Keeping Guns From Criminals Act This bill amends the federal criminal code to modify the criminal liability standard for certain firearm sales or transfers. Current law makes it a crime for any person to knowingly sell or transfer a firearm to a prohibited person (i.e., a person who is prohibited from possessing or receiving a firearm). This legislation eliminates the "knowingly" standard to impose criminal liability on any person who sells or transfers a firearm to a prohibited person, regardless of whether such seller or transferor knows that the buyer is a prohibited person. A defendant seller or transferor may assert as an affirmative defense against prosecution evidence to prove that the buyer or recipient passed a background check or possessed a valid concealed carry permit in the state of transfer. The affirmative defense does not apply if defendant knew or had reasonable cause to believe the buyer or recipient was a prohibited person.