An annulus is circumferential crimp or indentation creating the ring-like space between the primer and primer cup.

The military is known to use colored dye in this ring to both waterproof the round and identify cartridge characteristics.

Footnotes / Sources:

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

“A Cartridge Collector’s Glossary ⇒.” International Ammunition Association. Web. 27 May 2014.