being that it is a concept item that he put on the internet and that modern CNC is now affordable on small scale production, the only difficult parts of that weapon to make is the barrel and that is if you want one that has a reasonable lifespan and is reasonably accurate (due to barrel length I highly doubt the 400m accuracy stated on the concept).
great ideas. I see some possible issues in functionality but other than that pretty neat idea that is a possibility given the right materials and equipment to construct it.
Michael