No, it has nothing to do with the steel, and it isn't a strength issue. Great handling dynamics in double guns, especially double rifles, require good balance AND proper concentration of weight - 50% of the weight in one third of the length - or "weight between the hands". Barrel contour is used to concentrate weight at the action, and is just as important as an action of the correct size for the caliber. It's impossible to get it just right when you start with an action that is too small.

It's also next to impossible to do it at an affordable price. You can't profitably put a price point DR like the Heym, Merkel, Krieghoff, Chapuis, etc., into the hands of the consumer if you build every one on a caliber specific action. So, some calibers will handle better than others. That's the trade you make for affordable price. My point was that a .400 built on the action that Heym uses for the .470, with better barrel contour, might come pretty close to right.

9.5 lbs is a pound too light for a .470.
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