inside, than the military version.  There are ways to convert an AR15 to automatic or burst, but it generally requires a machine shop.  If you have a machine shop and the diagrams, you can illegally make your own anyway without much difficulty.

There have been very few illegal conversions used in crime, for the twofold reason of the complication of doing so, and that autos eat a LOT of ammo.  Regardless of movies, the standard 30 round magazine would be expended in 2.8 seconds.  As anyone who shoots auto can tell you, most of those will miss.  A burst longer than three rounds is rarely of effect.  That even drug dealers aren't mowing each other down with Uzis (not an "assault rifle," btw)(except on TV) should prove that this is really not a concern, even if you could do anything about it, which you can't, unless you plan to ban milling machines.  Except, of course, you can see a previous post where I built several AK style rifles in my garage.

The Congressional and some state definition of an "Assault weapon" attempts to FURTHER restrict guns.  However, there are only a few ways to make a rifle cycle, and any attempt to use those definitions means a ban on common hunting rifles.  Instead, the bans focused on cosmetics.

"Flash hider."  Not having a flash suppressor on a rifle doesn't stop it from shooting, or change its shooting characteristics in any way.  Many modern guns have some form of muzzle brake to reduce recoil or climb, or improve accuracy, and these remained legal.  The only difficulty was that they had to be permanently attached instead of threaded.  This made normal sport and hunting guns more awkward to build, without affecting the ability to shoot, changing the power, or anything else. 

"Grenade launcher."  This was a deliberately chosen term to promote hysteria.  Since the 1950s, NATO nations and many others have used 22mm as the diameter for the flash suppressor/compensator/brake.  There are projectiles that fit over the muzzle device, that are launched with a high power blank cartridge.  These are more powerful than regular blanks, and can damage the weapon.  To make use of this, one would need the blanks, and a 22mm projectile (some are explosive, some illumination, some marking).  Such projectiles are very hard to find and not even used much by the military, if at all.  In 25 years of service, I saw one dummy device.  And since 22mm muzzle brakes were still legal, it made no difference whatsoever.  The presence or absence does not affect the rifle's ability to fire.  Also, they remained legal on non-semi auto rifles, because apparently, grenade launchers aren't dangerous on those. 

"Collapsible or folding stock."  Folding stocks enable a weapon to be transported more easily in a case.  Collapsible, or more accurately, adjustable stocks, are a useful feature because the rifle can be adjusted to fit users of different size, or for various seasons of clothing.  The lack is irrelevant at best, frustrating to legitimate shooters at worse.  It makes no difference on the ability of the weapon to fire.

"Barrel shroud."  This stops you from burning your hand on a weapon that doesn't have an existing handguard.  Relevance?  I'm not really sure.  Of course, you have to be careful how you define it, or useful things like free-floated handguards used in competitive shooting become illegal.  But the gun still shoots.

"Pistol grip."  At one time, guns were straight sticks with barrels banded to them.  These days, we have a much better understanding of ergonomics.  A great many modern guns of every type have pistol grips that enable a more secure grip and better shooting.  The inconvenient workaround is a protrusion from the stock that doesn’t enable quite the same grip on the weapon.  In any case, it makes no difference to the ability of the weapon to fire.

Now, one might ask, "If they don’t affect anything, why do you object?"  This is the wrong question.  If they don't affect anything, by what right do you dictate?  What will you dictate next?  Since there was no negative effect on crime when the ban went away, why should we consider any such laws ever again?

There are ongoing attempts to ban weapons by name.  The problem is, a name can be changed, and, the Courts have ruled that it violates equal protection to name one brand and not another.  How hard is it to change brand names?  Not hard.

So, then it was "AR15s or copies."  No good.  The courts say you must define "copy."  Since the terms address cosmetics only, there's no way to do that.

In fact, California shooters have a brisk business in "off list lowers."  A manufacturer makes an AR compatible lower.  They buy it.  Eventually, CA gets around to adding that brand to its "list."  That means a window opens during which it can be registered as an "assault weapon," and all the evil parts that don't affect shooting can be added, thus creating a valuable new transferrable restricted weapon.

I guess if there wasn't such an obsession with banning them, there wouldn’t be such an obsession with owning them.

