Receiver sight for Crosman 180 2nd Variant

Receiver sights for the 180 are as hard to find as a needle in a haystack.

I have an idea of a method to adapt an existing receiver sight to the 180 2nd variant, but until the one I have coming I need a dimension. I need the outside diameter of the barrel/receiver at the rear where the Breech Plug is inserted. If someone could measure with a mic or good vernier I would appreciate it very much. I don't plan on altering the gun to do this. 

After I get the gun and if my idea works out, i'll post pictures.