Wayne;
I've visited a lot of bunkers and the big firing opening is always the front. The K & C bunker is equiped with a concrete support that not only serves as a support but also as a protection for who is entering or leaving the bunker from fire coming from the front. Your last picture is the position I would choose.
guy
Your right Guy the side walls are to protect against blast . We're lucky in Europe there loads of bunker with Jersey having the most per mile