Question: What's the difference between 26 inches and 25 1/2 inches?
That's at least true when the measurement at issue is of your shotgun in California. Because even if your barrel is the right length (e.g., it's not a "sawed-off" shotgun), if your shotgun's too short (e.g., a "shortened" shotgun), it's illegal. And, no, the Second Amendment doesn't save you. Even if a short shotgun would perhaps be marginally more effective for combat in a tight hallway or the like. There's a reasonable interest in avoiding shotguns that are more easily hidden (e.g., in one's coat). So use a tape measurer if you want to stay out of prison.
Oh, one more thing. If you elect not to use a tape measurer, it's probably not a good idea to repeatedly call up the DMV (as well as tell the police) that you might up in there and "light up the place". These folks tend to take these things pretty seriously.