Some of the things on the warrant against the Branch Davidians included statements about them owning the tools for making automatic weapons conversions for their otherwise legal guns. Listed tools included milling machines and lathes.
That's not unusual for warrants. If the FBI had other reasons to believe that they were illegally converting semi-auto guns to full-auto, then the machinery and tools are naturally going to be seized as potential evidence.
It's no different from executing a warrant on a drug lab - the police are going to seize scales, baggies and other paraphernalia in the lab as potential evidence, even though they may all have other, legal uses.