Members of the 1st Vulcan Combat Team (Provisional), fire the M61 20mm Vulcan anti-aircraft gun, mounted on M-113 Armored Personnel Carriers  (the M163 Vulcan Air Defense System) at Viet Cong bunkers during operational testing.
SP4 F.E. Mitchell, 221st Signal Company (Pic)
<a rel="nofollow" class="external text" href="https://www.fold3.com/image/1/270427941">NARA 111-CCV-50-CC58293 via Fold3</a>