Some great pictures and info from Springfield Armory National Historic Site facebook page.
As the city of Springfield grew, it became harder and harder to test firearms at Springfield Armory. A small test range existed onsite and a larger indoor range existed at the railhead but an outdoor site was needed, especially as the weapons to be tested became larger and the Armory became more involved in aircraft armament. Test firing was conducted on Cape Cod and the Shores of New Jersey but something closer was needed. Eventually, in the mid-twentieth-century, the Quabbin Reservoir became the outdoor test site for Springfield Armory. Quabbin Reservoir was created in the 1930’s as a drinking water supply for Western Massachusetts and is not only the largest inland body of water in the state but one of the largest human created public water supplies in the country. As the Armory utilized the site more and more in the early 1950’s, infrastructure had to be developed. Eventually, multiple firing platforms and storage facilities were constructed. Many different arms were tested there including the Vigilante and Vulcan (as discussed in previous posts) as well as numerous types of small arms. Today the former test site is a part of the Department of Conservation and Recreation (DCR).