Not all guns are considered equal and while all are dangerous weapons some are more so than others. My battle rifle happens to fall on the "more so" side. I know how to handle it, but the gun range can't take for granted that other people do. So if they allow me to use my battle rifle, they'd have to allow everyone with access to military grade weapons to fire them at the range.
That...and it's military grade. Having one and not being technically part of the military anymore also raises a few eyebrows.