It typically does not make a perfectly sharp line but I build a Dam of tape in the stock at the point I want the bedding to stop. this Dam needs to be thick enough to get you close to your barrel when inserted and tightened into the stock but not actually touching it. Now I top off the tape dam with some modeling clay which will compress as the barrel is inserted but somewhat prevent bedding material from flowing over the dam you have created with the tape. The trick is to get the tape dam as close to the barrel as you can without touching and this takes much trial and error. I attempt to get it so I can slide a piece of paper between the barrel and the dam when the action is tight in the stock, which means I need very little modeling clay.
Now it will still need some cleanup to look good because the modeling clay tends to stick to the bedding material. Using modeling clay that is the same color as your bedding helps with the looks somewhat. An exacto knife can be a big help at this point.
