The first gas operated machine gun, a hand made model by John M. Browning, known as the "gas flapper". This gun was conceived and developed around 1889-90, caliber .44/40 W.C.F., offered to the Colt Firearms Manufacturing Company in 1890.
Photographed by E. A. Metzger
ROCK ISLAND ARSENAL ORDNANCE CORPS (<a rel="nofollow" class="external text" href="http://www.browning.com/content/dam/browning/news/articles/2016/q2-2016/browning-flapper-article/2nd%20generation%20flapper%20operated%20machine%20gun%20john%20browning.JPG/_jcr_content/renditions/cq5dam.web.835.835.jpeg">Link</a>)