It's not a law that was passed under Obama ... it is on form 4473, which all legal buyers of firearms must fill out as part of their background check.
#11-F: "Have you ever been adjudicated as a mental defective OR have you ever been committed to a mental institution?"
Anyone who answers "Yes" to that question is prohibited from purchasing OR receiving a firearm.
It is also a federal felony to lie on the form.
So if everyone is following the law ... the mentally ill would not have access to firearms.
Of course, criminals don't follow the law. That's why they are criminals.
And the military doesn't follow the law. Because they believe they are above it, and protect their own. (LV shooter)
And the police don't follow the law. For the same reason as the military.
All these new laws about taking firearms away from people, on the word of family or friends ... those are unconstitutional.
How? They violate the 4th and 6th Amendments.