A bit late to the game, since this rifle has been out for 10 years. The lack of technical details available led me to write an internal and upgrade guide myself. Taken from a writeup I put together for my local group:
https://docs.google.com/document/d/19EZn2DgiSSANZsLn4gtlVTp4V9gO-Yv_RZc2uJPndm0/edit?usp=sharing
I'll leave all the UKSF impression-specific stuff out and just repost the technical bits here. If you're interested in building a UK-spec HK417 (a.k.a. the L2A1 SACTARASS), I believe I have most every relevant detail covered in the link.
Without further ado...