Can I buy a shotgun at 18+ in California?

Again,

Yes, you can purchase long guns (Rifles, Shotguns) at 18. 21 for handguns.

Per CA Penal Code Section 12071

All firearms purchases and transfers, including private party transactions and sales at gun shows, must be made through a licensed dealer under the Dealer Record of Sale (DROS) process. California imposes a 10-day waiting period before a firearm can be released to a buyer or transferee. A person must be at least 18 years of age to purchase a rifle or shotgun. To buy a handgun, a person must be at least 21 years of age, and either 1) possess an HSC plus successfully complete a safety demonstration with the handgun being purchased or 2) qualify for an HSC exemption.

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Yes you can, Federal law allows you to purchase a long gun (shotgun or rifle) at the age of 18 and handguns at the age of 21. California's requirements mirror federal law. You will need a clean criminal record, a valid (current) State issued ID and proof of residence. When you buy the shotgun you will fill out a Dealers record of sale aka DROS (State paperwork) and when you pick up the gun you will fill out Federal form 4473.

Sam is partially right. You can buy a long gun at 18. There is a waiting period and a background check. There is no firearms registration in California. (People who spell California with a "K" often are in error as to what California laws actually are.)

Yes you can. You have to be at least 18 years old. I believe in the peoples republic of Kalifornia there is a waiting period and also registration requirements.