Butch Searcy (Boron, CA) has made more side-by-side rifles (and some shotguns) than
any other US maker. He's been building them for thirty years and his total production is probably around 500. The last time I checked, he got his tubes from Pac-Nor and then
threaded them into a mono bloc that he manufactured. I have no idea if he would barrel
someone else's action - but he is certainly capable. Most of his rifles are .470s and .500s.