The Post and Public Advocate Bill de Blasio are pushing for the NYPD to stop buying their guns from Glock if the company won’t stop selling totally unnecessary high-capacity magazines like the one Jared Loughner used in Tucson. Surprisingly, Mayor Bloomberg, one of the nation’s foremost gun-control advocates, doesn’t much like the idea, but he has a good reason: The police department buys its guns from three manufacturers Glock, Smith & Wesson, and Sig Sauer and all three sell the magazines. How about this then: The first one to get rid of a product that’s mainly used to kill people efficiently gets all of our business.