Abstract: A method for forming a counter bore in the bore of a shotgun barrel and a choke tube insert for the counter bore. The insert includes an internal surface formed with a choke constriction for modifying the choke of the shotgun barrel and threads on its external surface for engaging complementary threads formed in the counter bore. Complementary angles are formed between the inner end of the choke tube and a ledge on the counter bore for preventing powder residue from accumulating between the threads. The method includes the steps of inserting a portion of a shotgun barrel in a frame and potting the barrel in a material which has a melting point substantially below the temperature which would affect the bluing of a shotgun barrel, but which when solidified rigidly holds the barrel when a counter bore is being reamed in its bore.