Jorge-
If you are looking for a scientific answer, please await the next response. I would not fire slugs or buck shot out of any of my older doubles. But I wouldn't do it in my newer one because I have a new 870 express designed for those applications.
I understand that the post war Sauer are very well built and use very high quality steel, but I have seen a number of the Sauer/Simson/Merkel, particularly those with the scalloped receivers, with cracks in their stocks, repairs and cross bolts. These cracks were most likely due to firing regular loads. Slug and buck loads do tend to kick harder, so for the well stated reservation in your inquiry, concerns over the stock, I would not shoot them in my doubles.
Stocks are typically the first thing to go on these shotguns, after all wood is the softer part of them and shotguns are often stored muzzle up after cleaning and oiling and the stocks can become weak from the soaking.
Doug
