"That's an offensive weapon, that is!"

In the 1990s, gun control advocates tried to get traction for Euro-style draconian gun laws in the US by holding up semi-automatic civilian versions of AR-15s and AK-47s on television, shouting "THESE ARE ON OUR STREETS!", and allowing people to think they were talking about machine guns. Since they needed a way to talk about the guns in question without discussing their actual features, they adopted the term "assault weapons", and pushed it so hard as a meme that it's still commonly used today, seven years after the silliness of the Federal Assault Weapons Ban was finally allowed to expire.

Galloping to our rescue, the [NSSF*] has for some years now been imploring gunnies to use the term "modern sporting rifle".

This is stupid. AR-15s are much more common in sporting use than most people assume, since they're quite good for target shooting and varmint hunting. They're too weak to be legal for deer in most states, but some very good marksmen can use them for that purpose, too. But trying to define it as a "sporting rifle" is a lie of omission, and worse, it's an utterly transparent one. I wouldn't care to speculate on whether more ARs are purchased for sport or for home defense or both. But it's entirely obvious that both roles are key to the rifle's success.

Denying that we're talking about a fighting rifle is destructive in two ways: it needlessly undermines our credibility on more important issues (why would a non-gun person trust us when we point out that concealed carry liberalization has never changed a state's murder trends if he's seen us being deceptive about ARs?), and it implies that fighting rifles are something we need to apologize for. The Second Amendment does not protect our access to guns with a "sporting purpose".

So what do we call these guns, given that we understandably don't want to use "assault weapon"?

Quite frankly, I don't see the need to ask this question. It’s the antis who feel the need for a specific term that describes "rifles with pistol grips that remind the mainstream of guns they see soldiers and revolutionaries shooting on CNN". We simply don’t have that need. When you need to talk about light semiautomatic carbines, just say "light semiautomatic carbines". In those very few cases when you need to talk exclusively about light semiautomatic carbines with one particular set of ergonomics, just say "light semiauto carbine with a pistol grip".

We don’t need verbal gymnastics here.

[* - I originally said "NRA". Though the National Rifle Association has also endorsed "modern sporting rifles", this particular link is to the National Shooting Sports Foundation, a prominent gun industry lobbying group.]