I've heard that the definition of "assault rifle" is mainly cosmetic; to what extent is that true?

Per Wikipedia, the US army defines an assault rifle as: "short, compact, selective-fire weapons that fire a cartridge intermediate in power between submachine gun and rifle cartridges."

Selective-fire means it's not just semi-automatic (one shot per trigger pull), it can also be set to fully auto (shoots as long as you pull the trigger) or burst (shoots a few rounds when you hold the trigger, then stops). These are essentially unavailable to the average civilian.

In popular usage, though, "assault rifle" means a short semi-automatic rifle with a plastic stock--anything that looks like a modern military rifle. Heck, I bet if you chopped down grandpa's bolt-action .30-06 deer rifle and put it in a black plastic stock, somebody would call it an assault rifle.

Personally, I think the AR-15 pattern rifle is a deeply boring and unattractive firearm. It's also become a symbol in a culture war: a terrifying "weapon of war" to some, a statement of rebellion and "owning the libs" to others. At the end of the day, though, there's just not a hell of a lot of difference between an AR-15 civilian rifle and, say, the Ruger Mini-14, except that the latter has a less scary wood stock.

'Per Wikipedia, the US army defines an assault rifle as: "short, compact, selective-fire weapons that fire a cartridge intermediate in power between submachine gun and rifle cartridges."'

The etymology supports this as well, going back to the first ones during WW2.

One thing to note, is that the law can define terms however they want. So places are applying their own novel definitions, largely involving cosmetic features or make/model.

Of course one other thing to point out is that the assault rifle bans are mostly a red herring given the relatively low contribution to homicides.

I don't know much about weapons, I'm European and we don't have much of them, but regardless of the definitions a gun is meant to kill people it has no other uses other than hunting. I find it very effective how in US the discourse can be shifted to which tools to kill should be limited more or less, rather than considering if society wouldn't be better off without them.

US police is violent, but how terrifying it can be when every car you pull for a routine control may contain a person who may shoot you?

"US police is violent, but how terrifying it can be when every car you pull for a routine control may contain a person who may shoot you?"

Arguably any car pulled over in any country has the potential for an occupant to shoot (or otherwise kill) you, even if guns are illegal and uncommon (as are murders). The likelihood just varies. In general, an officer should not be "terrified" of being shot on a routine stop. There are many technologies and procedures designed to minimize risk. Once you are no longer a rookie, the practice is routine. Being shot or shooting someone is fairly rare. I believe the lifetime chance of firing your weapon on duty was 2% or so, which is relatively low. Likewise, I remember non-violent fatalities were larger overall percentage (car accidents, drowning, suicide, illness, etc). Yet most officers are not terrified of covid nor car accidents. Situational context is a huge factor.

"I find it very effective how in US the discourse can be shifted to which tools to kill should be limited more or less, rather than considering if society wouldn't be better off without them."

Sporting purposes include target shooting in addition to hunting (see the Olympics, etc). The needs and ideals in one country are not necessarily the same as in other countries. It certainly does come up about repealing the second amendment and banning guns almost entirely. Some of the counter points are that much of the US is rural, where police response can take hours or even days (remote Alaska), and many of those residents require a firearm for various rural purposes, like protecting livestock from other animals.

On top of all that, it's a fairly large minority that owns guns. Many do not want to be burdened by additional regulations. It's similar to any other issue in a democracy - people vote for their own interests and benefits.