I'd look at the photos. MikeV said probably rifle converted from rf to cf in this barrel and I guess as I told before this barrel been overbored to smooth bore.
I never saw factory made small caliber rifle with smooth bore and proof mark crown/G, all this smooth bore rifle been overdrilled.
Geno, please reread my posts above. These garden guns were usually built to be free of licensing. This required a maximum rifled barrel in 6mm /.22" or a smothbore of 9mm maximum bore diameters. A 9mm rifled barrel required either a hunting licence to be even carried outside of approved shooting ranges. That's why these garden guns usually have a 6mm Flobert rifled barrel or/and a 9mm smooth one. Look at the proof rules above, 16b about the crown/G stamp: Rifling is not mentioned here, only barrels meant for a Einzelgeschoß = Bullet. Contemporary Busdorf in his book "Wilddieberei und Förstermorde" relates a murder case, to which key factors were two long, above the legal minimum of 20cm, single shot break-open target pistols, special ordered by a poacher and killer from Venus-Werke O.Will in Z-M, smoothbored and chambered for the 9.1x40R. Will had two of his standard offerings rebored smooth and converted to centerfire from .22 rimfire. The crook used it out to 20m with the case full of bp and a round ball. Must have been effective enough, because he killed not only roe deer and boars with these pistols, but a forester too.
MikeV mentions the proofmarks. at the time of proof the barrel was stamped 8.8 (mm) = .346" by the proofhouse, so a plug gauge of that diameter entered the barrel for 20cm, while a 8.9 = 350" one did not. This is a proper bore diameter for a 9.1x40R. "Overboring" such a barrel to remove the rifling would lead to a bore diameter of 9.1mm = .358". MikeV mentions a bore diameter of .336", perhaps a typo from .346, in any case much too small for a bored-out rifle barrel. As I wrote before, smooth barrels for the 9.1x40R were proofed for the bulleted cartridge with 1g bp and a 150gr lead bullet, as this load could be chambered and fired in them.
