The problem is also that the gun shield doesn't even properly protect the gunner from the front, as half his helmet pokes up over it.
It's one of the biggest mysteries of WW2 for me why the Germans didn't make the shield just a tiny bit bigger.
I know what you mean. I have no sources at all and can therefor just make some simple assumptions besides a simple oversight by design. One of which would be, and thats what I believe is the strongest, that the designers had situational awareness in mind. In firing position the gunner would have to raise his head just a little to observe the full battlefield again and could potentially switch to another target faster without changing his stance to much. With a larger shield he would have to leave or change firing positions and would take more time to be on target again.
There was a saying in my conscription time in the german army that burned down so that I remembered it pretty clear: "Wirkung vor Deckung !" (Effect comes before cover)
Little anecdote: On a simulated convoy attack we had to search for firing positions individually. Mine was a rather big hole besides the road and in front a big tree - best cover in this circumstances I thought to myself. My instructor yelled at me "Wirkung vor Deckung, Gefreiter Langer !!!" How would I be able to lay destructive fire at any of these vehicles ? Well... he was right.
That means that the best cover will not accomplish the mission, which was destroying/hold up those vehicles.
So I could imagine that in terms of the Sfkfz 251/1 it was a trade between reasonable cover and fire effectiveness.
And to be honest I never read about any complains that this was a big problem back than. I guess most of the crew casualties were inflicted by bigger calibers, that the armor wasn´t even designed to protect against. (If someone has some reports o this I would be grateful)
In game I would say that the player/the tac ai is sometimes more willing to take risks and shoot at opened up vehicles than it would be in real life. Shooting a head sized object at several hundred meters with just iron sights should be difficult and here it is just 1/2 to 1/4 of a head sized object due to the shield.
Most of the times I´ve lost an MG gunner on these vehicles were due to several mg salvos, and some to rifle fire (presumably sniper or lucky garand shooters). But in contrast to american halftracks the losses were not significant.