500 Nitro,
I have seen and handled a few, but all at the Hartmann &Weiss facility in Hamburg. In the past 10 years, I visit them regularly, at least once a year. The guns which I handled include over and under ( boss action ) guns, side by side guns, and a few side by side rifles. As far as I remember I handled DB rifles in .375 H&H, 500 N.E., 577 N.E. and a .600 N.E. Because they have their own shooting range build in house ( in the basement ) I also got the opportunity to shoot their .577 N.E. once, with a couple of rounds.
At my visits, Hartmann & Weiss most always has some British guns and rifles available for selling, so you can judge and compare them on the quality side by side.
I do not want to start a flaming, but I can clearly say that on the H&H rifles what I was able to inspect there is a quality difference even between old H&H side by side rifles and newly build in favour to the old ones. ( I am not talking about engraving, but gaps between wood and steel and so on ). The most disappointing rifle was just recently build, one of a pair Holland & Holland build 577 Rewa's. The visible gap between the side look and the frame on the left side could hold a toothpick!
I have never seen something like that on any of the Hartmann & Weiss guns or rifles which I handled in all the years.
Yes, it is very difficult to find a second hand double from Hartmann & Weiss. But if you think about the amount of shotguns and double rifles being build in a year and compared those to the amount of bolt action and falling block action rifle build in a year in the Hartmann & Weiss shop, you may find this as the reason for the very limited second hand double guns and double rifles available. All others like bolt or falling block actions you will find if you look carefully also second handed.
Again this is just my opinion, but for those who have doubts about the sidelock gap at the H&H I have pictures available.
Robert