An action bar is a gun part that transmits movement from one part of the mechanism to another. Most commonly its joins the fore end or gas system with the bolt. They may come singly or in pairs.
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Footnotes / Sources:
“Glossary ⇒.”ﾠSporting Arms and Ammunition Manufacturers’ Institute.ﾠEd. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.