All I can offer at the moment is SAAMI’s glossary definition:

Actuator – Part of the firing mechanism in certain automatic firearms, such as trigger actuator, which slides forward and back in preparing each round to be fired.

Footnotes / Sources:

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