http://www.nrw2000.de/ns/pics_ns/ruhrkessel.jpg

http://1.bp.blogspot.com/_V7Ehj7eG65Q/SBZBfavjMdI/AAAAAAAAAA4/08p7xBdHxYU/s1600-h/mg-peri.jpg

http://media.photobucket.com/image/s...iscopeRifl.jpg

Periscope-aimed weapons are old news. It works best if recoil is no problem:
Panzerfaust - recoilless
MMG - tripod handles recoil
Sniper rifle - single shot (recoil affects no follow-on shot)

Simple trench periscopes are even more common (although not officially supplied in many armies).