Federal agents on Thursday raided one of the nation’s largest manufacturers of ghost-gun parts, a sign that federal law enforcement is cracking down on kits that allow people to make weapons at home. The raid target, Nevada-based Polymer80, is suspected of illegally manufacturing and distributing firearms, failing to pay taxes, shipping guns across state lines and failing to conduct background investigations, according to an application for a search warrant unsealed Thursday after the raid took place. The probe focuses on Polymer80’s “Buy Build Shoot Kit,” which includes the parts to build a “ghost” handgun. The kit, which Polymer80 sells online,...