If you want a serious gunsmith (not a gun shop), personally I recommend Alain Depot. Well experienced and has several contracts with police & security agencies. His shop is in Granby, Google him for the exact address. I also read good words on a new fellow, Alain Combes in St-Rock-de-L'Achigan but seen none of his work.
Keep in mind that most gun shop in Montreal will outsource major work to real gunsmiths. I you need to thread your barrel to fit a muzzle break, I would deal with a gunsmith, not a gun shop. A gun shop may fix you with whatever they have on hand, not necessarily the best available out there . A gunsmith will find, order and fit what he thinks you want or specified.
Not all muzzle breaks are made equal (read my sig line...) and I would suggest to identify a few you could live with, otherwise the gunsmith will choose for you... May not be as pretty and quiet as you may have expected...
Hoping this helps.