Buying a fully assembled gun is a process subject to a host of regulations and restrictions, especially in states like California that tend to be more stringent.

But anyone can make a gun at home.

The do-it-yourself route is often favored by gun enthusiasts, including hobbyists and competitive shooters. It can also be a path to gun ownership for felons and people with mental illnesses or those who have been convicted of domestic violence.

The Giffords Law Center to Prevent Gun Violence, part of the gun safety group founded by Gabrielle Giffords, the former representative from Arizona who survived an assassination attempt in 2011, is calling on two internet service providers to disable websites that sell materials and tools to create homemade weapons. Such weapons, often referred to as ghost guns, were used in a mass shooting in Northern California this month.

The websites, ghostgunner.net and ghostguns.com, allow customers to bypass background checks and build unregistered firearms without serial numbers.