President Joe Biden promised to tighten controls on homemade firearms.
The justices will review a lower-court ruling that upheld New York’s restrictive gun permit law.
At least eight people were killed and several others were injured in a mass shooting at a FedEx warehouse in Indianapolis.
"If you can't tell the difference between those two things, it's crazy," the conservative TV evangelist said.
Investigators were still trying to determine the motive for the attack, authorities said.
The president has been pressured to act after two major mass shootings took 18 lives in March.
As of July 1, people can buy handguns from private non-licensed sources such as websites, gun shows and individuals without a permit or background check.
Parkland dad Fred Guttenberg ripped the South Carolina Republican for showing off "with the weapon used to kill my daughter."
South Dakota Sen. Mike Rounds posted a photo of a statue of himself holding a gun with his dog, raising alarm on Twitter.
The "Late Night" host issued a stark reminder of America's "epidemic of gun violence."