From Courthouse News:

Shearer, who is co-counsel in the lawsuit, said state gun laws have been undermined by the ATF’s refusal to treat the parts used to build homemade guns as firearms. This allows “unscrupulous” DIY firearm sellers to flood the gun market.
“The ATF has specifically exempted them from the federal definition of a firearm, meaning federal gun laws don’t apply to them. As a result, people can order and build these guns without passing background check or even verifying their age,” she said. “This has opened the biggest loophole you can imagine in our federal and state gun laws.”