The Montenegrin "centerfire" cartridge goes by many names, but usually "11mm" is part of the name. Bore diameter runs as tight as .442" and as large as .449". Cases can be formed from 45-70 or 7.62 x 54r (aka: 7.62 Moisin Nagant).
It might be possible to drop in a 44-40 and make it go bang, but it's definitely not the correct cartridge.
Accurate Bullet Mold company can make a correct diameter bullet mold custom fitted for your specific revolver. Their prices are quite reasonable.
But, everything I mention is specific to the centerfire cartridge. The pinfire is a different question, although I believe the details about bullet diameter are correct for both cartridges.
By the way, attached is a picture of King Nicolas the 1st with a Montenegrin Gasser revolver.
Addendum (correction??): After some digging, I found a couple Montenigrins that were stamped "For .44 Winchester Cartridge" on the top of the barrel. But, the stamping looks different than the other markings. And, these all seem to be licensed copies made in Belgium.