Ridge is on the ball for this one.

The Baikals get labeled in Euro as 12-71, 12-76, or 12-89, occasionally 12/71... Which is 12ga 2 3/4, 3" or 3.5" (71, 76, and 89mm).

BUT, they also happen to offer then in 66, 71, and 76cm barrel lengths, corresponding to 26, 28, and 30" barrels. Personally, I'd pass on the 76cm, then flip a coin between 66 and 71cm. For most of my hunting, 26" has treated me just fine, but I honestly don't notice much difference for a 28" (76cm) barrel. I DO start noticing a nose weight difference and swinging weight difference for 30" barrels though.

The Baikals I have owned were all double barrel side by side shotguns, which came with threaded barrels and full, modified, and improved choke tubes.

The current listings that I can find for the MP155 state that it comes with interchangeable choke tubes, so I'd assume that all 3 sizes are included.