They're very effective against honest citizens. Claims of effectiveness in reducing crime tend to have more holes in them than a block of Swiss cheese, though.
The California Department of Justice web site has current CA gun laws.
No. You can purchase a gun from a Georgia dealer, have it shipped to an FFL licenced dealer in California for receiving, and receive it in California, but you can't purchase a gun in Georgia and carry it with you as a California resident. This is due to California's laws, not Georgia's.
Oklahoma amended its gun law effective November 2012. Wikipedia has a good summary of it in an article titled "Gun Laws in Oklahoma."
California has strict gun laws and I doubt that you can legally own a 20MM rifle in California. Usually the only guns available in California are small handguns.
Many, but not all firearms in California must be registered with the state Dept. of Justice. Suggest you check with a local gun shop for a lesson in CA gun laws. Violate them, lose gun, go to jail.
It is always best to consult a lawyer that is familiar with the local gun laws, or even the local prosecutor.
Most retailers will state whether or not their products are California legal. Your nearest gun store should be able to tell you whether it is or not, but, given the complex nature of California laws, you should contact the California Department of Justice (see related link) if in doubt.
I'm not extremely familiar with California gun laws, but I don't think anyone in California is allowed to posses a non-registered firearm.
The California Department of Justice has a web site with current laws. Sections will note when new provisions started or old ones expired.
Provided you comply with all local, state and federal laws, yes.
You cant.Its illegal to have one in California
In California, individuals without a permit can legally possess up to 5 pounds of black powder or smokeless powder for personal use. However, it's important to check with local laws and regulations regarding storage and transportation of gun powder.