10-1000 USD or more depending on specifics
Not possible answer this without more information- you have told us who made the gun (DWM, the German national arms company) and where (Berlin)- but nothing about the gun. While this MAY be an 8mm Mauser type rifle, the 8mm cartridge was made in 2 different versions at different times. The safest route to an accurate answer will be to have a gunsmith examine your rifle. Many military surplus rifles have been rechambered for civilian cartridges different from the original caliber- a gunsmith can tell this.
It would depend on the type. There was a rifle, carbine, and engineer's model. If it was in good shape, I would say that the rifles would go for $150 to $200, the carbines for $160 to $210, and the engineers (which are fairly rare) for $250 to $300. Many of these rifles were sporterized and that would lower their value somewhat.
Really can't say- not enough information. POSSIBLY a Spanish Mauser, but could be others (Most of South America also speaks Spanish) 1806 is not a date, since DWM did not exist then, nor did cartridge rifles.
$200-$300
10-100 USD or so
50-1000 USD or so depending on specifics
Provide a detailed description of all markings, finish, barrel length, finish on all parts of the rifle and type of finish on the stock.
That little shotgun was made by Deutsche Werke, a German company that had operations in Kiel, Berlin, and Erfurt. The Erfurt plant made the shotgun and a youth 22 rifle with the same loading and firing mechanism. They were more famous for making the Ortgies pocket pistol, a semi-automatic in various calibers. These arms were made from 1919 until some time in the 1930s. The pistols can be found regularly on the online auctions, and the 22 rifles less often. The shotguns are even rarer, but not of great value. I saw two offered online in the past couple of years, and the price was around $250 - $450.
I too am looking for this answer for a rifle in Southern Va. The "S" on this one appears to be a snake with a wing.
It is most likely the makers stamp although it is hard to tell without seeing it properly.
why dont u ask ur grandfather!