"It seems there are very few gun manufacturers that actually make all their own parts."
It's too expensive to set up to mass produce quantities large enough to make it economical.
I'm aware of one gunmaker who advertises he makes all of the parts in house, but they make shotguns and only produce 2 or 3 dozen a year.
Depending on the level of engraving, you're probably looking at $200,000 and up.
" Every single part of the shotgun is designed and produced in house, from the smallest screw to the barrels, actions and everything else"
Check out the slide show of clients: King Juan Carlos, Steven Spielberg, Tom Selleck, etc. I like the caption under the pic of Spielberg's Jurassic Park shotgun - "...one of his..."