You might have a few issues with making a gun, from Shapeways FAQ Are there prohibited products? Explicit sexual content and weaponry look products are not accepted on Shapeways, neither private, nor public. Any file containing such content will be removed without further notice. We are allowed to make an exception for miniature weaponry (maximum size 10cm).
Apart from that, you can use a normal digital camera to take a series of photographs of the parts and upload the images to an online service or use software that specialises in creating a mesh from the images - google for '3d mesh from images'.