17mm what is a 16 ga or maybe 20 ga double with 12 ga shotgun barrels by B. Pohl in Wien
https://www.liveauctioneers.com/item/42531376_an-outstanding-original-cased-double-barrelled

"Description

dating: third quarter of the 19th Century
provenance: Vienna
Two pairs of barrels of very fine damask, one pair rifled, the other smooth, both with decorations in gold at the base, with rib, foresight and the signature "B. POHL IN WIEN" in gold, the rifled one with double, blued rear-sight, cal. 17 mm (the smoothbore is 12 gauge); long tang with monogram "MH", richly engraved and gilt with ibex and floral motifs; backward spring locks with hound and deer in gold between partially gilt floral motifs; receiver with hunter near a large, killed deer, with rifle and hat between thin and deep floral elements, at the trigger guard a bear, provided with a lever to open the barrels, with sculpted, horn tail, iron butt-plate with pheasant and floral motifs; butt of fine briarwood sculpted with deers and fox between bas-relieved floral motifs. In its wooden case covered with leather, with brass mounts; featuring the noble monogram "AP" with Dukes crown.

dimensions: length 115 cm"