You need to have an outer muzzle diameter of at least .735 to be able to have it threaded for 20 ga rem choke. The micrometer will tell you where you should cut your barrel. Mike Orlen is the only person i have found that will tap a 20 ga for rem chokes, and he does it @ very reasonable price. On my 20 ga H&R Topper with 22" barrel, its not quite thick enough at the muzzle for rem chokes, but it is thick enough to tap for invector 20 ga chokes(.725 minimum outer barrel diameter). Hope this helps.