How in the world do you keep something supplied that shoots 250 rounds per second? Is this a Mass Effect 1 rifle where you just slam a block of tungsten in the

How in the world do you keep something supplied that shoots 250 rounds per second? Is this a Mass Effect 1 rifle where you just slam a block of tungsten in the back? It feels like we've mostly converged on the ideal form of a general purpose battle rifle over the past fifty years.