The USLSS and then the USCG only had muzzle-loading lifesaving cannons. The official model for both services was the bronze gun "C" with two-diameter bronze barrel, 2.5 inch bore, and approx. 17 lb. shot. The barrel sat on a steel-bound oak carriage. There were probably over 100 different "commercial" models of line throwing cannons carried by merchant ships. They had to be USCG-approved, and I'm not sure if any breechloading line cannons were approved. Paul Barnett's book on thiis topic is excellent, well-illustrated and well-written (Paul used to be an English teacher before he took up cannon making.)