[Hectic]

The screen protector will stop the bb from entering the scope however the glass will still crack as the protector is flexible so it will slow the bb, likely prevent full penetration but you will still wind up with a broken ass sight.
lexan, the black watch kill flash or another similar product is you best (and really only) options for full protection . Those protectors would be great for the viewing side tho to protect from scratches and such but clarity could be a factor, especially in low lighting.