No, they don’t. I’ve made that drive several times between Vegas and SoCal. The truth is, several tens of thousands of vehicles make the trip up or down I-15 between Nevada and points south/southeast in California EVERY. SINGLE. DAY……and remember, I’m just talking about Southern California. There are LOTS of ways to enter the state.
On the way down, motorists cross with I-40 at Barstow, connecting the central Mojave desert to the Arizona border outside of Topock, Arizona, just across the Colorado River—yet another route for non compliant ammo imports into California. Further south, and just northeast of San Bernardino, drivers cross the interstate spur 210, leading into the San Gabriel Valley and points west—and literally millions of people. South of that, they’ll cross I-10, which connects the SoCal coastline at Los Angeles to the Arizona border across from Blythe, California (again, on the Colorado River). South of I-10, the 15 crosses the rest of San Bernardino county (the LARGEST county in the United States.
Further South, it crosses Riverside County all the way through, and on into San Diego county, where it intersects Interstate 8 in San Diego. I-8 connects San Diego to El Centro in Imperial County, before crossing into Arizona in Yuma.
So, between I-15, I-40, the 210, I-10, and I-8, we have four busy major access points into four of the largest counties in the state, home to more than half of California’s population of 39,613,493 people. Although I don’t know the actual number, it would be reasonable to assume that as many as 150,000-200,000 vehicles enter California by these entry points alone, every single day. And, once into the state, there are dozens of "rat lines" off the interstates into which the enterprising ammo transporter can disappear. THIS DOES NOT INCLUDE entry by less-well-travelled means either—for instance Highway 62 from Twentynine Palms or 177 from Desert Center into Parker, Arizona; nor does it include all of the entry points further north, above Las Vegas either….from the Death Valley for instance, or Reno, or a dozen other places.
There is literally no way for Californian authorities to interdict non compliant ammo imports. It just can’t be done. They might occasionally serendipitously bust someone while searching a vehicle for human or agricultural contraband at an CBP or USBP inspection station, but that would represent a tiny fraction of the potentially thousands of people running ammo into the state, whether for personal use or non compliant distribution……and the authorities KNOW this, which is why they make so much noise about "illegal" ammo. Noise is all they’ve got.
And please note my repeated use of "non compliant" here. The law itself is unconstitutional, and therefore illegal. People who violate it are patriots for giving the state the finger.
The biggest risk to California's non compliant ammo buyers isn’t buying or possessing it. It might come from using it—whether for range use or personal defense. Spent brass at the scene might possibly betray its provenance if examined by law enforcement for some reason or other, having witnessed or being able to prove that that individual user had fired it, and being able to however dubiously "prove" that it came from out of state after the import ban took effect. For the shooter who could prove that he’d bought in bulk before the import ban went into effect, there shouldn’t be any problem. Shouldn’t be. But this is commifornia, so you never know.