It’s an entirely innocuous statement. What are they supposed to say instead?

• “15 Assault Rifle” or “Assault Rifle 15” just makes no sense.
• “AR-15” isn’t understood by all people to mean an assault rifle, since not everyone knows gun names.
• “Assault Rifle” doesn’t tell people which kind of assault rifle it is, since the AR in AR-15 stands for ArmaLite.

It’s a statement that gives the most possible context to the reader about:

• What design of assault rifle it is
• What type of gun it is in general
• While also allowing readers to then learn the definition of the sole term “AR-15” over time