"High capacity magazines."  This is not only inciteful, it is dishonest.  The AR15 has had a STANDARD 30 round magazine since 1967 (replacing the original 20 round magazine).  The AK series have had 30 and 40 round magazines since 1947.  What you propose is REDUCED capacity magazines.  But, three 20 rounders are as effective as two thirty rounders, and a magazine change takes about a second. (This, btw, is a clear indicator the opponent in debate knows nothing functional about guns.)  The question of course is, if 20 round limits won't do the job, what's next?  15? 10 (which was the law during the ban)? 5?  Once I allow you to dictate a limit, it can be changed arbitrarily.  After all, you've already vilified my STANDARD CAPACITY magazines by claiming they're "high capacity."

However, this was an item that backfired massively, and why I make the snarky comment that amateurs need to not regulate guns because they'll get hurt.

The AR magazine is the NATO standard, or STANAG.  The British L85, the German G36, the French FAMAS, the Korean Daewoo, and quite a few other rifles take the exact same magazine.  There are approximately 63 bajillion of them in existence (and the manufacturers are cranking out more as we speak).  So a ban on new ones doesn't really have any effect on crime or availability.  There are tens of millions of AKs in the world, and billions of magazines.  And of course, under that pesky 5th Amendment, a ban on possession would mean the government having to buy them from us at market value of $12-$50 each.  Some idiot even proposed registering them, without any thought as to how, or who'd keep the records, or how transfers would be conducted.

SIDENOTE—gun haters love the concept of "registering" things.  To what end?  You can't inspect my premises without a warrant, and registering cars certainly hasn't had any effect on drunk drivers or accidents.

But, billions of magazines are out there.  And the same is true of common pistol magazines.  So if you're going to design a new gun, you can choose to design with a proprietary magazine of ten rounds, or adapt existing magazines.  The result of this was lots of manufacturers creating new carbines that took existing Glock magazines, and rifles that took AR magazines.

And…during this timeframe, Colt's patent on the AR15 expired, meaning anyone with a Class 07 Federal Firearms License (Manufacturer), could turn out as many AR receivers as they could mill, without paying license fees.  And they did.

The receiver is the numbered part that is legally the weapon in the US.  All other parts attach to it.  Regulation of those parts is impossible, because they're consumable replacement parts, and exist in the trizillions. 

The AR is almost unique in its flexibility.  It can be anything from a .22 caliber pistol to a .458 SOCOM entry carbine in a matter of seconds, just by changing receivers and stocks (assuming your gun is listed as a pistol on its original purchase form, because going the other way, from rifle to pistol, violates a law.  Why?  You tell me, it wasn't a shooter who came up with that idea).  Its basic design, btw, is from 1955, and the first civilian production guns from 1963, which means that in the coming year, it starts to achieve status as a "Curio And Relic," meaning it's more valuable as an historical artifact than as a rifle.  There are specific subrules for C&Rs that avoid much of the paperwork for modern guns.

The AR's lower receiver and trigger group can also be used to operate other weapons or devices.

It is also now used in quite a few modern hunting guns, such as this http://www.remington.com/en/product-families/firearms/centerfire-families/autoloading-model-r-15.aspx which as you can see, is not being marketed for "Shooting up schools."

So, the law changed nothing functional or practical, and instead led to an explosion of new weapons of various designs, and when it went away, nothing changed.

Except one thing:  Gun owners are not inclined to agree to an acceptable level of control, since that level can be changed, until the "right" to keep and bear arms means Airsoft guns.

And thanks to this ruling, we don't have to:

http://www.scotusblog.com/wp-content/uploads/2008/06/07-290.pdf

"Some have made the argument, bordering on the frivolous, that only those arms in existence in the 18th century are protected by the Second Amendment. We do not interpret constitutional rights that way. Just as the First Amendment protects modern forms of communications, e.g., Reno v. American Civil Liberties Union, 521 U. S. 844, 849 (1997), and the Fourth Amendment applies to modern forms of search, e.g., Kyllo v. United States, 533 U. S. 27, 35–36 (2001), the Second Amendment extends, prima facie, to all instruments that constitute bearable arms, even those that were not in existence at the time of the founding."."

So I ask again, politely:  Please address school security, mental health, and the need of the media to glamorize killers.  Because 94 million law abiding citizens didn't kill anyone last year.