The 50,078-acre Southwestern Proving Ground was used by the U.S. Army between 1942 and 1945 to test small arms ammunition, 20 to 155 mm projectiles, mortars, rockets, grenades, and up to 500-pound bombs. The proving ground was closed shortly after World War II and ordnance was cleared from the surface of the land. The land was then conveyed to state, municipal and private owners. Since the clearance after World War II, MEC have surfaced.

In the early 194Os, munitions filled with mustard gas were tested at SWPG. These included the 105mm projectile, 155mm projectile, and 30 lb bomb. Several additional chemical shells may have been used or stored at SWPG. The Chief of Ordnance wrote a letter to the Proof Department at SWPG regarding changes in the Safety Phase Test as part of the ballistic acceptance tests that were conducted there.