I was a Machine Gunner for 16 years. That was "NOT" blade slap.

That was "Mk 19" Training rounds fire.



Mk 19 grenade launcher
[link to en.wikipedia.org]


The Mk 19 can be mounted on virtually any vehicle, boat or helicopter using the Mk 64 Carriage and Cradle. It can also be mounted in a turret, such as the US Marine Corps 40/50 turret. Examples of vehicles on which the Mk 19 is mounted are the HMMWV, Chenowth Fast Attack Vehicle, M113 APC, Land Rover and US Marine Corps AAV7A1 amphibious vehicle.
[link to www.army-guide.com]