Thank you so much, Fernando, you're just great:  
 
Yes, this is the one I was talking about!  
 
And here is the original 
Österreichisches Kürassier-Tromblon M 1759, the very model that the barrel was taken from. 
Barrel length 65.6 cm, bore at muzzle 44 mm, at tang 26 mm.
I hope that Tobias Capwell has fun taking the measurements of the barrel of the Wallace 'wheellock blunderbuss' ...   
  
 
Scanned from Peter Krenn, 
Die Handfeuerwaffen des österreichischen Soldaten, Graz, 1985
I deaccessioned Baxter's 
Blunderbusses a few years ago from my library just because there was not one single honest 
and early piece in it.
Best,
Michael