Gangrene following a gunshot laceration of the femoral artery
Dry gangrene is caused by acute or chronic loss of blood flow to the distal part of a limb, and is most often seen these days in those with poorly-controlled diabetes and in life-long smokers. However, it can also occur if the limb suddenly loses circulation, such as in a thrombosis (blood clot), or a lacerated artery, as is seen here.
Without no circulation, tissues begin to die immediately, and spreads outwards until the point where bloodflow is adequate to keep tissue alive (in this case, probably around the point of laceration). Assuming no bacterial infection took hold above the gangrenous area, and the healthy tissue sealed itself off successfully, the end result without surgery would be the drying up and falling off of the necrotic tissue, in a process known as autoamputation.
However, the number of confounding factors in possible autoamputation scenarios is vast, and surgical intervention is called for whenever possible.
An American Text-Book of Surgery. Edited by J. William White and William W. Keen, 1894.