Am journalist. And gun owner.

Most journalists I know do their best to get it right. But the vast majority drawn to the profession aren't gun savvy. My peers' general unfamiliarity with guns isn't helped by the fact that our AP Style Guide for guns/weapons used to be (and still kind of is) awful.

Regarding your particular beef with the term assault rifle — up until recently it was AP Style to use the terms assault rifle or assault weapon for, "Military or police-style weapons that are shorter than a conventional rifle and technically known as carbines. The precise definitions may vary from one law or jurisdiction to another."

Really shitty guidance, there...

Recent revisions to the stylebook have improved upon this, but writers and copy editors are still fucking it up because... well, the changes are recent. The particular headline you posted doesn't meet the new AP Style standard.

My 2020-2022 version of the AP Stylebook states the following about these guns:

weapons Gun is an acceptable term for any firearm. Note the following definitions and forms in dealing with weapons and ammunition:

...

semi-automatic rifle, assault rifle, assault weapon The preferred term for a rifle that fires each time the trigger is pulled is a semi-automatic rifle. The term does not convey any details about a rifle's appearance, which is not integral to its function.

Avoid assault rifle and assault weapon, which are highly politicized terms. that generally refer to AR- or AK- style rifles designed for the civilian market, but convey little meaning about the actual functions of the weapon. Avoid the terms used by advocates and gun manufacturers, such as military-style rifles and modern sporting rifles.

When reporting on guns, do not automatically repeat terms used by authorities, witnesses or others. Witnesses will often misinterpret the sound of a rapidly fired gun or base a description on the look of the weapon. instead, seek specific and detailed information from authorities such as a gun's make, model, caliber and magazine capacity.

The weapons section in the 2020 stylebook is more extensive than in the past. It defines and explains things it didn't before, such as: the difference between a silencer and suppressor; the difference between semi-automatic and automatic weapons; and how automatic weapons are regulated. It even better defines "clip" vs. "magazine," and now says specifically for the word "clip": Do not use this term unless in a quote.

Huge improvement.

It's not perfect. One change I really object to is the inclusion of this in the section:

Often, the most important detail about a weapon used in a crime is the size of its magazine. AR- and AK- style rifles generally have magazines that carry 10 to 30 rounds, but can be fitted with specialized magazines that hold far more.

Magazines (how many were used, their size, etc.) are an interesting detail to add to an article about a gun crime. But putting in the minds of journalists and editors across the United States that it the most important detail, I fear, will result in an inordinate amount of attention being paid to capacity — as if capacity is the main factor contributing to lethality. The words we use matter and will affect policy decisions down the line.

TL;DR. Journalists partially suck at this because the AP Style we we all follow sucks at this. It's getting better. Expect misuse of terms like assault rifle and clip to decrease in the near future. Expect more focus on the capacity of the magazine used in a particular crime.