Gordon L. Rottman 
Browning .50-caliber Machine Guns [PDF ebook] 

The Browning 50-cal has become the longest serving weapon in the US inventory. The fifty has been employed in every imaginable role for a machine gun. It is considered such an effective and reliable weapon that few countries ever attempted to develop an equivalent weapon. Even the Japanese created a copy of it during World War II when the US was producing literally thousands every month to use in every theater. This is a history of the development of this famous weapon, its most critical operational use and the variants that have been produced to keep it at the forefront of the action.