I think the roller chain links are the fireing chambers for each round and rotate into position between the Barrels breech and the recoil shield, the chain acts a normal rotating cylinder would, its just a roller chain that must contain a live round each, from what I can make the russian handgun must fire the 7.62 Nagant revolver round, the fat apperance of each link is the nessary chamber to contain the cartridge's energy as it discharges.
its just a later version of the Josselyn patent of 1866, updated to fire a more substantal round than the original .22 rimfire.