The trouble with steel shot is that it will bulge even the toughest, thickest shotgun barrel at the choke, gradually destroying not only pattern density but the barrel itself. Plus, the battering also jams the choke tube threads into the barrel threads, making removal extremely difficult. Colonial has solved the problem by making their Super Steel tubes with the choke beyond the barrel and threads. Even Colonial's tough, thick, blued steel Super Steel tube will expand, but when it does, all that is damaged is the tube, not an expensive barrel. Just unscrew the tube and replace with a new one.
SPECS:
Blued steel. Approximately 2-5/8" (6.7cm) long, except "Long Invisible" which are 3-1/8" (7.9cm) long. Threads are as follows - Win-Choke® Invisible 12 ga. are .812"-32 tpi, 20 ga. are .687"-32 tpi; Tru-Choke™-style Tru-Tube 10 ga. are .865"-44, 12 ga. are .795"-44, 16 ga. are .718"-44, 20 ga. are .675"-44; Tru-Choke™- style Thinwall are .774"-44.
Catalog page 186