After carrying the M16 or one of its cousins across the globe for more than half a century, soldiers could get a peek at a new prototype assault rifle that fires a larger round by 2020. Army researchers are testing half a dozen ammunition variants in “intermediate calibers,” which falls between the current 7.62 mm and 5.56 mm rounds, to create a new light machine gun and inform the next-generation individual assault rifle/round combo. The weapon designs being tested will be “unconventional,” officials said, and likely not one that is currently commercially available. Some intermediate calibers being tested include the...