Had a closer look at Favé's Swedish-type gun illustrations as well as the fragments that come with St Remy ed 1745.

As I would complete the missing data from St Remy page 323 "Table des dimensions d'un Canon …[ ? ]
1) Longueur de la piece from rear of platte-bande to face of muzzle [ 4' ] 6'' – i.e. 18 ball diameters of [Paris?] 3'' - makes most sense here.
2) length of soul / bore is [ 4' ] 3'' 3''' - i.e. minus 2'' 9''' which is diameter of barrel wall at rear of 2nd reenforce as per below. Obviously, the walls were scaled less then 24/24 parts solid, or 'vollgütig' as you say in German.
3) Length of 1st reenforce is 1' 3'' 6'''
4) Length of 2nd is 9''
5) Length of Chase up to [centre of] muzzle astragal is 1' 11'' 6'''
6) Muzzle is 6''
7) Length of button section is 6''
barrel wall diametre at bottom end of 1st reenforce is 2'' 9'''

I placed the Favé barrel onto my grid system. Makes a rough fit. Button section, though is more like 2.5 ball diametres.
So, I need to correct myself. According to Favé and St Remy table of dimensions, the piece really is of approx 18 calibres.
Converting from decimal system into a 12 system, causes me endless trouble :-{

Cheers,
Christian