Charlie, the easiest answer for you is a handle bar mount off of a 4 wheeler adapted to your bike. But putting that heavy old 8 ga. up there I would be careful about weight. All that extra weight over the front wheel, when you hit the front brakes you will be going ass over tea kettle over the bars.( the visual in my mind is unbelievable)
The other option is a scabard hung saddle style under the top bar gun but out the rear left side of the frame.