antimonial lead vs hard lead

antimonial lead and hard lead both are nouns.

Word	Noun	Adjective	Verb	Adverb
antimonial lead	Yes	No	No	No
hard lead	Yes	No	No	No

As nouns, antimonial lead and hard lead are synonyms defined as:

antimonial lead and hard lead: a lead alloy that contains about 5% antimony

antimonial lead (noun)	hard lead (noun)
a lead alloy that contains about 5% antimony	a lead alloy that contains about 5% antimony
	unrefined lead that is hard because of the impurities it contains