you wrote "I guess that this period doesn't attract as many researchers as some!"
Without Napoleon, what is there to write about? ;-)
My interest about this particular timeframe is the Austrian Military Airgun, M1780. These were issued to the Tyrolean Sharpshooters. The only usage described is at the Siege of Valenciennes
There are only two mentions of these airguns in the literature during this campaign (Excluded Haller 1891)
Published anonymously in London 1796, “An Accurate and Impartial Narrative of the War” has on page 52;
“Air guns were made use of at that period by a corps in the service of his Imperial Majesty, constructed on a new and curious principle of mechanism; the ramrods acting upon them as pumps, to load the barrels. The inventor, if he still exists, is kept, as it is said, in the close confinement at Vienna, lest the secret should by any means transpire.”
Then there is a companion 1797 text, also written anonymously, published in Zurich, in German, Freimüthige Beyträge eines brittischen Offiziers zur Geschichte des gegenwärtigen Krieges (Frank contributions of a British officer on the history of the present war.)
My translation from the German, so, no guarentees;
“The (French) cannons, which were found on the walls of Condè, consisted for the most part of thirty-two pounders, were immediately moved to the trenches before Valenciennes, to be put there on the batteries of the third parallel. Here they did the besiegers very good service, shooting to demolish and breach the fortifications. It was there with the advanced trenches as close to the fortifications, that the Scharfschüzen and shooters out behind the sandbags, which stood on the parapet of the third parallel, could pick off the French, who appeared on the covered way, with little effort. There was nothing unusual in one moment both parties talking together and in the next to be seen firing at each other. We then had a corps of soldiers that stood in the imperial service, armed with a whole new type of airgun, which was made use of in such incidents. It was in such a way equipped, that the ramrod, in addition to the usual benefits, also had this, in that it pumped the air. The inventor of these guns still presently lives in Vienna, but he is in close custody, so that his secret is not made known.”
By the way, the bit about the inventor (Girardoni) in close custody is not true but it was the common belief at the time. Girardoni moved to the Vienna suburb of Penzing to build the guns and was only restricted to neither divulge the workings of the gun or sell it to anyone but the state.
Not too long after the end of the 1st coalition, airguns along the same lines as the Austrian model were being produced in England. One possibility is that the English -who clearly observed the airgun in action- picked up a sample on their way out. So, If you come across any mention of airguns in your research on the Duke of York, sure would appreciate a note.
The gunmaker best known for his airguns, including the English model of the M1780, in London during the period is Samuel Staudenmayer; who was the gunmaker to both the Prince Regent and the Duke of York. The Regent is known for enjoying airguns, some of which can be seen in Royal Sporting Guns at Windsor by Blackmore.