The White House on Monday defended the Obama administration’s move to ban certain bullets used in a wildly popular rifle, a proposal that has stirred opposition from Second Amendment advocates and a prominent member of Congress. The Bureau of Alcohol, Tobacco, Firearms and Explosives is considering a ban on M855 ball ammunition, which pro-gun groups have said is among the most popular cartridge used for the AR-15 rifle and is frequently used for sporting purposes like hunting, because of its so-called “armor-piercing” capabilities. “It would be fair to say, as we are looking at additional ways to protect our brave...