So I got to gunsmith part 4, and went to build the gun per the wiki, because that is easy. However doing that will set you back about 600k because everyone wants the Mini Monster suppressor (250K) and the 260mm barrel (200k)
One way to get around this is to just wait until you can buy everything from the traders. I did not want to do this, so I set out to complete this task without changing the barrel, and without using the Mini Monster suppressor. I came up with these 2 different builds that are both very close to the same total cost. It is about 1/2 of the wiki price.
Hope this helps at least someone out.