World Field Target Federation target checking device, plans source code and instructions for manufacture and assembly.
There are 8 parts that need to be printed, the STL files can be found in the STL directory. All dimentions are in mm and the parts are oriented so that no support structures are required.
For best results the parts should be printed in PLA/PHA filament (ColorFabb recommended) The Left and Right hand Cylinder parts may be prone to some curling on the edges and as a result it is recommended that they are printed individually.
Note: This is the spring supplier on eBay that I have used to make the official WFTF checkers : http://www.ebay.co.uk/itm/12-7mm-OD-13mm-Stainless-Carbon-steel-Compression-spring-up-to-600mm-long-/231995371162?var=&hash=item3604006e9a:m:mkqkKRZ80jLM1G65dUfObjA