Information is definitely out-dated, and you shouldn't really take it to heart.
The XS1 Scope was an optical attachment for the Railgun.
It added a scope which you could use to zoom in to see zombies.
As part of the Railgun package, K177 included the XS1 Scope with every gun given out. This particular scope was designed specifically for use in conjunction with the Railgun. No other gun can properly be used with this scope.
Unfortunately, the people chosen to wield the Railgun quickly grew arrogant with its use. Scientists forcibly and quickly modified the Railgun to be a fraction of its former glory. When that wasn't enough, the Railgun was taken away entirely. The XS1 Scope had no other choice but to leave with the only gun it was made for.