To do it right you would need to know what the I.D. is at that point. Is the choke area tapered or straight? If straight, a pin gage that Jent described would help you in finding what that I.D. is. Since the tool used cannot be put in from the muzzle end unless it is like the one shown with the threaded rod that tightens up the two pieces and expands it, you would have to put it in from the breech and tap it out.
As suggested, unless you have the means to do it, let those that know do it and then let us know how you make out.
