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, you can buy virtually any kind of firearm online, including the Bushmaster AR-15 allegedly used by James Holmes—a semi-auto rifle that'll blast a shot with every pull of the trigger, shooting as quickly as you can move your finger.
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 will sell you this semi-automatic Adcor Defense B.E.A.R., for a little over $1,500. Shipping is free. An AR-15 can be snapped up for under $900. You have other online sites to buy as well: Sportsmanguide.com, GunBroker.com, etc.
If that's beyond your budget, you can still get semi-automatic action for around $500, with this Heckler & Koch Rimfire 416—a variant of a weapon deployed by US Army forces in Iraq.
A semi-auto Glock 17 will run you $500 each. Holmes allegedly carried two, along with a Remington 870 shotgun: available for $325. There's even a "VIP Club" for expedited shipping But amassing an armory isn't as easy as stuffing all these weapons in your shopping cart and beaming over your credit card information. Guns can't be shipped directly to your house. A federally licensed dealer needs to act as a proxy. Most websites, like Bud's, already have a list of approved "FFL" dealers in your area for you to choose from a dropdown menu. The website will take your money and ship the gun to the dealer, the dealer collects a fee. Then you show up at the dealership for a telephone or Internet background check, and if you're not a convicted felon, off you go. But don't worry, this background check is a piece of cake. In fact, despite being mandated from one of the few tough gun laws on the books, you'll hardly notice the National Instant Criminal Background Check System (NICS) at all:
"The FFLs will provide the descriptive information requested on the ATF Form 4473, which is required by law to be completed and signed by every prospective firearm transferee. The FFL will receive a response that the transfer may proceed or is delayed. This response is typically provided within 30 seconds. If no matching records are returned by any of the databases, the transaction is automatically proceeded."
The guns are then legally yours for the shooting, so long as you're not a criminal, fugitive or insane. You might, however, have to do a little IRL legwork if you want to pick up more than one without setting off alarms.
Laws vary from state to state as to how many guns you can purchase at once from the same dealer—this might explain why James Holmes bought his first Glock in May, and then went back again shortly before his alleged shooting spree to buy another. But as there's no central government database to track any of this stuff, if you send your online purchases to a few different physical FFL dealers nearby, you won't have any trouble at all. Ben Van Houten of the Law Center to Prevent Gun Violence explains just how easily this gear can be picked up: