stamp mill vs stamping mill

stamp mill and stamping mill both are nouns.

Word	Noun	Adjective	Verb	Adverb
stamp mill	Yes	No	No	No
stamping mill	Yes	No	No	No

As nouns, stamp mill and stamping mill are synonyms defined as:

stamp mill and stamping mill: a mill in which ore is crushed with stamps

stamp mill (noun)	stamping mill (noun)
a mill in which ore is crushed with stamps	a mill in which ore is crushed with stamps