I've played all the campaigns in CM-Black Sea, and I have noticed that soldiers occupying turrets in HUMMWVs are unrealistically vulnerable to small arms fire. I've had HUMMWVs engage dismounted troops at 400m, and had the turret gunners killed within 30 seconds, and then their replacements killed the next turn.
I've an active duty combat arms soldier in the US Army for 19 years, and I have had experience with using HUMMWVs downrange on combat deployments. It seems like it would be easy to pick off a turret gunner, but in reality, it is very difficult for someone to hit a guy in a turret, even back when they only had a forward facing plate (or in some cases no protection at all). The gunners in the HUMMWV turrets with 360 degree protection and bullet proof view ports are effectively invulnerable to small arms fire at anything beyond close range plunging fire (except for a one in a million round that slips in through a small gap), especially if they lay low in the turret. They actually do quite well in urban combat against fire coming from multiple directions and elevations in my experience. Gunners do occasionally get hit, but it is rare.
The game is fantastic. However I think this mechanic needs to be fixed to reflect reality. As it is I've found the HUMMWVs so vulnerable that they cannot even be pushed forward against realtively light resistance (which is not reality).