Hi Jean-Marc,
Eighteenth c. hunting guns are not my main topic but I will have a try at this.
This is a wheellock hunting rifle of common type which actually continued being made almost unchanged for quite some time. The earliest possible date I would assign to your gun is ca. 1720-30, based on the style of the lock engraving.
Georg Friedrich Dünnewald is recorded working from 1742-60. Though actually residing in Friedrichstadt, Saxony, Eastern Germany, he is known to have signed his works '
a Dresde(n)' although he failed to join the Dresden guild.
Thus, we should date your rifle to the mid-18th century.
Best, and have fun with your nice rifle,
Michael