You must have Quick Weapon Select on to do this trick.

If you want to switch sniper scopes (Plasma Rifle 
and Tek Bow), first pick one of them and turn on 
the scope. Then switch to the other weapon and 
QUICKLY press D-pad Right (the button to turn on/off 
the scope) which will mix up the scopes.

This can be done from Tek Bow to Plasma Rifle or 
Plasma Rifle to Tek Bow. You may have to try it 
several times before accomplishing this task.