Baikal shotguns take the "Tru-choke" designed threads. I bought mine from Als' Corner Store in Arnprior, Ontario (Google them). Excellent service and good guys to deal with. I can personally vouch that the Tru-chokes I bought from them (a flush thread full choke steel rated and an extended extra-full choke steel rated) thread flawlessly and work excellently in my Baikal. I do use high-temp choke tube grease and that helps.
Note: If you order online the "small diameter" Tru-chokes are not for factory Baikal or other factory shotguns, as they are for thin barreled shotguns that a gunsmith threaded with a smaller diameter tap than normal.