Up until a certain point of Armored Core, sniper rifles were something of the neglected child of the rifle family. While they dished out absurd amounts of damage and had incredible range, their small lock boxes and slow firing rate made them nearly useless at medium to close range, which is where most AC fights happen.
So there they sat in the shops, unwanted, unloved, until From Software hit upon two changes that suddenly made it a very, very bad idea to bumrush anybody packing them. The first were left-arm mounts, which allowed users to expand the SR’s miniscule lockbox by synergizing it with the much larger lockbox of handguns in the left hand.