CALIFORNIA: First things First: check "Roster of Handguns" for a list of all handguns legal in California. http://certguns.doj.ca.gov/
With the exception of military-grade machine guns, or California Assault Weapons, you can buy virtually any kind of firearm online. However, as of January 1, 2017 Assault weapons such as AR-15, AK-47/74 and others have now been listed as "Assault Weapons", which means they are not legal in California. However, you can still purchase an AK-47 from Atlantic Firearms, California Compliant, but it has to be upgraded to a featherless rifle. Atlantic Firearms will install a grip rap on you AK-47 for an additional cost. If you don't purchase any 10 round magazines at the same time it will ship without magazines. It is listed as a RAK 47, U.S.A. Model 7.62 x39
Let's take BudsGunShop.com—one of countless online weapons dealers. It's entirely on the level, with a brick and mortar HQ in Lexington, Kentucky. It even has a Pinterest button. Bud's Shipping is free and no sales tax if you are, or become a member, which isn't a large amount. You have other online sites to buy as well: Sportsmanguide.com, GunBroker.com, Atlantic Firearms, Grapagun.com, Impactguns.com, etc.
If would like to start a hobby, and stay with in a budget, you can still purchase AR-15 Striped Lowers $69.00 up. You can purchase different calibers, 223/556/308/9mm etc.
As you may already know, you can't have any firearms shipped directly to your home. A federally licensed dealer needs to act as a proxy. Most firearm websites, like Bud's Gun Shop, already have a list of approved FFL dealers in your area for you to choose from. The firearm website distributor will take your money and ship your firearm's to the dealer of your choosing. The dealer charges a transfer fee from $35.00 to $100.00. Fore example: The FFL Dealer will charge a transfer fee of $35.00 or more, plus a DROS fee, "background check" of $25.00. Also, you will need your Firearm Safety Certificate, "FSC", or CCW and two types of ID with your picture and current address. Most, or all FFL Dealers can provide you with the FSC test should you need to take it. Oh! that's an additional $25.00 fee.
You will be asked to fill out a 4473 ATF form prior to completing your DROS online background check. This process doesn't take very long. After that, you wait 10 days prior to picking up your firearm, and if you're not a convicted felon, you will be on you way to the range.
There are rare occasion a dealer will receive a DELAY notice back on a DROS. This response is typically provided within the 10 day waiting period. DROS/DOJ has 30 days to resolve the issue. After 30 days the FFL dealer has the choice to release the firearm or not. In most cases there isn't any problem releasing the firearm to the buyer.
Laws vary from state to state as to how many firearms you can purchase at once from the same dealer. Always check with you state laws and FFL dealers if you are interested in purchasing multiple firearms. For example: you can purchase multiple long guns and one pistol and run them on one DROS background check. This can save you the cost of running separate DROS for each firearm. However, in California, The FFL dealer has to submit an additional form to ATF when multiple firearms are purchased at the same time.
FYI, you can only purchase one Handgun per every 30 days.