[Pibwl]

I guess it protected the layer, behind his sights, quite well, against accidental shots, and even not precisely aimed MG bursts. Of course, it won't be sufficient against snipers and well-aimed MG fire, but I don't think it's a purpose of the shield. They're not meant to protect in a long duel. Even if the layer and loader are cramped and hidden behind the shield, a precise rifle fire would make impossible delivering ammo and would pick the rest of a crew. Note BTW, that Pak-40 has spaced twin layer shield, pretty good against rifle bullets.