At last some progress, first off the HaT cavalry guns, rather too wide and also only provided with the howitzer version. To make the normal gun version I used some left over Airfix RHA barrels. They are rather too wide so it is going to be difficult to find some suitable plastic riders.
As I already had sufficient (converted) limbers for my Napoleonic Austrian army I decided to convert the drivers into the earlier 1798 style with grey coats and round hats for my Revolutionary Wars Austrians. Given the rather rounded shakos it was an easy job to cut them off, reduce by 2mm, add a circular chad from my hole punch and reassemble. On further checking I found that they were foot not horse artillery limbers
The caissons were split three ways one each for the SYW, Revolution and later Napoleonic wars.
Certainly I'm going to look out for some more round hat conversions.