Because according to the sergeant that deals with the press, they can tell what the caliber and make of a rifle is just by the sound.
Castano says judging by the noise of the weapon, the gun was likely an assault rifle.
He could not confirm media reports the weapon was an AK-47
So was it an assault rifle or wasn’t it? Now, I’m mostly kidding around with the LA County Sheriff’s Office here, because I’m pretty strongly biased in their favor – but come on, I’ve heard loads of AKs being fired and I couldn’t tell the difference between the sound of an AK, an SKS, or a bolt rifle chambered in 7.62×39.
Making statements like that to the press just feeds the assault rifle fervor – just look at the title of the linked article.