Low brass is a colloquial misnomer for shotshells having a low metal head.

Low cup is the correct nomenclature for shotshells having a low metal head.

related:

• base, low
• cup, low

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.