My main reference are the pre WWII Czechslovak border defenses - these do have gun shields & apertures that get wider to the outside:

https://commons.wikimedia.org/wiki/Category:85mm_kan%C3%B3n_... https://commons.wikimedia.org/wiki/Category:4cm_kan%C3%B3n_v...

Not that the apertures are stepped instead of smooth - I guess the avoid the funneling effect mentioned above, possibly changing a near miss to deflection and hit on the gun shield. Should not happen if the steps get in the way.