I feel the same way I do about the AR15 that I do about Margaret Thatcher: I don't like either of them, but I respect what they are in relation to their respective country's culture. The AR15 is probably more symbolic toward US culture than Margaret Thatcher will ever be to Britain's.
It is a symbol of militarism since it was designed over 60 years ago for the US military, with variants used by military forces worldwide. Part of its attraction is that it is the civilian version of the US military's weapon. And its deadliness is one of its attractive features. It is proven in combat and mass shootings.
The AR15 platform allows for it to be built in a myriad of different ways. It is also fairly easy to build with various kits being sold; from complete upper and lower receiver assemblies to the parts for making a ghost gun. Although, ghost gun means a firearm made "80%" lower receiver and parts. It is the AR15's ability to be built by anyone which should cause people to pause.
I personally would not want to invest the time and effort into making an actual ghost gun. Complete stripped lower receivers are also available, which is the lower receiver block without the parts. That allows someone to create their custom gun. It's easy to customise a completed lower receiver as well. Just look up a video on how to do that mod to your gun.
And there are the AR15 pistols as well, which I am mentioning since there is the debate as to how often these weapons are used in crime:
Mass shootings involving rifles like the AR-15 can produce dozens of victims at one time, and combined with extensive media coverage of these events, many people have been led to believe that such rifles pose a significant threat to public safety.Given the amount of variations on the AR15, there are a fair amount of pistol versions. One manufacturer lists barrel lengths from 8 inches to 20 inches for their upper receiver assembly. An interesting riff on all this since Orlando, Florida, authorities revised their initial description of one of the weapons used in the June 2016 attack at Pulse nightclub. After initially describing it as an “AR-15-type assault rifle,” police said it was a different type of firearm, the Sig Sauer MCX.
However, such shootings are extremely rare, and a look at the FBI data informs us that homicide with these types of rifles represents an extremely small fraction of overall homicide violence. Banning or confiscating such firearms from the civilian population would likely produce little to no reduction in violent crime rates in America.
One the the variants of the MCX is the Rattler SBR (short barrelled rifle). While SBRs are NFA weapons, it's pretty easy to bang one up using the AR15 platform. I would also toss in that semi-auto pistols that accept high capacity magazines are banned in some places. Additionally, a submachinegun is a machinegun that fires pistol calibre ammunition. That means that submachineguns are basically pistols that can have a very high rate of fire.
But the main reason I would say that the AR15 is America's gun is that it will probably never be regulated despite the carnage it is capable of causing. Despite the deadly shooting in Las Vegas to the 20 toddlers killed at Sandy Hook, these weapons are more than freely available to anyone who wants one. You can buy an 80% receiver with no background check to build whatever version of an AR15 you want.
That means that anyone who is adept with metalworking tools, or just adept with tools if it's a polymer 80, can crank out a weapon intended for the battlefield.
That should cause you to pause and think no matter what your opinion of these weapons happens to be.
 Short barrelled rifles are another topic which I am not going to get into.