The argument I’m replying to is that criminals are buying guns in neighboring states because the laws are looser and they can get away with it.

But the laws regarding buying a gun outside one’s home state are federal, and don’t change from state to state. A California resident buying a Glock 43 in Texas is no more legal than them buying it in California. In fact - it’s moreso. Buying it in California is just buying a gun illegally by California law. Buying it in Texas is violating California, Texas, and Federal law, and then illegally smuggling the gun afterwards.