I assumed firearms would be transported with higher standards, especially given the known state of LA. Maybe via armored car?
Not really. Armored trucks are used to transport goods based on the value of the goods, not how dangerous they are.
Guns aren't cheap, a good handgun can be for or five hundred dollars and a rifle twice that, but in comparison to cash or jewelry the other stuff those trucks are used to haul, the value per volume (remember, armored trucks can't carry a lot compared to a normal truck or a semi) is very, very low.