5" Gauge GWR Self-Contained buffer

Self contained buffer in designed in FreeCAD, it was a 10min job last night so not parameterised in any way. Uses a 7x12.5 compression spring and a M3x20 cheese head screw. You'll need to tap an M3 thread in the Head and fettle a little.

The design was inspired by this webpage [Self Contained Buffers](http://www.cherryclan.com/2015/08/self-contained-sprung-buffers).

I added the "b" version which has extra supports. This also has a template for centering the mounting hole with a 3mm drill before drilling and tapping M2.5.