All depends on luck and skill if the ambusher is sotted or not, or if the ambush fails and the passenger troops then notice the attackers and squirt some lead into their faces.
However, putting a sniper or 3 man scout on escort duty is not a good idea for thier health, if the enemy is using 12+ man sections .
Then again a hit penetrating the "cargo" bay of an armored vehicle doesn't neccessarily kill thw whole vehicle.
I remember I report on telly I saw about some poor Soviet fella whos BTR was hit by a Mujaheddin RPG in Afghanistan.
He was the only memeber of the infantry squad who was unarmed. And so was the crew of the BTR which wasn't even immobilized by the attack...
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If you find yourself in a fair fight, planning went wrong.
I have seen this alot while playing. I have had apc hit by rifle fire while ambushed and troops killed inside. I have noticed that certain classes of veh this seems to happen with. Just the other day i was playing with a v-150 in the green oob and this happened. I have seen it alot with the bradley scout version also. The scouts always seem to die in an ambush.